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ACT4455YH-T Datasheet(PDF) 8 Page - Active-Semi, Inc |
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ACT4455YH-T Datasheet(HTML) 8 Page - Active-Semi, Inc |
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8 / 15 page  ACT4455 Rev 2, 21-Nov-12 Innovative Power TM - 8 - www.active-semi.com Copyright © 2012 Active-Semi, Inc. STABILITY COMPENSATION Figure 2: Stability Compensation : CCOMP2 is needed only for high ESR output capacitor The feedback loop 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: 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): The following steps should be used to compensate the IC: STEP 1. Set the cross over frequency at 1/10 of the switching frequency via RCOMP: 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 6.36 / (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 capacitor has sufficiently low ESR, a small value CCOMP2 such as 100pF may improve stability against PCB layout parasitic effects. Table 1 shows some calculated results based on the compensation method above. Table 1: Typical Compensation for Different Output Voltages and Output Capacitors : CCOMP2 is needed for high ESR output capacitor. Output Cable Resistance Compensation To compensate for resistive voltage drop across the charger's output cable, the ACT4455 integrates a simple, user-programmable cable voltage drop compensation using the impedance at the FB pin. Use the curve in Figure 3 to choose the proper feedback resistance values for cable compensation. RFB1 is the high side resistor of voltage divider. (8) COMP VEA OUT VDC G A I V 808 . 0 A = (9) COMP VEA EA 1 P C A π 2 G f = (10) OUT OUT OUT 2 P C V π 2 I f = (11) COMP1 COMP 1 Z C R 2 1 f π = (12) COMP2 COMP 3 P C R π 2 1 f = (13) (Ω) OUT OUT 8 C V 10 48 . 0 × = V 808 . 0 G G 10 f C V 2 R COMP EA SW OUT OUT COMP × = π (14) (F) COMP 5 COMP R 10 18 . 3 C − × = OUT OUT 6 COMP C V 10 67 . 6 C − × = (F) (15) ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × × ≥ − OUT OUT 6 ESRCOUT V 012 . 0 , C 10 1 . 1 Min R (16) (Ω) (17) COMP ESRCOUT OUT 2 COMP R R C C = VOUT COUT RCOMP CCOMP CCOMP2 2.5V 47μF SP CAP 5.6kΩ 5.6nF None 3.3V 47μF SP CAP 7.5kΩ 4.7nF None 5V 47μF SP CAP 11kΩ 3.3nF None 2.5V 680μF/6.3V/30mΩ 15kΩ 3.3nF 220pF 3.3V 680μF/6.3V/30mΩ 15kΩ 3.3nF 220pF 5V 680μF/6.3V/30mΩ 15kΩ 4.7nF 220pF |
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