ACT4065ASH-T datasheet(6/10 Pages) ACTIVE-SEMI

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ACT4065A

Rev 0, 23-Apr-12

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:

The dominant pole P1 is due to CCOMP:

The second pole P2 is the output pole:

The first zero Z1 is due to RCOMP and CCOMP:

And finally, the third pole is due to RCOMP and

CCOMP2 (if CCOMP2 is used):

Follow the following steps to compensate the IC:

STEP 1. Set the cross over frequency at 1/5 of the

switching frequency via RCOMP:

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:

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

over frequency is 6.1/ (VOUTCOUT). Therefore:

STEP 3. If the output capacitors 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 using CCOMP2 is

required. The condition for using CCOMP2 is:

And the proper value for CCOMP2 is:

Though CCOMP2 is unnecessary when the output ca-

pacitor has sufficiently low ESR, a small value

CCOMP2 such as 100pF 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 volt-

ages and Output Capacitors

Figure 3 shows a sample ACT4065A application

circuit generating a 2.5V/2A output.

VOUT

COUT

RCOMP CCOMP

CCOMP2

2.5V

22μF Ceramic

12kΩ

2.2nF

None

3.3V

22μF Ceramic

12kΩ

1.5nF

None

22μF Ceramic

15kΩ

2.2nF

None

2.5V

47μF SP Cap

15kΩ

1.5nF

None

3.3V

47μF SP Cap

15kΩ

1.8nF

None

47μF SP Cap

15kΩ

2.7nF

None

2.5V

470µF/6.3V/30mΩ

15kΩ

1.5nF

47pF

3.3V

470µF/6.3V/30mΩ

15kΩ

2.2nF

47pF

470µF/10V/30mΩ

15kΩ

2.7nF

47pF

: CCOMP2 is needed only for high ESR output capacitors

Stability compensation

Figure 2:

Stability Compensation

COMP

VEA

OUT

VDC

808

(4)

COMP

VEA

(5)

OUT

(6)

COMP

(7)

COMP2

COMP

(8)

(9)

808

COMP

OUT

COMP

(Ω)

(10)

(F)

(11)

(F)

(12)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

≥

−

OUT

ESROUT

012

Min

(Ω)

COMP

ESROUT

OUT

COMP

(13)

OUT

COMP

−

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