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LTC1624

FEATURES DESCRIPTION U

N-Channel MOSFET Drive Implements Boost, Step-Down, SEPIC

and Inverting Regulators Wide VIN Range: 3.5V to 36V Operation Wide VOUT Range: 1.19V to 30V in Step-Down

Configuration ±1% 1.19V Reference Low Dropout Operation: 95% Duty Cycle 200kHz Fixed Frequency Low Standby Current Very High Efficiency Remote Output Voltage Sense Logic-Controlled Micropower Shutdown Internal Diode for Bootstrapped Gate Drive Current Mode Operation for Excellent Line and

Load Transient Response Available in an 8-Lead SO Package

Notebook and Palmtop Computers, PDAs Cellular Telephones and Wireless Modems Battery-Operated Digital Devices DC Power Distribution Systems Battery Chargers

TYPICAL APPLICATION U

The LTC®1624 is a current mode switching regulatorcontroller that drives an external N-channel power MOSFETusing a fixed frequency architecture. It can be operated inall standard switching configurations including boost,step-down, inverting and SEPIC. Burst ModeTM operationprovides high efficiency at low load currents. A maximumhigh duty cycle limit of 95% provides low dropout operationwhich extends operating time in battery-operated systems.

The operating frequency is internally set to 200kHz, allowingsmall inductor values and minimizing PC board space. Theoperating current level is user-programmable via an externalcurrent sense resistor. Wide input supply range allowsoperation from 3.5V to 36V (absolute maximum).

A multifunction pin (ITH / RUN ) all ows ex terna lcompensation for optimum load step response plusshutdown. Soft start can also be implemented with theITH /RUN pin to properly sequence supplies.

Figure 1. High Efficiency Step-Down Converter

+

+

SENSE–

ITH

/RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

100pF

CC470pF

RC6.8k

D1MBRS340T3

CB0.1µF

R235.7k

R120k

COUT100µF10V× 2

M1Si4412DY

L110µH

RSENSE0.05Ω

CIN22µF35V

× 2

VOUT3.3V2A

VIN4.8V TO 28V

1624 F01

, LTC and LT are registered trademarks of Linear Technology Corporation.Burst Mode is a trademark of Linear Technology Corporation.

High Efficiency SO-8N-Channel SwitchingRegulator Controller

APPLICATIONSU

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LTC1624

ABSOLUTE MAXIMUM RATINGS W W W U

Input Supply Voltage (VIN) .........................36V to – 0.3VTopside Driver Supply Voltage (BOOST)....42V to – 0.3VSwitch Voltage (SW)..................................36V to –0.6V

Differential Boost Voltage(BOOST to SW) ....................................7.8V to – 0.3V

SENSE – VoltageVIN < 15V.................................. (VIN + 0.3V) to –0.3VVIN ≥ 15V .......................... (VIN +0.3V) to (VIN – 15V)

ITH /RUN, VFB Voltages ............................ 2.7V to –0.3VPeak Driver Output Current < 10µs (TG) .................... 2AOperating Temperature Range LTC1624CS ............................................ 0°C to 70°C LTC1624IS......................................... – 40°C to 85°CJunction Temperature (Note 1) ............................. 125°CStorage Temperature Range ................. –65

°C to 150

°C

Lead Temperature (Soldering, 10 sec).................. 300°C

PACKAGE/ORDER INFORMATION W U U

ORDER PARTNUMBER

S8 PART MARKING

TOP VIEW

VIN

BOOST

TG

SW

SENSE–

ITH /RUN

VFB

GND

S8 PACKAGE8-LEAD PLASTIC SO

1

2

3

4

8

7

6

5

TJMAX = 125°C, θJA = 110°C/ W

LTC1624CS8LTC1624IS8

16241624I

Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS TA = 25°C, VIN = 15V, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Main Control Loop

IIN VFB Feedback Current (Note 2) 10 50 nA

VFB Feedback Voltage (Note 2) 1.1781 1.19 1.2019 V

∆VLINE REG Reference Voltage Line Regulation VIN = 3.6V to 20V (Note 2) 0.002 0.01 %/V∆VLOAD REG Output Voltage Load Regulation (Note 2)

ITH Sinking 5µA 0.5 0.8 %ITH Sourcing 5µA –0.5 –0.8 %VOVL Output Overvoltage Lockout 1.24 1.28 1.32 V

IQ Input DC Supply Current (Note 3)Normal Mode 550 900 µAShutdown VITH/RUN = 0V 16 30 µA

VITH/RUN Run Threshold 0.6 0.8 V

IITH/RUN Run Current Source VITH/RUN = 0.3V –0.8 –2.5 –5.0 µARun Pullup Current VITH/RUN = 1V –50 –160 –350 µA

∆VSENSE(MAX) Maximum Current Sense Threshold VFB = 1.0V 145 160 185 mVTG Transition Time

TG tr Rise Time CLOAD = 3000pF 50 150 ns

TG tf Fall Time CLOAD = 3000pF 50 150 nsfOSC Oscillator Frequency 175 200 225 kHz

VBOOST Boost Voltage SW = 0V, IBOOST = 5mA, VIN = 8V 4.8 5.15 5.5 V

∆VBOOST Boost Load Regulation SW = 0V, IBOOST = 2mA to 20mA 3 5 %

TJ = TA + (PD • 110°C/W)Note 2: The LTC1624 is tested in a feedback loop which servos VFB tothe midpoint for the error amplifier (VITH = 1.8V).

Note 3: Dynamic supply current is higher due to the gate charge beingdelivered at the switching frequency. See Applications Information.

The denotes specifications which apply over the full operatingtemperature range.

LTC1624CS: 0°C ≤ TA ≤ 70°CLTC1624IS: –40°C ≤ TA ≤ 85°C

Note 1: TJ is calculated from the ambient temperature TA and powerdissipation PD according to the following formula:

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LTC1624

TYPICAL PERFORMANCE CHARACTERISTICS U W

Efficiency vs Load CurrentVOUT = 3.3V

LOAD CURRENT (A)

E F F I C I E N C Y ( % )

100

95

90

85

80

75

700.001 0.1 1 10

1624 G08

0.01

VOUT = 5VVIN = 10VRSENSE = 0.033Ω

LOAD CURRENT (A)


100

95

90

85

80

75

700.001 0.1 1 10

1624 G07

0.01

VOUT = 3.3VRSENSE = 0.033Ω

VIN = 5V

VIN = 10V

Efficiency vs Input VoltageVOUT = 3.3V

INPUT VOLTAGE (V)

0


100

95

90

85

80

75

70

15 25

1624 G09

5 10 20 30

ILOAD = 1A

VOUT = 3.3VRSENSE = 0.033Ω

ILOAD = 0.1A

Efficiency vs Load CurrentVOUT = 5V

Efficiency vs Input VoltageVOUT = 5V

Input Supply Current vsInput Voltage

Boost Line Regulation

INPUT VOLTAGE (V)

0

B O O S T V O L T A

G E ( V )

6

5

4

3

2

1

015 25

1624 G04

5 10 20 30 35

IBOOST = 1mAVSW = 0V

BOOST LOAD CURRENT (mA)

0

B O O S T V O L T A

G E ( V )

6

5

4

3

2

1

015 25

1624 G06

5 10 20 30

VSW = 0V

VIN = 15V

VIN = 5V

Boost Load Regulation

TEMPERATURE (°C)

–40

B O O S T V O L T A

G E ( V )

6.0

5.5

5.0

4.5

4.035 85

1624 G15

–15 10 60 110 135

ILOAD = 1mA

LOAD CURRENT (A)

V I N – V O U T ( V )

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

1624 G11

0 0.5 1.0 1.5 2.0 2.5 3.0

RSENSE = 0.033Ω VOUT DROP OF 5%

INPUT VOLTAGE (V)

0

S U P P L Y C U R R E N T ( µ A )

700

600

500

400

300

200

100

015 25

1624 G05

5 10 20 30 35

SLEEP MODE

VFB = 1.21V

SHUTDOWN

INPUT VOLTAGE (V)

