LTC3832 datasheet(16/24 Pages) LINER

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LTC3832/LTC3832-1

sn3832 3832fs

LTC3832 application might exhibit 5A input ripple cur-

rent. Sanyo OS-CON capacitors, part number 10SA220M

(220

µF/10V), feature 2.3A allowable ripple current at

°C; three in parallel at the input (to withstand the input

ripple current) meet the above requirements. Similarly,

Sanyo POSCAP 4TPB470M (470

µF/4V) capacitors have a

maximum rated ESR of 0.04

Ω; three in parallel lower the

net output capacitor ESR to 0.013

Ω.

Feedback Loop Compensation

The LTC3832 voltage feedback loop is compensated at the

COMP pin, which is the output node of the error amplifier.

The feedback loop is generally compensated with an RC +

C network from COMP to GND as shown in Figure 10a.

Loop stability is affected by the values of the inductor, the

output capacitor, the output capacitor ESR, the error

amplifier transconductance and the error amplifier com-

pensation network. The inductor and the output capacitor

create a double pole at the frequency:

OUT

=π

[]

(

)(

)

The ESR of the output capacitor and the output capacitor

value form a zero at the frequency:

ESR C

ESR

OUT

=π

[]

(

)(

)

The compensation network used with the error amplifier

must provide enough phase margin at the 0dB crossover

frequency for the overall open-loop transfer function. The

zero and pole from the compensation network are:

fZ = 1/[2π(RC)(CC)] and

fP = 1/[2π(RC)(C1)] respectively

Figure 10b shows the Bode plot of the overall transfer

function.

When low ESR output capacitors (Sanyo OS-CON) are

used, the ESR zero can be high enough in frequency that

it provides little phase boost at the loop crossover fre-

quency. As a result, the phase margin becomes

inadequate and the load transient is not optimized. To

resolve this problem, a small capacitor can be connected

APPLICATIO S I FOR ATIO

3832 F10a

LTC3832

VREF

SENSE–

SENSE+

VFB

COMP

ERR

Figure 10a. Compensation Pin Hook-Up

3832 F10b

3832 F10c

fLC

fZC2

fCO

fP fPC2

fESR

fCO

FREQUENCY

20dB/DECADE

fSW = LTC3832 SWITCHING

FREQUENCY

fCO = CLOSED-LOOP CROSSOVER

FREQUENCY

fSW = LTC3832 SWITCHING

FREQUENCY

fCO = CLOSED-LOOP CROSSOVER

FREQUENCY

Figure 10b. Bode Plot of the LTC3832 Overall Transfer Function

Figure 10c. Bode Plot of the LTC3832 Overall

Transfer Function Using a Low ESR Output Capacitor

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