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| Data Sheet Rev PrA, 6/2006 ACT4012 Wide Input 2A Step Down Converter FEATURES 2A Output Current Up to 92% Efficiency Up 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 GENERAL DESCRIPTION The ACT4012 is a current-mode step-down DC-DC converter that generates up to 2A output current at 410kHz switching frequency. The device utilizes Active-Semi's proprietary ISOBCD20 process for operation with input voltage up to 20V. Consuming only 8A in shutdown mode, the ACT4012 is highly efficient with peak efficiency at 92% when in operation. Protection features include cycle-by-cycle current limit, thermal shutdown, and frequency foldback at short circuit. The ACT4012 is available in SOP-8 package and requires very few external devices for operation. 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 12V VIN IN BS SW 2.5V/2A ACT4012 ENABLE EN G FB COMP + Figure 1. Typical Application Circuit Active-Semi, Inc. -1- www.active-semi.com ACT4012 ORDERING INFORMATION PART NUMBER ACT4012SH ACT4012SH-T TEMPERATURE RANGE -40C to 85C -40C to 85C PACKAGE SOP-8 SOP-8 PINS 8 8 PACKING TUBE TAPE & REEL PIN CONFIGURATION BS IN SW G 1 2 3 4 8 7 N/C EN COMP FB ACT4012SH 6 5 SOP-8 PIN DESCRIPTION PIN NUMBER 1 2 3 4 5 6 7 8 PIN NAME BS IN SW G FB COMP EN N/C PIN DESCRIPTION Bootstrap. This pin acts as the positive rail for the high-side switch's gate driver. Connect a 10nF between this pin and SW. Input Supply. Bypass this pin to G with a low ESR capacitor. See Input Capacitor in Application Information section. Switch Output. Connect this pin to the switching end of the inductor. Ground and Heatsink. Connect to a large, uncovered PCB copper area for best heat dissipation. Feedback Input. The voltage at this pin is regulated to 1.293V. Connect to the resistor divider between output and ground to set output voltage. Compensation Pin. See Compensation Technique in Application Information section. Enable Input. When higher than 1.3V, this pin turns the IC on. When lower than 0.7V, this pin turns the IC off. Output voltage is discharged when the IC is off. This pin has a small internal pull up current to a high level voltage when pin is not connected. Do not allow EN to exceed 6V. Not Connected. Active-Semi, Inc. -2- www.active-semi.com ACT4012 ABSOLUTE MAXIMUM RATINGS (Note: Exceeding these limits may damage the device. Exposure to absolute maximum rating conditions for long periods may affect device reliability.) PARAMETER IN Supply Voltage SW Voltage BS Voltage EN, FB, COMP Voltage Continuous SW Current Junction to Ambient Thermal Resistance ( JA) Maximum Power Dissipation Operating Junction Temperature Storage Temperature Lead Temperature (Soldering, 10 sec) VALUE -0.3 to 25 -1 to VIN + 1 VSW - 0.3 to VSW + 8 -0.3 to 6 Internally limited 105 0.76 -40 to 150 -55 to 150 300 UNIT V V V V A C/W W C C C ELECTRICAL CHARACTERISTICS (VIN = 12V, TA= 25C unless otherwise specified.) PARAMETER Input Voltage Feedback Voltage High-Side Switch On Resistance Low-Side Switch On Resistance SW Leakage Current Limit COMP to Current Limit Transconductance Error Amplifier Transconductance Error Amplifier DC Gain Switching Frequency Short Circuit Switching Frequency Maximum Duty Cycle Minimum Duty Cycle Enable Threshold Voltage Enable Pull Up Current Supply Current in Shutdown IC Supply Current in Operation Thermal Shutdown Temperature SYMBOL TEST CONDITIONS VIN VOUT = 5V, ILOAD = 0A to 1A VFB 4.75V VIN 20V, VCOMP = 1.5V RONH RONL VEN = 0 ILIM GCOMP GEA AVEA fSW DMAX ICOMP = 10A MIN 7 1.267 TYP 1.293 0.4 10 0 2.85 1.8 550 4000 410 50 90 1 2 8 0.7 160 MAX 20 1.319 10 2.4 UNIT V V A A A/V A/V V/V kHz kHz % % V A 350 VFB = 0 VFB = 1.1V VFB = 1.4V Hysteresis = 0.1V Pin pulled up to 4.5V typically when left unconnected VEN = 0 VEN = 3V, VFB = 1.4V Hysteresis = 10C 470 0.7 0 1.3 20 A mA C Active-Semi, Inc. -3- www.active-semi.com ACT4012 IN 2A EN ENABLE REGULATOR & REFERENCE BS CURRENT SENSE AMPLIFIER COMP ERROR AMPLIFIER + - 1.293V FB + - + PWM COMPARATOR - + - 0.4 HIGH-SIDE POWER SWITCH SW FOLDBACK CONTROL OSCILLATOR & RAMP LOGIC 10 LOW-SIDE POWER SWITCH THERMAL SHUTDOWN G Figure 2. Functional Block Diagram FUNCTIONAL DESCRIPTION As seen in Figure 2, Functional Block Diagram, the ACT4012 is a current mode