0


100

95

90

85

80

75

7015 25

1624 G10

5 10 20 30

ILOAD = 1A

VOUT = 5VRSENSE = 0.033Ω

ILOAD = 0.1A

VIN – VOUT Dropout Voltagevs Load Current

Boost Voltage vs Temperature

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LTC1624

TYPICAL PERFORMANCE CHARACTERISTICS U W

V I T H

/ R

U N

( V

)

1.2

0.8

2.4

00 IOUT(MAX)

IOUT

(a) 1624 G01

IOUT(MAX)

SHUTDOWN

ACTIVEMODE

VITH vs Output Current

TEMPERATURE (°C)

–40 I T H / R U N P I N S O U R C E C U R R E N T W I T H V I T H =

1 V ( µ A ) I T

H / R U

N P I N S O U R C E C U R R E N T WI T H V I T H = 0 V ( µA )

300

250

200

150

100

50

0

5

4

3

2

1

035 85

1624 G14

–15 10 60 110 135

ITH /RUN = 1V

ITH /RUN = 0V

ITH /RUN Pin Source Current vsTemperatureIITH vs VITH

I I T H

( µ

A )

200

150

50

03

VITH (V)

1.2 2.40.80

(b) 1624 G02

SHUTDOWN

ACTIVEMODE

Frequency vs Feedback Voltage

FEEDBACK VOLTAGE

0

F R E Q U E N C Y ( k H z )

250

200

150

100

50

01.00

1624 G03

0.25 0.50 0.75 1.25

TEMPERATURE (°C)

–40

C U R R E N T S E N S E T H R E S H O L D ( m V )

170

168

166

164

162

160

158

156

154

152

15010 60 85

1448 G13

–15 35 110 135

Maximum Current SenseThreshold vs Temperature

TEMPERATURE (°C)

–40

F R E Q U E N C Y ( k H z )

250

200

150

100

50

010 60 85

1448 G12

–15 35 110 135

VOUT IN REGULATION

VFB = 0V

PIN FUNCTIONS U U U

SENSE– (Pin 1): Connects to the (–) input for the currentcomparator. Built-in offsets between the SENSE – and VINpins in conjunction with RSENSE set the current trip thresh-

olds. Do not pull this pin more than 15V below VIN or morethan 0.3V below ground.

ITH /RUN (Pin 2): Combination of Error Amplifier Compen-sation Point and Run Control Inputs. The current com-parator threshold increases with this control voltage.Nominal voltage range for this pin is 1.19V to 2.4V. Forcingthis pin below 0.8V causes the device to be shut down. In

shutdown all functions are disabled and TG pin is held low.

VFB (Pin 3): Receives the feedback voltage from an exter-

nal resistive divider across the output.GND (Pin 4): Ground. Connect to the (–) terminal of COUT,the Schottky diode and the (–) terminal of CIN.

SW (Pin 5): Switch Node Connection to Inductor. In step-down applications the voltage swing at this pin is from aSchottky diode (external) voltage drop below ground toVIN.

Operating Frequency vsTemperature

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LTC1624

TG (Pin 6): High Current Gate Drive for Top N-ChannelMOSFET. This is the output of a floating driver with avoltage swing equal to INTVCC superimposed on the

switch node voltage SW.

BOOST (Pin 7): Supply to Topside Floating Driver. Thebootstrap capacitor CB is returned to this pin. Voltage

swing at this pin is from INTVCC to VIN + INTVCC in step-down applications. In non step-down topologies the volt-age at this pin is constant and equal to INTVCC if SW = 0V.

VIN (Pin 8): Main Supply Pin and the (+) Input to theCurrent Comparator. Must be closely decoupled to ground.

PIN FUNCTIONS U U U

(Refer to Functional Diagram)OPERATIOU

Main Control Loop

The LTC1624 uses a constant frequency, current modearchitecture. During normal operation, the top MOSFET isturned on each cycle when the oscillator sets the RS latchand turned off when the main current comparator I1 resetsthe RS latch. The peak inductor current at which I1 resetsthe RS latch is controlled by the voltage on the ITH /RUNpin, which is the output of error amplifier EA. The VFB pin,described in the pin functions, allows EA to receive anoutput feedback voltage from an external resistive divider.When the load current increases, it causes a slightdecrease in VFB relative to the 1.19V reference, which inturn causes the ITH /RUN voltage to increase until the

average inductor current matches the new load current.While the top MOSFET is off, the internal bottom MOSFETis turned on for approximately 300ns to 400ns to rechargethe bootstrap capacitor CB.

The top MOSFET driver is biased from the floating boot-strap capacitor CB that is recharged during each off cycle.The dropout detector counts the number of oscillatorcycles that the top MOSFET remains on and periodicallyforces a brief off period to allow CB to recharge.

The main control loop is shut down by pulling the ITH /RUN

pin below its 1.19V clamp voltage. Releasing ITH /RUNallows an internal 2.5µA current source to charge com-pensation capacitor CC. When the ITH /RUN pin voltagereaches 0.8V the main control loop is enabled with the ITH / RUN voltage pulled up by the error amp. Soft start can be

implemented by ramping the voltage on the ITH /RUN pinfrom 1.19V to its 2.4V maximum (see Applications Infor-mation section).

Comparator OV guards against transient output over-shoots >7.5% by turning off the top MOSFET and keepingit off until the fault is removed.

Low Current Operation

The LTC1624 is capable of Burst Mode operation in whichthe external MOSFET operates intermittently based onload demand. The transition to low current operationbegins when comparator B detects when the ITH /RUNvoltage is below 1.5V. If the voltage across RSENSE does

not exceed the offset of I2 (approximately 20mV) for onefull cycle, then on following cycles the top and internalbottom drives are disabled. This continues until the ITHvoltage exceeds 1.5V, which causes drive to be returned tothe TG pin on the next cycle.

INTVCC Power/Boost Supply

Power for the top and internal bottom MOSFET drivers isderived from VIN. An internal regulator supplies INTVCCpower. To power the top driver in step-down applicationsan internal high voltage diode recharges the bootstrap

capacitor CB during each off cycle from the INTVCC supply.A small internal N-channel MOSFET pulls the switch node(SW) to ground each cycle after the top MOSFET hasturned off ensuring the bootstrap capacitor is kept fullycharged.

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LTC1624

FUNCTIONAL DIAGRA U U W

(Shown in a step-down application)

– +

– +

– +

– +

+ +

– +

– +

I 2

I 1

8

4 k

8 k

4 k

V I N

3 µ

A R U N

C B

V O U T

C O U T

C I N

V I N

B O O S T

F L O A T I N G

D

R I V E R

T G

S W

D 1

L 1

N - C

H A N N E L

M O S F E T

N

- C H A N N E L

M

O S F E T

I N T V C C

R S E N S E

S E N S E –

D B

5 . 6

V

I N T V C C

R E G

V I N

0 . 8

V

1 . 1

9 V

2 0 0 k H z

2

0 0 k H z

1 . 1

9 V

I T H

/ R U N

1 . 2

8 V

1 . 1

9 V

1 8 0 k

1

. 5 V

3 µ

A

3

0 k

8 k

2 3

4 5 6 7 G

N D

1 6 2 4 F D

2 . 5 µ

A

S L O

P E

C O M

P

S

L O P E

C

O M P

S T

– +

B

R S

Q

O V

O S C

D R O P

-

O U T

D E T

1 - S

H O T

4 0 0 n s

S W I T C H

L O G I C

I N T V C C

V F B

V F B

R 2

R 1

R C C

C

C O S C

E A

1

g m

= 1

mΩ

1 . 1

9 V

R E F

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LTC1624

APPLICATIONS INFORMATION W U U U

The LTC1624 can be used in a wide variety of switchingregulator applications, the most common being the step-down converter. Other switching regulator architectures

include step-up, SEPIC and positive-to-negative converters.

The basic LTC1624 step-down application circuit is shownin Figure 1 on the first page. External component selectionis driven by the load requirement and begins with theselection of RSENSE. Once RSENSE is known, the inductorcan be chosen. Next, the power MOSFET and D1 areselected. Finally, CIN and COUT are selected. The circuitshown in Figure 1 can be configured for operation up to aninput voltage of 28V (limited by the external MOSFETs).

