BUCK2-外围芯片手册ACT4060_

ACT4060 Wide Input 2A Step Down Converter FEATURES 2A Output Current Up to 95% Efficiency 4.75V to 20V Input Range 8A Shutdown Supply Current 410kHz Switching Frequency Adjustable Output Voltage Cycle-by-Cycle Current Limit Protection Thermal Shutdown Protection Frequency Foldback at Short Circuit Stability with Wide Range of Capacitors, Including Low ESR Ceramic Capacitors SOP-8 Package APPLICATIONS TFT LCD Monitors Portable DVDs Car-Powered or Battery-Powered Equipments Set-Top Boxes Telecom Power Supplies DSL and Cable Modems and Routers Termination Supplies GENERAL DESCRIPTION The ACT4060 is a current-mode step-down DC/DC converter that generates up to 2A of output current at 410kHz switching frequency. The device utilizes Active-Semis proprietary ISOBCD20 process for operation with input voltages up to 20V. Consuming only 8A in shutdown mode, the ACT4060 is highly efficient with peak operating efficiency at 95%. Protection features include cycle-by-cycle current limit, thermal shutdown, and frequency foldback at short circuit. The ACT4060 is available in a SOP-8 package and requires very few external devices for operation. Active-Semi, Inc.- 1www.micro- Figure 1. Typical Application Circuit Data Sheet Rev7 , 7/2006 ACT4060 IN BS ENFB GCOMP SW + ENABLE Up to 20V 2.5V/2A ACT4060 ORDERING INFORMATION PART NUMBERTEMPERATURE RANGEPACKAGEPINSPACKING ACT4060SH-40C to 85CSOP-88TUBE ACT4060SH-T-40C to 85CSOP-88TAPE it should never be directly connected to IN. THERMAL SHUTDOWN The ACT4060 automatically turns off when its junction temperature exceeds 160C. Active-Semi, Inc.- 4 - www.micro- IN EN COMP FB BS SW REGULATOR however, low ESR tantalum or electrolytic types may also be used provided that the RMS ripple current rating is higher than 50% of the output current. The input capacitor should be placed close to the IN and G pins of the IC, with the shortest traces possible. In the case of tantalum or electrolytic types, they can be further away if a small parallel 0.1F ceramic capacitor is placed right next to the IC. OUTPUT CAPACITOR The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is: ESRRIPPLEOUTMAXRIPPLE RKIV= OUT 2 SW IN LCf28 V + (3) where IOUTMAX is the maximum output current, KRIPPLE is the ripple factor, RESR is the ESR of the output capacitor, fSW is the switching frequency, L is the inductor value, and COUT is the output capacitance. In the case of ceramic output capacitors, RESR is very small and does not contribute to the ripple. Therefore, a lower capacitance value can be used for ceramic capacitors. In the case of tantalum or electrolytic capacitors, the ripple is dominated by RESR multiplied by the ripple current. In that case, the output capacitor is chosen to have sufficiently low ESR. For ceramic output capacitors, typically choose a capacitance of about 22F. For tantalum or electrolytic capacitors, choose a capacitor with less than 50m ESR. RECTIFIER DIODE Use a Schottky diode as the rectifier to conduct current when the High-Side Power Switch is off. The Schottky diode must have a current rating higher than the maximum output current and a reverse voltage rating higher than the maximum input voltage. Active-Semi, Inc.- 5 - www.micro- ACT4060 FB VOUT RFB1 RFB2 ACT4060 STABILITY COMPENSATION *CCOMP2 is needed only for high ESR output capacitor Figure 4. Stability Compensation The feedback loop of the IC is stabilized by the components at the COMP pin, as shown in Figure 4. The DC loop gain of the system is determined by the following equation: COMPVEA OUT VDC GA I V3.1 A= (4) The dominant pole P1 is due to CCOMP: COMPVEA EA 1P CA2 G f= (5) The second pole P2 is the output pole: OUTOUT OUT 2P CV2 I f= (6) The first zero Z1 is due to RCOMP and CCOMP: COMPCOMP 1Z CR2 1 f= (7) And finally, the third pole is due to RCOMP and CCOMP2 (if CCOMP2 is used): 2COMPCOMP 3P CR2 1 f= (8) The following steps should be used to compensate the IC: STEP 1. Set the crossover frequency at 1/10 of the switching frequency via RCOMP: V3.1GG10 fCV2 R COMPEA SWOUTOUT COMP = )(CV107.1 OUTOUT 8 =(9) but limit RCOMP to 15k maximum. STEP 2. Set the zero fZ1 at 1/4 of the crossover frequency. If RCOMP is less than 15k, the equation for CCOMP is: )F( R 108.1 C COMP 5 COMP =(10) If RCOMP is limited to 15k, then the actual cross over frequency is 3.4 / (VOUTCOUT). Therefore: )F(CV102.1C OUTOUT 5 COMP =(11) STEP 3. If the output capacitors ESR is high enough to cause a zero at lower than 4 times the crossover frequency, an additional compensation capacitor CCOMP2 is required. The condition for using CCOMP2 is: ESRCOUT R )(V012.0, C 101.1 Min OUT OUT 6 (12) And the proper value for CCOMP2 is: COMP ESRCOUTOUT 2COMP R RC C= (13) 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 2 shows some calculated results based on the compensation method above. Table 2. Typical Compensation for Different Output Voltages and Output Capacitors VOUTCOUTRCOMPCCOMPCCOMP2 2.5V22F Ceramic8.2k2.2nFNone 3.3V22F Ceramic12k1.5nFNone 5V22F Ceramic15k1.5nFNone 2.5V47F SP Cap15k1.5nFNone 3.3V47F SP Cap15k1.8nFNone 5V47F SP Cap15k2.7nFNone 2.5V470F/6.3V/30m15k15nF1nF 3.3V470F/6.3V/30m15k22nF1nF 5V470F/10V/30m15k27nFNone Figure 5 shows a sample ACT4060 application circuit generating 2.5V/2A output. Active-Semi, Inc.- 6 - www.micro- ACT4060 COMP CCOMP RCOMP CCOMP 2* ACT4060 Figure 5. ACT4060 2.5V/2A Output Application Active-Semi, Inc.- 7 - www.micro- IC1 ACT4060 IN BS ENFB GCOMP SW ENABLE Up to 20V 2.5V/2A C1 10F/35V R3 8.2k R2 13k C5 (OPTIONAL) C2 2.2nF L1 10H/3A C3 10nF D1 + R1 12K C4 22F/10V Ceramic,or 47F/6.3 SP Cap ACT4060 TYPICAL PERFORMANCE CHARACTERISTICS Active-Semi, Inc.- 8 - www.micro- ACT4060 PACKAGE OUTLINE SOP-8 PACKAGE OUTLINE AND DIMENSIONS SYMBOL DIMENSION IN MILLIMETERS DIMENSION IN INCHES MINMAXMINMAX A1.3501.7500.0530.069 A10.1000.2500.0040.010 A21.3501.5500.0530.061 B0.3300.5100.0130.020 C0.1900.2500.0070.010 D4.7805.0000.1880.197 E3.8004.0000.1500.157 E15.8006.3000.2280.248 e1.270 TYP0.050 TYP L0.4001.2700.0160.050 0808 Active-Semi, Inc.- 9 - www.micro- Active-Semi, Inc. reserves the righ