pulse width modulation (PWM) converter. The converter operates as follows: A switching cycle starts when the rising edge of the Oscillator clock output causes the HighSide Power Switch to turn on and the Low-Side Power Switch to turn off. With the SW side of the inductor now connected to IN, the inductor current ramps up to store energy in the its magnetic field. The inductor current level is measured by the Current Sense Amplifier and added to the Oscillator ramp signal. If the resulting summation is higher than the COMP voltage, the output of the PWM Comparator goes high. When this happens or when Oscillator clock output goes low, the High-Side Power Switch turns off and the Low-Side Power Switch turns on. At this point, the SW side of the inductor swings to a diode voltage below ground, causing the inductor current to decrease and magnetic energy to be transferred to output. This state continues until the cycle starts again. The High-Side Power Switch is driven by logic using BS bootstrap pin as the positive rail. This pin is charged to VSW + 6V when the LowSide Power Switch turns on. The COMP voltage is the integration of the error between FB input and the internal 1.293V reference. If FB is lower than the reference voltage, COMP tends to go higher to increase current to the output. Current limit happens when COMP reaches its maximum clamp value of 2.55V. The Oscillator normally switches at 410kHz. However, if FB voltage is less than 0.7V, then the switching frequency decreases until it reaches a minimum of 50kHz at VFB = 0.5V. SHUTDOWN CONTROL The ACT4012 has an enable input EN for turning the IC on or off. When EN is less than 0.7V, the IC is in 8A low current shutdown mode and output is discharged through the LowSide Power Switch. When EN is higher than 1.3V, the IC is in normal operation mode. EN is internally pulled up with a 2A current source and can be left unconnected for always-on operation. Note that EN is a low voltage input with a maximum voltage of 6V; it should never be directly connected to IN. THERMAL SHUTDOWN The ACT4012 automatically turns off when its junction temperature exceeds 160C. Active-Semi, Inc. -4- www.active-semi.com ACT4012 APPLICATION INFORMATION OUTPUT VOLTAGE SETTING VO U T INPUT CAPACITOR The input capacitor needs to be carefully selected to maintain sufficiently low ripple at the supply input of the converter. A low ESR capacitor is highly recommended. Since large current flows in and out of this capacitor during switching, its ESR also affects efficiency. The input capacitance needs to be higher than 10F. The best choice is the ceramic type; 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 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. ACT4012 FB RFB 1 RFB 2 Figure 3. Output Voltage Setting Figure 3 shows the connections for setting the output voltage. Select the proper ratio of the two feedback resistors RFB1 and RFB2 based on the output voltage. Typically, use RFB2 10k and determine RFB1 from the output voltage: V RFB1 = R FB2 OUT - 1 1.293 V OUTPUT CAPACITOR The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is: VRIPPLE = I OUTMAX K RIPPLE R ESR + VIN 28 * f SW 2 LCOUT (1) INDUCTOR SELECTION The inductor maintains a continuous current to the output load. This inductor current has a ripple that is dependent on the inductance value: higher inductance reduces the peak-to-peak ripple current. The trade off for high inductance value is the increase in inductor core size and series resistance, and the reduction in current handling capability. In general, select an inductance value L based on ripple current requirement: L= VOUT * ( VIN - VOUT ) VIN fSW IOUTMAX K RIPPLE (3) (2) where VIN is the input voltage, VOUT is the output voltage, fSW is the switching frequency, IOUTMAX is the maximum output current, and KRIPPLE is the ripple factor. Typically, choose KRIPPLE = 30% to correspond to the peak-to-peak ripple current being 30% of the maximum output current. With this inductor value (Table 1), the peak inductor current is IOUT * (1 + KRIPPLE / 2). Make sure that this peak inductor current is less that the 3A current limit. Finally, select the inductor core size so that it does not saturate at 3A. Table 1. Typical Inductor Values where IOUTMAX is the maximum