Step-Down Converter: RSENSE Selection forOutput Current

RSENSE is chosen based on the required output current.The LTC1624 current comparator has a maximum thresh-old of 160mV/RSENSE. The current comparator thresholdsets the peak of the inductor current, yielding a maximumaverage output current IMAX equal to the peak value lesshalf the peak-to-peak ripple current, ∆IL.

Allowing a margin for variations in the LTC1624 andexternal component values yields:

R mVI

SENSEMAX

= 100

The LTC1624 works well with values of RSENSE from0.005Ω to 0.5Ω.

Step-Down Converter: Inductor Value Calculation

With the operating frequency fixed at 200kHz smallerinductor values are favored. Operating at higher frequen-cies generally results in lower efficiency because ofMOSFET gate charge losses. In addition to this basic

trade-off, the effect of inductor value on ripple current andlow current operation must also be considered.

The inductor value has a direct effect on ripple current. Theinductor ripple current ∆IL decreases with higher induc-tance and increases with higher VIN or VOUT:

∆I V Vf L

V V

V VL

IN OUT OUT D

IN D

= −

( )( )+

+

where VD is the output Schottky diode forward drop.

Accepting larger values of ∆IL allows the use of lowinductances, but results in higher output voltage rippleand greater core losses. A reasonable starting point for

setting ripple current is ∆IL = 0.4(IMAX). Remember, themaximum ∆IL occurs at the maximum input voltage.

The inductor value also has an effect on low currentoperation. Lower inductor values (higher ∆IL) will causeBurst Mode operation to begin at higher load currents,which can cause a dip in efficiency in the upper range oflow current operation. In Burst Mode operation lowerinductance values will cause the burst frequency todecrease. In general, inductor values from 5µH to 68µHare typical depending on the maximum input voltage and

output current. See also Modifying Burst Mode Operationsection.

Step-Down Converter: Inductor Core Selection

Once the value for L is known, the type of inductor must beselected. High efficiency converters generally cannotafford the core loss found in low cost powdered iron cores,forcing the use of more expensive ferrite, molypermalloyor Kool Mµ® cores. Actual core loss is independent of coresize for a fixed inductor value, but it is very dependent oninductance selected. As inductance increases, core losses

go down. Unfortunately, increased inductance requiresmore turns of wire and, therefore, copper losses willincrease.

Ferrite designs have very low core loss and are preferredat high switching frequencies, so design goals can con-centrate on copper loss and preventing saturation. Ferritecore material saturates “hard,” which means that induc-tance collapses abruptly when the peak design current isexceeded. This results in an abrupt increase in inductorripple current and consequent output voltage ripple. Donot allow the core to saturate!

Molypermalloy (from Magnetics, Inc.) is a very good, lowloss core material for toroids, but it is more expensive thanferrite. A reasonable compromise from the same manu-facturer is Kool Mµ. Toroids are very space efficient,especially when you can use several layers of wire.Because they generally lack a bobbin, mounting is moredifficult. However, designs for surface mount that do notincrease the height significantly are available.

Kool Mu is a registered trademark of Magnetics, Inc.

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ratings that are ideal for input capacitor applications.Consult the manufacturer for other specific recommend-ations.

INTVCC Regulator

An internal regulator produces the 5V supply that powersthe drivers and internal circuitry within the LTC1624.Good VIN bypassing is necessary to supply the hightransient currents required by the MOSFET gate drivers.

High input voltage applications in which large MOSFETsare being driven at high frequencies may cause the maxi-mum junction temperature rating for the LTC1624 to beexceeded. The supply current is dominated by the gate

charge supply current as discussed in the EfficiencyConsiderations section. The junction temperature can beestimated by using the equations given in Note 1 of theElectrical Characteristics table. For example, the LTC1624is limited to less than 17mA from a 30V supply:

TJ = 70°C + (17mA)(30V)(110°C/W) = 126°C

To prevent maximum junction temperature from beingexceeded, the input supply current must be checkedoperating in continuous mode at maximum VIN.

Step-Down Converter: Topside MOSFET DriverSupply (CB, DB)

An external bootstrap capacitor CB connected to the BOOSTpin supplies the gate drive voltage for the topside MOSFET.Capacitor CB in the functional diagram is charged throughinternal diode DB from INTVCC when the SW pin is low.When the topside MOSFET is to be turned on, the driverplaces the CB voltage across the gate to source of theMOSFET. This enhances the MOSFET and turns on thetopside switch. The switch node voltage SW rises to VINand the BOOST pin rises to V IN + INTVCC. The value of the

boost capacitor CB needs to be 50 times greater than thetotal input capacitance of the topside MOSFET. In mostapplications 0.1µF is adequate.

Significant efficiency gains can be realized by supplyingtopside driver operating voltage from the output, since theVIN current resulting from the driver and control currentswill be scaled by a factor of (Duty Cycle)/(Efficiency). For5V regulators this simply means connecting the BOOST

monly used for design because even significant deviationsdo not offer much relief. Note that capacitor manufacturer’sripple current ratings are often based on only 2000 hours

of life. This makes it advisable to further derate thecapacitor, or to choose a capacitor rated at a highertemperature than required. Several capacitors may also beparalleled to meet size or height requirements in thedesign. Always consult the manufacturer if there is anyquestion.

The selection of COUT is driven by the required effectiveseries resistance (ESR). Typically, once the ESR require-ment is satisfied the capacitance is adequate for filtering.The output ripple (∆VOUT) is determined by:

∆ ∆V I ESRfC

OUT LOUT

≈ +

1

4

where f = operating frequency, COUT = output capacitanceand ∆IL = ripple current in the inductor. The output rippleis highest at maximum input voltage since ∆IL increaseswith input voltage. With ∆IL = 0.4IOUT(MAX) the outputripple will be less than 100mV at maximum VIN, assuming:

COUT Required ESR < 2RSENSE

Manufacturers such as Nichicon, United Chemicon andSANYO should be considered for high performancethrough-hole capacitors. The OS-CON semiconductordielectric capacitor available from SANYO has the lowestESR(size) product of any aluminum electrolytic at a some-what higher price. Once the ESR requirement for COUT hasbeen met, the RMS current rating generally far exceedsthe IRIPPLE(P-P) requirement.

In surface mount applications multiple capacitors mayhave to be paralleled to meet the ESR or RMS currenthandling requirements of the application. Aluminum elec-

trolytic and dry tantalum capacitors are both available insurface mount configurations. In the case of tantalum it iscritical that the capacitors are surge tested for use inswitching power supplies. An excellent choice is the AVXTPS series of surface mount tantalums, available in caseheights ranging from 2mm to 4mm. Other capacitor typesinclude SANYO OS-CON, Nichicon WF series and Sprague595D series and the new ceramics. Ceramic capacitors arenow available in extremely low ESR and high ripple current

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LTC1624

pin through a small Schottky diode (like a CentralCMDSH-3) to VOUT as shown in Figure 10. However, for3.3V and other lower voltage regulators, additional cir-

cuitry is required to derive boost supply power from theoutput.

For low input voltage operation (VIN < 7V), a Schottkydiode can be connected from VIN to BOOST to increase theexternal MOSFET gate drive voltage. Be careful not toexceed the maximum voltage on BOOST to SW pinsof 7.8V.

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

V V R

ROUT = +

1 19 1

2

1.

The external resistive divider is connected to the output asshown in Figure 2, allowing remote voltage sensing. Whenusing remote sensing, a local 100Ω resistor should beconnected from L1 to R2 to prevent VOUT from runningaway if the sense lead is disconnected.


ITH /RUN

CC

RC

1624 F03

D1

3.3VOR 5V ITH /RUN

CC

RC

(a) (b)

D1

C1

R1

ITH /RUN

CC

RC

(c)

Figure 3. ITH / RUN Pin Interfacing

Soft start can be implemented by ramping the voltage onITH /RUN during start-up as shown in Figure 3(c). As thevoltage on ITH/RUN ramps from 1.19V to 2.4V the internalpeak current limit is also ramped at a proportional linearrate. The peak current limit begins at approximately10mV/RSENSE (at VITH/RUN = 1.4V) and ends at:

160mV/RSENSE (VITH/RUN = 2.4V)The output current thus ramps up slowly, charging theoutput capacitor. The peak inductor current and maximumoutput current are as follows:

IL(PEAK) = (VITH/RUN – 1.3V)/(6.8RSENSE)

IOUT(MAX) = ILPEAK – ∆IL / 2

with ∆IL = ripple current in the inductor.

