ACT4070B datasheet(6/9 Pages) ACTIVE-SEMI

6 / 9 page

ACT4070B

Rev 0, 23-Apr-12

Innovative Power

- 6 -

www.active-semi.com

Stability compensation

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:

COMP

VEA

OUT

VDC

808

The dominant pole P1 is due to CCOMP:

COMP

VEA

COMP

And finally, the third pole is due to RCOMP and

CCOMP2 (if CCOMP2 is used):

COMP2

COMP

Follow the following steps to compensate the IC:

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

the switching frequency via RCOMP:

but limit RCOMP to 15kΩ maximum.

OUT

The first zero Z1 is due to RCOMP and CCOMP:

The second pole P2 is the output pole:

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:

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

over frequency is 4.8/(VOUTCOUT). Therefore:

OUT

COMP

−

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:

And the proper value for CCOMP2 is:

COMP

ESROUT

OUT

COMP

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

22μF Ceramic

12kΩ

1.5nF

220pF

1.8V

100μF SP CAP

15kΩ

1.5nF

220pF

2.5V

100μF SP CAP

15kΩ

2.2nF

220pF

100μF SP CAP

15kΩ

4.7nF

220pF

Figure 3 shows a sample ACT4070B application

circuit generating a 2.5V/3A output.

(11)

(13)

(4)

(5)

(6)

(7)

(8)

OUT

COMP

OUT

COMP

808

(Ω)

(9)

006

Min

OUT

ESROUT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≥

−

(12)

(Ω)

COMP

(10)

(F)

: CCOMP2 is needed for board parasitic and high ESR output

capacitor.

: CCOMP2 is needed only for high ESR output capacitor

Figure 2:

Stability Compensation

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