IR3521 datasheet(34/45 Pages) IRF

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IR3521

Page 34

V3.03

VOLTAGE LOOP COMPENSATION

The adaptive voltage positioning (AVP) is usually adopted in the computer applications to improve the transient

response and reduce the power loss at heavy load. Like current mode control, the adaptive voltage positioning

loop introduces extra zero to the voltage loop and splits the double poles of the power stage, which make the

voltage loop compensation much easier.

Adaptive voltage positioning lowers the converter voltage by RO*IO, where RO is the required output impedance of

the converter.

The selection of compensation types depends on the output capacitors used in the converter. For the applications

using Electrolytic, Polymer or AL-Polymer capacitors and running at lower frequency, type II compensation shown

in Figure 21(a) is usually enough. While for the applications using only ceramic capacitors and running at higher

frequency, type III compensation shown in Figure 21(b) is preferred.

For applications where AVP is not required, the compensation is the same as for the regular voltage mode

control. For converter using Polymer, AL-Polymer, and ceramic capacitors, which have much higher ESR zero

frequency, type III compensation is required as shown in Figure 18(b) with RDRP and CDRP removed.

RCP

CCP1

EAOUT

CCP

RFB

RDRP

VO+

VDRP

VDAC

EAOUT

CFB

CDRP

RCP

EAOUT

CCP1

CCP

RFB

RDRP

VO+

VDRP

VDAC

EAOUT

RFB1

(a) Type II compensation

(b) Type III compensation

Figure 21 Voltage loop compensation network

Type II Compensation for AVP Applications

Determine the compensation at no load, the worst case condition. Choose the crossover frequency fc between

1/10 and 1/5 of the switching frequency per phase. Assume the time constant of the resistor and capacitor across

the output inductors matches that of the inductor, and determine RCP and CCP from (20) and (21), where LE and

CE are the equivalent inductance of output inductors and the equivalent capacitance of output capacitors

respectively.

)

(

)

(









(20)





(21)

CCP1 is optional and may be needed in some applications to reduce the jitter caused by the high frequency noise.

A ceramic capacitor between 10pF and 220pF is usually enough.

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