During normal operation the voltage on the ITH /RUN pinwill vary from 1.19V to 2.4V depending on the load current.

Pulling the ITH /RUN pin below 0.8V puts the LTC1624 intoa low quiescent current shutdown (IQ< 30µA). This pin canbe driven directly from logic as shown in Figures 3(a)and 3(b).

Efficiency Considerations

The percent efficiency of a switching regulator is equal tothe output power divided by the input power times 100%.It is often useful to analyze individual losses to determine

ITH /RUN Function

The ITH /RUN pin is a dual purpose pin that provides theloop compensation and a means to shut down the LTC1624.

Soft start can also be implemented with this pin. Soft startreduces surge currents from VIN by gradually increasingthe internal current limit. Power supply sequencing canalso be accomplished using this pin.

An internal 2.5µA current source charges up the externalcapacitor CC. When the voltage on ITH /RUN reaches 0.8Vthe LTC1624 begins operating. At this point the erroramplifier pulls up the ITH /RUN pin to its maximum of 2.4V(assuming VOUT is starting low).

Figure 2. Setting the LTC1624 Output Voltage

LTC1624

VFB

GND

100pF

R2

L1

R1

VOUT

1624 F02

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LTC1624

what is limiting the efficiency and which change wouldproduce the most improvement. Percent efficiency can beexpressed as:

%Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentageof input power.

Although all dissipative elements in the circuit producelosses, four main sources usually account for most of thelosses in LTC1624 circuits:

1. LTC1624 VIN current2. I2R losses3. Topside MOSFET transition losses

4. Voltage drop of the Schottky diode1. The VIN current is the sum of the DC supply current IQ,

given in the Electrical Characteristics table, and theMOSFET driver and control currents. The MOSFETdriver current results from switching the gatecapacitance of the power MOSFET. Each time a MOSFETgate is switched from low to high to low again, a packetof charge dQ moves from INTVCC to ground. Theresulting dQ/dt is a current out of VIN which is typicallymuch larger than the control circuit current. Incontinuous mode, IGATECHG = f (QT + QB), where QT and

QB are the gate charges of the topside and internalbottom side MOSFETs.

By powering BOOST from an output-derived source(Figure 10 application), the additional VIN currentresulting from the topside driver will be scaled by afactor of (Duty Cycle)/(Efficiency). For example, in a20V to 5V application, 5mA of INTVCC current results inapproximately 1.5mA of VIN current. This reduces themidcurrent loss from 5% or more (if the driver waspowered directly from VIN) to only a few percent.

2. I2R losses are predicted from the DC resistances of theMOSFET, inductor and current shunt. In continuousmode the average output current flows through L but is“chopped” between the topside main MOSFET/currentshunt and the Schottky diode. The resistances of thetopside MOSFET and RSENSE multiplied by the dutycycle can simply be summed with the resistance of L toobtain I2R losses. (Power is dissipated in the senseresistor only when the topside MOSFET is on. The I2R

loss is thus reduced by the duty cycle.) For example, at50% DC, if RDS(ON) = 0.05Ω, RL = 0.15Ω and RSENSE =0.05Ω, then the effective total resistance is 0.2Ω. Thisresults in losses ranging from 2% to 8% for VOUT = 5Vas the output current increases from 0.5A to 2A. I2Rlosses cause the efficiency to drop at high outputcurrents.

3. Transition losses apply only to the topside MOSFET(s),and only when operating at high input voltages (typically20V or greater). Transition losses can be estimatedfrom:

Transition Loss = 2.5(VIN)1.85 (IMAX)(CRSS)(f)

4. The Schottky diode is a major source of power loss athigh currents and gets worse at high input voltages.The diode loss is calculated by multiplying the forwardvoltage drop times the diode duty cycle multiplied bythe load current. For example, assuming a duty cycle of50% with a Schottky diode forward voltage drop of0.5V, the loss is a relatively constant 5%.

As expected, the I2R losses and Schottky diode lossdominate at high load currents. Other losses includingCIN and COUT ESR dissipative losses and inductor corelosses generally account for less than 2% total additional

loss.

Checking Transient Response

The regulator loop response can be checked by looking atthe load transient response. Switching regulators takeseveral cycles to respond to a step in DC (resistive) loadcurrent. When a load step occurs, VOUT immediately shiftsby an amount equal to (∆ILOAD • ESR), where ESR is theeffective series resistance of COUT. ∆ILOAD also begins tocharge or discharge COUT which generates a feedbackerror signal. The regulator loop then acts to return VOUT to

its steady-state value. During this recovery time VOUT canbe monitored for overshoot or ringing that would indicatea stability problem. The ITH external components shown inthe Figure 1 circuit will provide adequate compensation formost applications.

A second, more severe transient, is caused by switching inloads with large (>1µF) supply bypass capacitors. Thedischarged bypass capacitors are effectively put in parallel


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LTC1624

with COUT, causing a rapid drop in VOUT. No regulator candeliver enough current to prevent this problem if the loadswitch resistance is low and it is driven quickly. The only

solution is to limit the rise time of the switch drive so thatthe load rise time is limited to approximately (25 • CLOAD).Thus a 10µF capacitor would require a 250µs rise time,limiting the charging current to about 200mA.

Automotive Considerations: Plugging into theCigarette Lighter

As battery-powered devices go mobile there is a naturalinterest in plugging into the cigarette lighter in order toconserve or even recharge battery packs during operation.But before you connect, be advised: you are plugging intothe supply from hell. The main battery line in an automo-bile is the source of a number of nasty potential transients,including load dump, reverse battery and double battery.

Load dump is the result of a loose battery cable. When thecable breaks connection, the field collapse in the alternatorcan cause a positive spike as high as 60V which takesseveral hundred milliseconds to decay. Reverse battery isjust what it says, while double battery is a consequence oftow-truck operators finding that a 24V jump start crankscold engines faster than 12V.

The network shown in Figure 4 is the most straightforwardapproach to protect a DC/DC converter from the ravagesof an automotive battery line. The series diode preventscurrent from flowing during reverse battery, while thetransient suppressor clamps the input voltage during loaddump. Note that the transient suppressor should notconduct during double battery operation, but must stillclamp the input voltage below breakdown of the converter.Although the LTC1624 has a maximum input voltage of


36V, most applications will be limited to 30V by theMOSFET BVDSS.

Modifying Burst Mode Operation

The LTC1624 automatically enters Burst Mode operationat low output currents to boost efficiency. The point whencontinuous mode operation changes to Burst Mode op-eration scales as a function of maximum output current.The output current when Burst Mode operation com-mences is approximately 8mV/RSENSE (8% of maximumoutput current).

With the additional circuitry shown in Figure 5 the LTC1624can be forced to stay in continuous mode longer at low

output currents. Since the LTC1624 is not a fully synchro-nous architecture, it will eventually start to skip cycles asthe load current drops low enough. The point when theminimum on-time (450ns) is reached determines the loadcurrent when cycle skipping begins at approximately 1%of maximum output current. Using the circuit in Figure 5the LTC1624 will begin to skip cycles but stays in regula-tion when IOUT is less than IOUT(MIN):

It f

L

V V V V

V VOUT MIN

ON MIN

IN OUT IN D

OUT D( )

( )=

−

( )

+

+

2

2

where tON(MIN) = 450ns, f = 200kHz.

The transistor Q1 in the circuit of Figure 5 operates as acurrent source developing an 18mV offset across the

+

+

1000pF 100Ω18mV

R*

COUT

L1

RSENSE CIN

VOUT

D1MBRS340T3

Q12N2222

VIN

1624 F05

(VOUT – 0.7V)

180µA*R =

VIN

LTC1624

SENSE

–

TG

SW

+–

Figure 5. Modifying Burst Mode OperationFigure 4. Plugging into the Cigarette Lighter

LTC1624

VIN

50A IPKRATING

12V

TRANSIENT VOLTAGESUPPRESSOR

GENERAL INSTRUMENT1.5KA24A

1624 F04

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LTC1624

100Ω resistor in series with the SENSE – pin. This offsetcancels the internal offset in current comparator I2 (referto Functional Diagram). This comparator in conjunction

with the voltage on the ITH /RUN pin determines when toenter into Burst Mode operation (refer to Low CurrentOperation in Operation section). With the additional exter-nal offset present, the drive to the topside MOSFET isalways enabled every cycle and constant frequency opera-tion occurs for IOUT > IOUT(MIN).

