ACT4075YH-T 查看數據表（PDF） - Active-Semi, Inc

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ACT4075YH-T

Wide Input 2.5A Step Down Converter

Active-Semi, Inc 

ACT4075

Rev PrA, 13-Aug-07

Stability compensation

Figure 2:

Stability Compensation

: CCOMP2 is needed only for high ESR output capacitor

The feedback system of the IC is stabilized by the

components at COMP pin, as shown in Figure 2.

The DC loop gain of the system is determined by

the following equation:

AVDC

 1.222V

IOUT

AVEAGCOMP

(4)

The dominant pole P1 is due to CCOMP:

fP 1



G EA

2  AVEA C COMP

(5)

The second pole P2 is the output pole:

fP 2



IOUT

2 VOUT COUT

(6)

The first zero Z1 is due to RCOMP and CCOMP:

fZ1



1

2 RCOMP CCOMP

(7)

And finally, the third pole is due to RCOMP and

CCOMP2 (if CCOMP2 is used):

fP 3



1

2 RCOMP CCOMP2

(8)

Follow the following steps to compensate the IC:

STEP 1. Set the cross over frequency at 1/10 of the

switching frequency via RCOMP:

RCOMP

 2πVOUT COUT fSW

10GEAGCOMP 1.222V



1 .25

x10

V 8

OUT

COUT

(Ω) (9)

but limit RCOMP to 15kΩ maximum.

STEP 2. Set the zero fZ1 at 1/4 of the cross over

frequency. If RCOMP is less than 15kΩ, the equation

for CCOMP is:

CCOMP

 1.6 x10 5

RCOMP

(F)

(10)

If RCOMP is limited to 15kΩ, then the actual cross

over frequency is 4.8/(VOUTCOUT). Therefore:

CCOMP



8.8

x10

V 6

OUT

COUT

(F)

(11)

STEP 3. If the output capacitor’s ESR is high

enough to cause a zero at lower than 4 times the

cross over frequency, an additional compensation

capacitor CCOMP2 is required. The condition for us-

ing CCOMP2 is:

RESROUT

 Min1.1CxO1U0T 6 ,0.012VOUT 

(Ω)

(12)

And the proper value for CCOMP2 is:

CCOMP

 COUT RESROUT

RCOMP

(13)

Though CCOMP2 is unnecessary when the output

capacitor has sufficiently low ESR, a small value

CCOMP2 such as 220pF may improve stability

against PCB layout parasitic effects.

Table 2 shows some calculated results based on

the compensation method above.

Table 2:

Typical Compensation for Different Output

Voltages and Output Capacitors

VOUT

COUT

RCOMP CCOMP CCOMP2

1.8V 22μF Ceramic 4kΩ 3.3nF 220pF

2.5V 22μF Ceramic 5.6kΩ 3.3nF 220pF

5V

22μF Ceramic 12kΩ 1.5nF 220pF

1.8V 100μF SP CAP 15kΩ 1.5nF 220pF

2.5V 100μF SP CAP 15kΩ 2.2nF 220pF

5V 100μF SP CAP 15kΩ 4.7nF 220pF

: CCOMP2 is needed for board parasitic and high ESR output

capacitor.

Figure 3 shows a sample ACT4075 application

circuit generating a 2.5V/2.5A output.

Innovative Products. Active Solutions.

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www.active-semi.com

Copyright © 2007 Active-Semi, Inc.

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