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LTC3703

3703f

Once VPROG is determined, RIMAX is chosen as follows:

RIMAX = VPROG/12µA

Note that in a boost mode architecture, it is only possible

to provide protection for “soft” shorts where VOUT > VIN.

For hard shorts, the inductor current is limited only by the

input supply capability. Refer to Current Limit Program-

ming for buck mode for further considerations for current

limit programming.

Boost Converter: Feedback Loop/Compensation

Compensating a voltage mode boost converter is unfortu-

nately more difficult than for a buck converter. This is due

to an additional right-half plane (RHP) zero that is present

in the boost converter but not in a buck. The additional

phase lag resulting from the RHP zero is difficult if not

impossible to compensate even with a Type 3 loop, so the

best approach is usually to roll off the loop gain at a lower

frequency than what could be achievable in buck con-

verter.

A typical gain/phase plot of a voltage-mode boost con-

verter is shown in Figure 16. The modulator gain and

phase can be measured as described for a buck converter

or can be estimated as follows:

GAIN (COMP-to-VOUT DC gain) = 20Log(VOUT2/VIN)

Dominant Pole: fP =

OUT

•

Since significant phase shift begins at frequencies above

the dominant LC pole, choose a crossover frequency no

greater than about half this pole frequency. The gain of the

compensation network should equal –GAIN at this fre-

quency so that the overall loop gain is 0dB here. The

compensation component to achieve this, using a Type 1

amplifier (see Figure 12), is:

G = 10–GAIN/20

C1 = 1/(2

π • f • G • R1)

Run/Soft-Start Function

The RUN/SS pin is a multipurpose pin that provide a soft-

start function and a means to shut down the LTC3703.

Soft-start reduces the input supply’s surge current by

gradually increasing the duty cycle and can also be used

for power supply sequencing.

Pulling RUN/SS below 1V puts the LTC3703 into a low

quiescent current shutdown (IQ ≅ 50µA). This pin can be

driven directly from logic as shown in Figure 17. Releasing

the RUN/SS pin allows an internal 4

µA current source to

charge up the soft-start capacitor CSS. When the voltage

on RUN/SS reaches 1V, the LTC3703 begins operating at

its minimum on-time. As the RUN/SS voltage increases

from 1V to 3V, the duty cycle is allowed to increase from

0% to 100%. The duty cycle control minimizes input

supply inrush current and elimates output voltage over-

shoot at start-up and ensures current limit protection even

with a hard short. The RUN/SS voltage is internally clamped

at 4V.

Figure 16. Transfer Function of Boost Modulator

GAIN

(dB)

PHASE

(DEG)

3703 F16

–90

–180

PHASE

GAIN

–12dB/OCT

Figure 17. LTC3703 Startup Operation

2ms/DIV

VOUT

5V/DIV

VIN = 50V

ILOAD = 2A

CSS = 0.01µF

RUN/SS

2V/DIV

2A/DIV

3703 F17

APPLICATIO S I FOR ATIO

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