Step-Down Converter: Design Example

As a design example, assume VIN = 12V(nominal),VIN = 22V(max), VOUT = 3.3V and IMAX = 2A. RSENSE canimmediately be calculated:

RSENSE = 100mV/2A = 0.05Ω

Assume a 10µH inductor. To check the actual value of theripple current the following equation is used:

∆I V Vf L

V V

V VL

IN OUT OUT D

IN D

= −

( )( )+

+

The highest value of the ripple current occurs at themaximum input voltage:

∆I V VkHz HV VV V

L = −( )

++ =22 3 3

200 103 3 0 522 0 5

1 58. . .. .µAP-P

The power dissipation on the topside MOSFET can beeasily estimated. Choosing a Siliconix Si4412DY resultsin: RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum inputvoltage with T(estimated) = 50°C:

P

V V

V VA C C

V A pF kHz mW

MAIN =++ ( )

+ ( ) ° − °( )[ ]( )

+ ( ) ( )( )( )=

3 3 0 5

22 0 52 1 0 005 50 25 0 042

2 5 22 2 100 200 62

2

1 85

. ..

. .

. .

Ω

The most stringent requirement for the Schottky diodeoccurs when VOUT= 0V (i.e. short circuit) at maximum VIN.In this case the worst-case dissipation rises to:

P I VV

V VD SC AVG D

IN

IN D

= ( ) +

( )


With the 0.05Ω sense resistor ISC(AVG) = 2A will result,increasing the 0.5V Schottky diode dissipation to 0.98W.

CIN is chosen for an RMS current rating of at least 1.0A at

temperature. COUT is chosen with an ESR of 0.03Ω for lowoutput ripple. The output ripple in continuous mode will behighest at the maximum input voltage. The output voltageripple due to ESR is approximately:

VORIPPLE = RESR(∆IL) = 0.03Ω (1.58AP-P) = 47mVP-P

Step-Down Converter: Duty Cycle Limitations

At high input to output differential voltages the on-timegets very small. Due to internal gate delays and responsetimes of the internal circuitry the minimum recommended

on-time is 450ns. Since the LTC1624’s frequency is inter-nally set to 200kHz a potential duty cycle limitation exists.When the duty cycle is less than 9%, cycle skipping mayoccur which increases the inductor ripple current but doesnot cause VOUT to lose regulation. Avoiding cycle skippingimposes a limit on the input voltage for a given outputvoltage only when VOUT < 2.2V using 30V MOSFETs.(Remember not to exceed the absolute maximum voltageof 36V.)

VIN(MAX) = 11.1VOUT + 5V For DC > 9%

Boost Converter Applications

The LTC1624 is also well-suited to boost converter appli-cations. A boost converter steps up the input voltage to ahigher voltage as shown in Figure 6.

Figure 6. Boost Converter

+

CB

L1

M1R2

R1

RSENSE CIN

D1

VIN

1624 F06

VIN

VFB

LTC1624

SENSE–

BOOST

TG

SWGND

+COUT

VOUT

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LTC1624

Boost Converters: Power MOSFET Selection

One external N-channel power MOSFET must be selectedfor use with the LTC1624 for the switch. In boost applica-tions the source of the power MOSFET is grounded alongwith the SW pin. The peak-to-peak gate drive levels are setby the INTVCC voltage. The gate drive voltage is equal toapproximately 5V for VIN > 5.6V and a logic level MOSFETcan be used. At VIN voltages below 5V the gate drivevoltage is equal to VIN – 0.6V and a sublogic level MOSFETshould be used.

Selection criteria for the power MOSFET include the “ON”resistance RDS(ON), reverse transfer capacitance CRSS,input voltage and maximum output current. When the

LTC1624 is operating in continuous mode the duty cyclefor the MOSFET is given by:

Main Switch Duty Cycle = 1−+

V

V V

IN

OUT D

The MOSFET power dissipation at maximum output cur-rent is given by:


Boost Converter: Inductor Selection

For most applications the inductor will fall in the range of10

µH to 100

µH. Higher values reduce the input ripple

voltage and reduce core loss. Lower inductor values arechosen to reduce physical size.

The input current of the boost converter is calculated at fullload current. Peak inductor current can be significantlyhigher than output current, especially with smaller induc-tors and lighter loads. The following formula assumescontinuous mode operation and calculates maximum peakinductor current at minimum VIN:

I I

V

V

I

L PEAK OUT MAXOUT

IN MIN

L MAX

( ) ( ) ( )

( )=

+

∆

2

The ripple current in the inductor (∆IL) is typically 20% to30% of the peak inductor current occuring at VIN(MIN) andIOUT(MAX).

∆IV V V V

kHz L V VL

IN OUT D IN

OUT DP-P( ) =

+ −( )( )( ) +( )200

with ∆IL(MAX) = ∆IL(P-P) at VIN = VIN(MIN).

Remember boost converters are not short-circuit pro-tected, and that under output short conditions, inductorcurrent is limited only by the available current of the inputsupply, IOUT(OVERLOAD). Specify the maximum inductorcurrent to safely handle the greater of IL(PEAK) orIOUT(OVERLOAD). Make sure the inductor’s saturation cur-rent rating (current when inductance begins to fall)exceeds the maximum current rating set by RSENSE.

Boost Converter: RSENSE Selection for Maximum

Output CurrentRSENSE is chosen based on the required output current.Remember the LTC1624 current comparator has a maxi-mum threshold of 160mV/RSENSE. The current compara-tor threshold sets the peak of the inductor current, yieldinga maximum average output current IOUT(MAX) equal toIL(PEAK) less half the peak-to-peak ripple current (∆IL),divided by the output-input voltage ratio (see equation forIL(PEAK)).

P IV

V VR

k V I C kHz

where I IV V

V

MAIN IN MAX

IN MIN

OUT DDS ON

OUT IN MAX RSS

IN MAX OUT MAXOUT D

IN MIN

=

− +

+( ) +

( )

( )( )

= +

( ) ( )

( )

( )

( ) ( )( )

2

1 85

1 1

200

δ

.

δ is the temperature dependency of RDS(ON) and k is aconstant inversely related to the gate drive current.

MOSFETs have I2R losses, plus the PMAIN equationincludes an additional term for transition losses that arehighest at high output voltages. For VOUT < 20V the highcurrent efficiency generally improves with larger MOSFETs,while for VOUT > 20V the transition losses rapidly increaseto the point that the use of a higher RDS(ON) device withlower CRSS actual provides higher efficiency. For addi-tional information refer to Step-Down Converter: PowerMOSFET Selection in the Applications Informationsection.

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LTC1624

Allowing a margin for variations in the LTC1624 (withoutconsidering variation in RSENSE), assuming 30% ripplecurrent in the inductor, yields:

R mV

I

V

V VSENSE

OUT MAX

IN MIN

OUT D

=+

( )

( )100

Boost Converter: Output Diode

The output diode conducts current only during the switchoff-time. Peak reverse voltage for boost converters isequal to the regulator output voltage. Average forwardcurrent in normal operation is equal to output current.

Remember boost converters are not short-circuit pro-tected. Check to be sure the diode’s current rating exceedsthe maximum current set by RSENSE. Schottky diodes suchas Motorola MBR130LT3 are recommended.

Boost Converter: Output Capacitors

The output capacitor is normally chosen by its effectiveseries resistance (ESR), because this is what determinesoutput ripple voltage.

Since the output capacitor’s ESR affects efficiency, uselow ESR capacitors for best performance. Boost regula-tors have large RMS ripple current in the output capacitorthat must be rated to handle the current. The outputcapacitor ripple current (RMS) is:

C I V V

VOUT OUT

OUT IN

IN

IRIPPLE RMS( ) ≈ −

Output ripple is then simply: VOUT = RESR (∆IL(RMS)).