output current, KRIPPLE is the ripple factor, RESR is the ESR resistance of the output capacitor, fSW is the switching frequency, L in the inductor value, 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 type. In the case of tantalum or electrolytic type, 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 type, typically choose a capacitance of about 22F. For tantalum or electrolytic type, 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 current rating higher than the maximum output current and the reverse voltage rating higher than the maximum input voltage. VOUT L 1.5V 1.8V 2.5V 3.3V 5V 6.8H 6.8H 10H 15H 22H Active-Semi, Inc. -5- www.active-semi.com ACT4012 STABILITY COMPENSATION COMP STEP 2. Set the zero fZ1 at 1/4 of the cross over frequency. If RCOMP is less than 15k, the equation for CCOMP is: CCOMP = C * C OMP 2 ACT4012 CC OMP 1 .8 x 10 - 5 RCOMP (F ) (10) RC OMP If RCOMP is limited to 15k, then the actual cross over frequency is 3.4 / (VOUTCOUT). Therefore: CCOMP = 1.2 x 10 - 5 VOUT COUT (F ) (11) *CCOMP2 is needed only for high ESR output capacitor Figure 4. Stability Compensation The feedback system of the IC is stabilized by the components at COMP pin, as shown in Figure 4. The DC loop gain of the system is determined by the following equation: 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: R ESRCOUT AVDC = 1.3V AVEAGCOMP IOUT G EA (4) The dominant pole P1 is due to CCOMP: fP1 = 2 AVEA CCOMP 1 .1 x 10 - 6 Min ,0.012 * VOUT C OUT ( ) (12) (5) And the proper value for CCOMP2 is: CCOMP 2 = COUT RESRCOUT RCOMP The second pole P2 is the output pole: fP 2 = IOUT 2VOUT COUT (13) (6) The first zero Z1 is due to RCOMP and CCOMP: fZ 1 = 1 2RCOMP CCOMP 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 (7) And finally, the third pole is due to RCOMP and CCOMP2 (if CCOMP2 is used): fP 3 = 1 2RCOMP CCOMP 2 (8) IC: Follow the following steps to compensate the STEP 1. Set the cross over frequency at 1/10 of the switching frequency via RCOMP: RCOMP = 2 VOUT COUT fSW 10 G EAG COMP * 1.3V = 1.7 x 10 8 VOUT COUT ( ) (9) VOUT 2.5V 3.3V 5V 2.5V 3.3V 5V 2.5V 3.3V 5V COUT RCOMP 22F Ceramic 8.2k 22F Ceramic 12k 22F Ceramic 15k 47F SP Cap 15k 47F SP Cap 15k 47F SP Cap 15k 470F/6.3V/30m 15k 470F/6.3V/30m 15k 470F/10V/30m 15k CCOMP 2.2nF 1.5nF 1.5nF 1.5nF 1.8nF 2.7nF 15nF 22nF 27nF CCOMP2 None None None None None None 1nF 1nF None but limit RCOMP to 15k maximum. Figure 5 shows a sample ACT4012 application circuit generating 2.5V/2A output. Active-Semi, Inc. -6- www.active-semi.com ACT4012 7 to 20V VIN IN C3 10nF BS L1 10 H/3A 2.5V/2A IC1* ACT4012 G SW R1 12 K FB ENABLE EN COMP C2 2.2nF R3 8.2k C5 (OPTIONAL) C4 22F/10V Ceramic + C1 10F/25V R2 13k D1 *Heat dissipation copper area required. Leave more than 2 in2 of uncovered PCB area immediately adjacent to the G pin. Figure 5. ACT4012 2.5V/2A Output Application Active-Semi, Inc. -7- www.active-semi.com ACT4012 TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 5, unless otherwise specified .) Active-Semi, Inc. -8- www.active-semi.com ACT4012 PACKAGE OUTLINE SOP-8 PACKAGE OUTLINE AND DIMENSIONS DIMENSION IN MILLIMETERS MIN MAX 1.350 1.750 0.100 0.250 1.350 1.550 0.330 0.510 0.190 0.250 4.780 5.000 3.800 4.000 5.800 6.300 1.270 TYP 0.400 1.270 0 8 DIMENSION IN INCHES MIN MAX 0.053 0.069 0.004 0.010 0.053 0.061 0.013 0.020 0.007 0.010 0.188 0.197 0.150 0.157 0.228 0.248 0.050 TYP 0.016 0.050 0 8 SYMBOL A A1 A2 B C D E E1 e L Active-Semi, Inc. reserves the right to modify the circuitry or specifications without notice. Users should evaluate each product to make sure that it is suitable for their applications. Active-Semi products are not intended or authorized for use as critical components in life-support devices or systems. Active-Semi, Inc. does not assume any liability arising out of the use of any product or circuit described in this data sheet, nor does it convey any patent license. Active-Semi and its logo are trademarks of Active-Semi, Inc. For more information on this and other products, contact sales@active-semi.com or visit www.active-semi.com. For other inquiries, please send to: 1270 Oakmead Parkway, Suite 310, Sunnyvale, California 94085-4044, USA Active-Semi, Inc. -9- www.active-semi.com |
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