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LTC1553

APPLICATIONS INFORMATION

application might exhibit 5A input ripple current. SANYO

OS-CON part number 10SA220M (220

µF/10V) capacitors

feature 2.3A allowable ripple current at 85

°C; three in

parallel at the input (to withstand the input ripple current)

will meet the above requirements. Similarly, AVX

TPSE337M006R0100 (330

µF/6V) have a rated maximum

ESR of 0.1

Ω; seven in parallel will lower the net output

capacitor ESR to 0.014

Ω.Forlowcostapplication,SANYO

MV-GX series of capacitors can be used with acceptable

performance.

Feedback Loop Compensation

The LTC1553 voltage feedback loop is compensated at the

COMP pin, attached to the output node of the internal gm

error amplifier. The feedback loop can generally be com-

pensated properly with an RC + C network from COMP to

GND as shown in Figure 10a.

Loop stability is affected by the values of the inductor,

output capacitor, output capacitor ESR, error amplifier

transconductance and error amplifier compensation net-

work. The inductor and the output capacitor creates a

double pole at the frequency:

fLC =

π√(LO)(COUT)

The ESR of the output capacitor forms a zero at the

frequency:

fESR =

π(ESR)(COUT)

The compensation network at the error amplifier output is

to provide enough phase margin at the 0dB crossover

frequency for the overall closed-loop transfer function.

The zero and pole from the compensation network are:

fZ =

π(RC)(CC) and

fP =

π(RC)(C1) respectively.

Figure 10b shows the Bode plot of the overall transfer

function.

The compensation value used in this design is based on

the following criteria: fSW = 12fCO, fZ = fLC and fP = 5fCO. At

the closed-loop frequency fCO, the attenuation due the LC

filter and the input resistor divider is compensated by the

gain of the PWM modulator and the gain of the error

amplifier (gmERR)(RC). Although a mathematical approach

to frequency compensation can be used, the added

Figure 10b. Bode Plot of the LTC1553 Overall Transfer Function

Figure 10a. Compensation Pin Hook-Up

– 20dB/DECADE

fCO

fESR

FREQUENCY

1553 F10b

fSW = LTC1553 SWITCHING

FREQUENCY

fCO = CLOSED-LOOP CROSSOVER

FREQUENCY

fLC

1553 F10

DAC

LTC1553

SENSE

COMP

ERR

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