Boost Converter: Input Capacitors

The input capacitor of a boost converter is less critical dueto the fact that the input current waveform is triangular,and does not contain large square wave currents as foundin the output capacitor. The input voltage source imped-ance determines the size of the capacitor that is typically10µF to 100µF. A low ESR is recommended although notas critical as the output capacitor and can be on the orderof 0.3Ω. Input capacitor ripple current for the LTC1624used as a boost converter is:


CV V V

kHz L VIN

IN OUT IN

OUT

IRIPPLE ≈ ( ) −( )( )( )( )0 3

200

.

The input capacitor can see a very high surge current whena battery is suddenly connected and solid tantalum capaci-tors can fail under this condition. Be sure to specify surgetested capacitors.

Boost Converter: Duty Cycle Limitations

The minimum on-time of 450ns sets a limit on how closeVIN can approach VOUT without the output voltage over-shooting and tripping the overvoltage comparator. Unlessvery low values of inductances are used, this should neverbe a problem. The maximum input voltage in continuousmode is:

VIN(MAX) = 0.91VOUT + 0.5V For DC = 9%

SEPIC Converter Applications

The LTC1624 is also well-suited to SEPIC (Single EndedPrimary Inductance Converter) converter applications.The SEPIC converter shown in Figure 7 uses two induc-tors. The advantage of the SEPIC converter is the inputvoltage may be higher or lower than the output voltage.

The first inductor L1 together with the main N-channelMOSFET switch resemble a boost converter. The secondinductor L2 and output diode D1 resemble a flyback orbuck-boost converter. The two inductors L1 and L2 can beindependent but also can be wound on the same core since

Figure 7. SEPIC Converter

+

+

CB

L1

L2M1 R2

R1

RSENSE CIN

D1C1

VIN

1624 F07

VIN

VFB

LTC1624

SENSE

–

BOOST

TG

SWGND

+COUT

VOUT

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LTC1624

10µH inductance is obtained with a current rating of 4A.Splitting the two windings creates two 10µH inductorswith a current rating of 2A each. Therefore substitute

(2)(10µH) = 20µH for L in the equations.Specify the maximum inductor current to safely handleIL(PEAK). Make sure the inductor’s saturation current rat-ing (current when inductance begins to fall) exceeds themaximum current rating set by RSENSE.

SEPIC Converter: RSENSE Selection for MaximumOutput Current

RSENSE is chosen based on the required output current.Remember the LTC1624 current comparator has a maxi-

mum threshold of 160mV/RSENSE. The current compara-tor threshold sets the peak of the inductor current, yieldinga maximum average output current IOUT(MAX) equal toIL1(PEAK) less half the peak-to-peak ripple current, ∆IL,divided by the output-input voltage ratio (see equation forIL1(PEAK)).

Allowing a margin for variations in the LTC1624 (withoutconsidering variation in RSENSE), assuming 30% ripplecurrent in the inductor, yields:

R mVI

V

V VSENSE

OUT MAX

IN MIN

OUT D= +

( )

( )100

SEPIC Converter: Output Diode

The output diode conducts current only during the switchoff-time. Peak reverse voltage for SEPIC converters isequal to VOUT + VIN. Average forward current in normaloperation is equal to output current. Peak current is:

I I

V V

V ID PEAK OUT MAX OUT D

IN MIN L1 1( ) ( ) ( )= +

+

+ ∆

Schottky diodes such as MBR130LT3 are recommended.

SEPIC Converter: Input and Output Capacitors

The output capacitor is normally chosen by its effectiveseries resistance (ESR), because this is what determines


output ripple voltage. The input capacitor needs to be sizedto handle the ripple current safely.

Since the output capacitor’s ESR affects efficiency, uselow ESR capacitors for best performance. SEPIC regula-tors, like step-down regulators, have a triangular currentwaveform but have maximum ripple at VIN(MAX). The inputcapacitor ripple current is:

I I

RIPPLE RMSL

( ) = ∆

12

The output capacitor ripple current is:

I IRIPPLE RMS OUT V

VOUT

IN( ) =

The output capacitor ripple voltage (RMS) is:

VOUT(RIPPLE) = 2(∆IL)(ESR)

The input capacitor can see a very high surge current whena battery is suddenly connected, and solid tantalumcapacitors can fail under this condition. Be sure to specifysurge tested capacitors.

SEPIC Converter: Coupling Capacitor (C1)

The coupling capacitor C1 in Figure 7 sees a nearlyrectangular current waveform. During the off-time thecurrent through C1 is IOUT(VOUT /VIN) while approximately– IOUT flows though C1 during the on-time. This currentwaveform creates a triangular ripple voltage on C1:

∆V I

kHz C

V

V V VC

OUT OUT

IN OUT D1

200 1=

( )( )

+ +

The maximum voltage on C1 is then:

VC1(MAX) = VIN + ∆VC1 /2 (typically close to VIN(MAX)).

The ripple current though C1 is:

I IV

VRIPPLE C OUT

OUT

IN1( ) =

The maximum ripple current occurs at IOUT(MAX) andVIN(MIN). The capacitance of C1 should be large enough so

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LTC1624


Positive-to-Negative Converter: Output VoltageProgramming

Setting the output voltage for a positive-to-negative con-verter is different from other architectures since the feed-back voltage is referenced to the LTC1624 ground pin andthe ground pin is referenced to –VOUT. The output voltageis set by a resistive divider according to the followingformula:

V V R

RV

DC

DCOUT IN= +

≈ −

−

1 19 1

1

2 1.

The external resistive divider is connected to the output asshown in Figure 8.

Positive-to-Negative Converter: PowerMOSFET Selection

One external N-channel power MOSFET must be selectedfor use with the LTC1624 for the switch. As in step-downapplications the source of the power MOSFET is con-nected to the Schottky diode and inductor. The peak-to-peak gate drive levels are set by the INTVCC voltage. Thegate drive voltage is equal to approximately 5V for VIN >5.6V and a logic level MOSFET can be used. At VIN voltages

below 5V the INTVCC voltage is equal to VIN – 0.6V and asublogic level MOSFET should be used.

Selection criteria for the power MOSFET include the “ON”resistance RDS(ON), reverse transfer capacitance CRSS,input voltage and maximum output current. When theLTC1624 is operating in continuous mode the duty cyclefor the MOSFET is given by:

MainV

V V

D

OUT D

Switch Duty Cycle =V

V

OUT

IN

+

+ +

with VOUT being the absolute value of VOUT.

The MOSFET power dissipation and maximum switchcurrent are given by:

that the voltage across C1 is constant such that VC1 = VINat full load over the entire VIN range. Assuming the enegrystorage in the coupling capacitor C1 must be equal to the

enegry stored in L1, the minimum capacitance of C1 is:

CL I V

VMIN

OUT OUT

IN MIN

11

2 2

4( )( )

= ( ) ( )

SEPIC Converter: Duty Cycle Limitations

The minimum on-time of 450ns sets a limit on how highan input-to-output ratio can be tolerated while not skip-ping cycles. This only impacts designs when very low

output voltages (VOUT < 2.5V) are needed. Note that aSEPIC converter would not be appropriate at these lowoutput voltages. The maximum input voltage is (remem-ber not to exceed the absolute maximum limit of 36V):

VIN(MAX) = 10.1VOUT + 5V For DC > 9%

Positive-to-Negative Converter Applications

The LTC1624 can also be used as a positive-to-negativeconverter with a grounded inductor shown in Figure 8.Since the LTC1624 requires a positive feedback signal

relative to device ground, Pin 4 must be tied to theregulated negative output. A resistive divider from thenegative output to ground sets the output voltage.Remember not to exceed maximum VIN ratings VIN +VOUT ≤ 36V.

PMAIN = ISW MAX ×

I OUT MAX I + δ RDS ON +

k V IN MAX + VOUT1.85

CRSS 200kHz

( )

( )

( )

( ) ( )( )( )( )

{ {Figure 8. Positive-to-Negative Converter

+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

8

7

6

5

1

2

3

4100pF

CCRC

D1

CB

R1

R2

COUT

M1

L1

RSENSE CIN

–VOUT

VIN

1624 F08

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LTC1624

where IV V V

VOUT MAX

IN OUT D

IN

: ISW MAX( ) ( )= + +

δ is the temperature dependency of RDS(ON) and k is aconstant inversely related to the gate drive current. Themaximum switch current occurs at VIN(MIN) and the peakswitch current is ISW(MAX)+ ∆IL /2. The maximum voltageacross the switch is VIN(MAX) + VOUT.

