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ACT4012A Datasheet(PDF) 6 Page - Active-Semi, Inc |
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ACT4012A Datasheet(HTML) 6 Page - Active-Semi, Inc |
6 / 10 page ACT4012A Rev0, 16-May-08 Innovative Power TM - 6 - www.active-semi.com Copyright © 2008 Active-Semi, Inc. STABILITY COMPENSATION 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 3.4 / (VOUTCOUT). Therefore: 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 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 Voltages and Output Capacitors VOUT COUT RCOMP CCOMP CCOMP2 2.5V 22µF Ceramic 12kΩ 1.5nF None 3.3V 22µF Ceramic 15kΩ 1.5nF None 5V 22µF Ceramic 15kΩ 1.5nF None 2.5V 47µF SP CAP 15kΩ 1.5nF None 3.3V 47µF SP CAP 15kΩ 1.8nF None 5V 47µF SP CAP 15kΩ 2.7nF None 2.5V 470µF/6.3V/30mΩ 15kΩ 15nF 1nF 3.3V 470µF/6.3V/30mΩ 15kΩ 22nF 1nF 5V 470µF/6.3V/30mΩ 15kΩ 27nF None : CCOMP2 is needed for high ESR output capacitor. Figure 3 shows an example ACT4012A application circuit gener- ating a 5V/1.5A output. COMP ESRCOUT OUT 2 COMP R R C C = (13) Figure 2: Stability Compensation (10) (F) COMP 5 COMP R 10 8 . 1 C − × = (11) (F) OUT OUT 5 COMP C V 10 2 . 1 C − × = : CCOMP2 is needed only for high ESR output capacitor The feedback system of the IC is stabilized by the components at the 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: 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 1/10 of the switching frequency via RCOMP: but limit RCOMP to 15kΩ maximum. The first zero Z1 is due to RCOMP and CCOMP: The second pole P2 is the output pole: COMP2 COMP 3 P C R π 2 1 f = (8) (Ω) (9) V 3 . 1 G G 10 f C V π 2 R COMP EA SW OUT OUT COMP × = OUT OUT 8 C V 10 2 × = (4) COMP VEA OUT VDC G A I V 3 . 1 A = (5) COMP VEA EA 1 P C A π 2 G f = (6) OUT OUT OUT 2 P C V π 2 I f = (12) (Ω) ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × × ≥ − OUT OUT 6 ESRCOUT V 012 . 0 , C 10 1 . 1 Min R (7) COMP2 COMP 1 Z C R π 2 1 f = CCOMP2 CCOMP RCOMP ACT4012A COMP 1 |
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