MOSFETs have I2R losses plus the PMAIN equationincludes an additional term for transition losses that arehighest at high total input plus output voltages. For(VOUT+ VIN) < 20V the high current efficiency generally

improves with larger MOSFETs, while for (VOUT+ VIN)> 20V the transition losses rapidly increase to the pointthat the use of a higher RDS(ON) device with lower CRSSactual provides higher efficiency. For additional informa-tion refer to the Step-Down Converter: Power MOSFETSelection in the Applications Information section.

Positive-to-Negative Converter: Inductor Selection

For most applications the inductor will fall in the range of10µH to 100µH. Higher values reduce the input and outputripple voltage (although not as much as step-down con-

verters) and also reduce core loss. Lower inductor valuesare chosen to reduce physical size and improve transientresponse but do increase output ripple.

Like the boost converter, the input current of the positive-to-negative converter is calculated at full load current.Peak inductor current can be significantly higher thanoutput current, especially with smaller inductors (withhigh ∆IL values). The following formula assumes continu-ous mode operation and calculates maximum peak induc-tor current at minimum VIN:

I IV V V

V

IL PEAK OUT MAX

IN OUT D

IN

L( ) ( )=

+ +

+

∆2

The ripple current in the inductor (∆IL) is typically 20% to50% of the peak inductor current occuring at VIN(MIN) andIOUT(MAX) to minimize output ripple. Maximum ∆IL occursat minimum VIN.


∆IV V V

kHz L V V VL

IN OUT D

IN OUT DP-P( ) =

( ) +( )

( )( ) + +

( )200

Specify the maximum inductor current to safely handleIL(PEAK). Make sure the inductor’s saturation current rat-ing (current when inductance begins to fall) exceeds themaximum current rating set by RSENSE.

Positive-to-Negative Converter: RSENSE Selection forMaximum Output Current

RSENSE is chosen based on the required output current.Remember the LTC1624 current comparator has a maxi-mum threshold of 160mV/RSENSE. The current compara-tor threshold sets the peak of the inductor current, yieldinga maximum average output current IOUT(MAX) equal toIL(PEAK) less half the peak-to-peak ripple current with theremainder divided by the duty cycle.

Allowing a margin for variations in the LTC1624 (withoutconsidering variation in RSENSE) and assuming 30% ripplecurrent in the inductor, yields:

R mV

I

V

V V VSENSE

OUT MAX

IN MIN

IN MIN OUT D

=+ +

( )

( )

( )

100

Positive-to-Negative Converter: Output Diode

The output diode conducts current only during the switchoff-time. Peak reverse voltage for positive-to-negativeconverters is equal to VOUT+ VIN. Average forwardcurrent in normal operation is equal to ID(PEAK) – ∆IL /2.Peak diode current (occurring at VIN(MIN)) is:

I IV V

V

I

D PEAK OUT MAX

OUT D

IN

L

( ) ( )=

+( )+

+1

2

∆

Positive-to-Negative Converter: Input andOutput Capacitors

The output capacitor is normally chosen by its effectiveseries resistance (ESR), because this is what determinesoutput ripple voltage. Both input and output capacitorsneed to be sized to handle the ripple current safely.

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LTC1624

TYPICAL APPLICATIONS U

Figure 9. LTC1624 Layout Diagram (See Board Layout Checklist)

+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

100pF

CC 560pF

RC 4.7k

D1MBRS340T3

D2CMDSH-3

CB 0.1µF

8

7

6

5

1

2

3

4

R235.7k

1%

R1

11k1%

COUT 100µF10V

× 2

M1Si4412DY

L1*10µH

RSENSE 0.033Ω

CIN 22µF35V× 2

VOUT 5V3A

VIN 5.3V TO 28V

1624 F10

*COILTRONICS CTX10-4

0.1µF

Figure 10. 5V/3A Converter with Output Derived Boost Voltage

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

8

7

6

5

1

2

3

4100pF

BOLD LINES INDICATEHIGH CURRENT PATHS

CCRC

D1

CB0.1µF

R2

R1

COUT

M1L1

+

–

RSENSECIN VIN

+

–

VOUT

1624 F09

+

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LTC1624


+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

100pF

CC330pF

RC3.3k

D1

MBRS130LT3

CB0.1µF

8

7

6

5

1

2

3

4R2

35.7k1%

R13.92k

1%

COUT100µF16V× 2

M1Si4412DY

L1*22µH

RSENSE

0.04Ω

CIN22µF35V

× 2

VOUT12V0.75A

VIN5.2V TO 11V

1624 F13

*SUMIDA CDRH125-220

0.1µF

Figure 11. Wide Input Range 1.8V/1.5A Converter

+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

100pF

CC 470pF

RC 6.8k

D1MBRS140T3

CB 0.1µF

8

7

6

5

1

2

3

4

R235.7k

1%

R13.92k

1%

COUT 100µF16V× 2

M1Si4412DY L1*

47µH

RSENSE 0.068Ω

CIN 22µF35V× 2

VOUT 12V1A

VIN 12.3V TO 28V

1624 F12

*SUMIDA CDRH125-470

0.1µF

Figure 12. 12V/1A Low Dropout Converter

Figure 13. 12V/0.75A Boost Converter

+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

100pF

CC 470pF

RC 6.8k

D1MBRS340T3

CB 0.1µF

8

7

6

5

1

2

3

4

R235.7k

1%

R169.8k

1%

COUT 100µF10V× 2

M1Si6436DY L1*

10µH

RSENSE

0.068Ω

CIN 22µF35V× 2

VOUT 1.8V1.5A

VIN 4.8V TO 22V

1624 F11

*SUMIDA CDR105B-100

0.1µF

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LTC1624


+

+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

100pF

CC330pF

RC4.7k

D1MBRS130LT3

CB0.1µF

8

7

6

5

1

2

3

4R2

35.7k1%

R13.92k

1%

COUT100µF16V× 2

M1Si4412DY

L1a*

L1b*

RSENSE0.068Ω

CIN

22µF35V 22µF35V

VOUT12V0.5A

VIN5V TO 15V

1624 F14


0.1µF

+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

100pF

CC470pF

RC6.8k

D1MBRS340T3

CB0.1µF

8

7

6

5

1

2

3

4

R235.7k

1%

R120k1%

COUT100µF10V× 2

M1Si6426DQ L1*

20µH

RSENSE0.068Ω

CIN22µF35V× 2

VOUT3.3V1.5A

VIN3.5V TO 18V

1624 F16


0.1µF

Figure 16. Low Dropout 3.3V/1.5A Converter

Figure 14. 12V/0.4A SEPIC Converter

Figure 15. Inverting –5V/2A Converter

+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

0.1µF

100pF

CC 1000pFRC

3.3k

R3

100k

VCCVCC

SHUTDOWN

D1MBRS340T3

D2CMDSH-3CB

0.1µF

8

7

6

5

1

2

3

4

R278.7k

1%

R124.9k1%

COUT 100µF10V× 2

M2VN2222

M3TP0610L

M1Si4410DY L1*

33µH

RSENSE 0.025Ω

CIN 22µF35V× 2

VOUT –5V2A

VIN 5V TO 22V

1624 F15*COILCRAFT DO5022P-333

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24/2824

LTC1624


+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

100pF

CC330pF

RC6.8k

D1MBRS130LT3D2

CMDSH-3

CB0.1µF

8

7

6

5

1

2

3

4

+

R235.7k

1%

R111k1%

COUT100µF16V× 2

M1Si6426DQ L1b*

L1a*

RSENSE0.05Ω

CIN22µF35V× 2 22µF

35V

VOUT5V1A

VOUT

VIN3.6V TO 18V

1624 F17

* COILTRONICS CTX20-4

0.1µF

Figure 17. 5V/1A SEPIC Converter with Output Derived Boost Voltage

Figure 18. 24V to 12V/10A Buck Converter with Output-Derived Boost Voltage

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

CIN1, CIN2 1000µF35V× 2

VIN 13V TO

28V

D1*

CB 0.1µF

C53.3µF50V

C4, 0.1µF

8

7

6

5

1

2

3

4

RSENSE2, 0.015Ω

R111k1%

RC 20k

CC 100pF

M1*

COUT 2700µF16V

R5220Ω

R2100k1%

Z1IN 755

L1 VOUT 12V10A

1624 F18

CIN1, CIN2 = SANYO 35MV1000GXC5, C7 = WIMA MKS2COUT = SANYO 16MV2700GXD1 = MOTOROLA MBR2535CTL1 = PULSE ENGINEERING PO472

M1 = INTERNATIONAL RECTIFIER IRL3803RSENSE1, RSENSE2 = IRC LR2010-01-R015-F* BOTH D1 AND M1 MOUNTED TO SAME

THERMALLOY #6399B HEAT SINK

RSENSE1, 0.015Ω

+

C73.3µF50V

C90.1µF

C10220pF

D2MBR0540

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LTC1624


Figure 20. 12V to 24V/5A Boost Converter

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

C3100pF

C41500pF

CIN100µF16V

VIN9V TO

15V

VOUT24V5A

D1*

CB0.1µF

C50.1µF

8

7

6

5

1

2

3

4

R2, 1M, 1%

RSENSE0.005Ω, 5%

R152.3k

R5750Ω0.5W

RC27k

CC4700pF

M1*

COUT11000µF35V

+ COUT21000µF35V

Z1IN7557.5V

L110µH

1624 F20

CIN = KEMET T495X107M016ASCOUT1, COUT2 = SANYO 35MV 1000GXD1 = MOTOROLA MBR2535CT

L1 = MAGNETICS CORE #55930AZ WINDING = 8T#14BIFM1 = INTERNATIONAL RECTIFIER IRL 3803RSENSE = IRC OAR-3, 0.005Ω, 5%

+

*BOTH D1 AND Q1 MOUNTED ON THERMALLOY MODEL 6399 HEAT SINK

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

C3100pF

CIN 22µF35V

VIN 20V TO

32V

D1

CB 0.1µF

C50.1µF

8

7

6

5

1

2

3

4

R2, 1M, 1%

RSENSE 0.025Ω

R113.3k

RC 6.8k

CC 820pF

M1

COUT 100µF100V

L147µH

VOUT 90V0.5A

1624 F19

CIN = KEMET T495X226M035ASCOUT = SANYO 100MV100GXD1 = MOTOROLA MBRS1100

L1 = COILCRAFT D05022P-473M1 = INTERNATIONAL RECTIFIER IRL 540NSRSENSE = IRC LR2010-01-R025-F

+

Figure 19. 24V to 90V at 0.5A Boost Converter

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LTC1624


SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

C4100pF

CIN1, CIN222µF35V× 2

VIN

13V TO

28V+VIN

D1MBRS340

CB0.1µF

C100.1µF

C110.1µF

C121µF

C9100pF

C130.1µF

+VIN

C5, 0.1µF

8

7

6

5

1

2

3

4

RSENSE0.033Ω

R13.92k

R610k

R756k

R81MCURRENTADJ

R235.7k

RC10k

CC330pF

M1

COUT100µF16V× 2

L127µH

R40.025Ω VOUT

12V3A

1624 F21

CIN1, CIN2 = KEMET T495X226M035ASL1 = SUMIDA CDRH127-270RSENSE = IRC LR2010-01-R033-FR4 = IRC LR2010-01-R025-FM1 = SILICONIX Si4412DYQ2 = MOTOROLA MMBT A14

C14, 0.01µF

+

SENSE

IOUT

GND

–IN

AVE

PROG

VCC

+IN

1

2

3

4

8

7

6

5

LTC1620

OUT

NC/ADJ

GND

NC

IN

NC

NC

SHDN

1

2

3

4

8

7

6

5

LT1121-5

+

Q2

Figure 21. 12V/3A Adjustable Current Power Supply for Battery Charger or Current Source Applications

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LTC1624


Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

1 2 3 4

0.150 – 0.157**

(3.810 – 3.988)

8 7 6 5

0.189 – 0.197*(4.801 – 5.004)

0.228 – 0.244

(5.791 – 6.197)

0.016 – 0.050

0.406 – 1.270

0.010 – 0.020

(0.254 – 0.508)

× 45°

0°– 8° TYP0.008 – 0.010

(0.203 – 0.254)

SO8 0996

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

TYPDIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

*

**

PACKAGE DESCRIPTION U

Dimensions in inches (millimeters) unless otherwise noted.

S8 Package8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

Figure 22. High Current 3.3V/6.5A Converter

+

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

1000pF

0.1µF

100pF

CC680pF

RC3.3k

D1MBRD835L

CB0.1µF

8

7

6

5

1

2

3

4

R235.7k

1%

R120k1%

COUT100µF10V× 3

M1**L1*8µH

RSENSE0.015Ω

CIN

22µF35V× 3

VOUT3.3V6.5A

VIN4.8V TO 28V

1624 F22

* PANASONIC 12TS-7ROLB** SILICONIX SUD50N03-10

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LTC1624

TYPICAL APPLICATION U

PART NUMBER DESCRIPTION COMMENTS

LTC1147 High Efficiency Step-Down Controller 100% DC, Burst Mode Operation, 8-Pin SOand PDIP

LTC1148HV/LTC1148 High Efficiency Synchronous Step-Down Controllers 100% DC, Burst Mode Operation, VIN < 20V

LTC1149 High Efficiency Synchronous Step-Down Controller 100% DC,Std Threshold MOSFETs, VIN < 48V

LTC1159 High Efficiency Synchronous Step-Down Controller 100% DC, Logic Level MOSFETs, VIN < 40V

LTC1174 Monolithic 0.6A Step-Down Switching Regulator 100% DC, Burst Mode Operation, 8-Pin SO

LTC1265 1.2A Monolithic High Efficiency Step-Down Switching Regulator 100% DC, Burst Mode Operation, 14-Pin SO

LTC1266 High Efficiency Synchronous Step-Down Controller, N-Channel Drive 100% DC, Burst Mode Operation, VIN < 20V

LT®1375/LT1376 1.5A, 500kHz Step-Down Switching Regulators High Frequency

LTC1433/LTC1434 Monolithic 0.45A Low Noise Current Mode Step-Down Switching Regulators 16- and 20-Pin Narrow SSOP

LTC1435 High Efficiency Low Noise Synchronous Step-Down Controller, Burst Mode Operation, 16-Pin Narrow SON-Channel Drive

LTC1436/LTC1436-PLL High Efficiency Low Noise Synchronous Step-Down Controllers, Adaptive PowerTM Mode, 20- and 24-Pin SSOPN-Channel Drive

LTC1474/LTC1475 Ultralow Quiesent Current Step-Down Monolithic Switching Regulators 100% DC, 8-Pin MSOP, VIN < 20V

Adaptive Power is a trademark of Linear Technology Corporation.

RELATED PARTS

Figure 23. 5V to 3.3V/10A Converter (Surface Mount)

+

SENSE–

ITH /RUN

VFB

GND

VIN

BOOST

TG

SW

LTC1624

D1

10k

Q2

CB0.1µF

C2100pF

CIN100µF10V

× 4C4, 0.1µF

C3, 0.033µF

8

7

6

5

1

2

3

4

RSENSE0.0082Ω

RC6.8k

CC820pF

M1

COUT470µF

6.3V× 2

L11.68µH +VOUT

3.3V10A

+VIN4.5V TO5.5V

VOUT

RTN

1624 F23

CIN (× 4) = KEMET T495D107M010ASCOUT (× 2) = AVX TPSV477M006R0055D1 = MOTOROLA MBRB2515LD2 = MOTOROLA MBR0520

L1 = PULSE ENGINEERING PE53691M1 = INTERNATIONAL RECTIFIER IRL3803SQ2 = MOTOROLA MMBTA14LT1 RSENSE = IRC OAR3-R0082

R3, 10Ω

C8100pF

D2

+

R120k

R235.7k

ltc1624f

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