Data Sheet Rev PrA, 6/2006 ACT4012 Wide Input 2A Step Down Converter FEATURES 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. 2A Output Current Up to 92% Efficiency Up to 20V Input Range 8µA 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 Consuming only 8μA 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 2.5V/2A BS VIN IN SW 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 -40°C to 85°C -40°C to 85°C PACKAGE SOP-8 SOP-8 PINS 8 8 PACKING TUBE TAPE & REEL PIN CONFIGURATION BS 1 IN 2 SW 3 G 4 ACT4012SH 8 N/C 7 EN 6 COMP 5 FB SOP-8 PIN DESCRIPTION PIN NUMBER PIN NAME 1 BS 2 IN 3 SW 4 G 5 FB 6 COMP 7 EN 8 N/C Active-Semi, Inc. 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. -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 (θ 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 UNIT V V V V A 105 °C/W 0.76 -40 to 150 -55 to 150 300 W °C °C °C ELECTRICAL CHARACTERISTICS (VIN = 12V, TA= 25°C 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 Active-Semi, Inc. SYMBOL TEST CONDITIONS VIN VOUT = 5V, ILOAD = 0A to 1A VFB 4.75V ≤ VIN ≤ 20V, VCOMP = 1.5V RONH RONL VEN = 0 ILIM MIN 7 1.267 2.4 GCOMP GEA AVEA fSW DMAX ΔICOMP = ±10µA 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 = 10°C -3- 0.7 TYP 1.293 0.4 10 0 2.85 MAX 20 1.319 10 UNIT V V Ω Ω µA A 1.8 A/V 550 4000 410 50 90 µA/V V/V kHz kHz % % V 470 0 1.3 1 2 µA 8 0.7 160 20 µA mA °C www.active-semi.com ACT4012 IN 2μA ENABLE EN COMP REGULATOR & REFERENCE BS CURRENT SENSE AMPLIFIER ERROR AMPLIFIER – + 1.293V + – FB FOLDBACK CONTROL + OSCILLATOR & RAMP – + – PWM COMPARATOR 0.4Ω HIGH-SIDE POWER SWITCH SW LOGIC 10Ω LOW-SIDE POWER SWITCH THERMAL SHUTDOWN G Figure 2. Functional Block Diagram 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. 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 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 8μA 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 2μA 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 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. Active-Semi, Inc. The ACT4012 automatically turns off when its junction temperature exceeds 160°C. -4- www.active-semi.com ACT4012 APPLICATION INFORMATION 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. OUTPUT VOLTAGE SETTING VO U T RFB 1 ACT4012 The input capacitance needs to be higher than 10µF. 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 0.1µF ceramic capacitor is placed right next to the IC. FB 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 ≈ 10kΩ 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: (1) VRIPPLE = I OUTMAX K RIPPLE R ESR 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 ) + 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. 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. Table 1. Typical Inductor Values L 1.5V 1.8V 6.8μH 6.8μH Active-Semi, Inc. 2.5V 3.3V (3) For ceramic output type, typically choose a capacitance of about 22µF. For tantalum or electrolytic type, choose a capacitor with less than 50mΩ ESR. 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. VOUT 28 • f SW 2 LCOUT 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. (2) VIN fSW IOUTMAX K RIPPLE VIN 5V 10μH 15μH 22μH -5- www.active-semi.com ACT4012 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: STABILITY COMPENSATION COMP ACT4012 CC OMP RC OMP * C OMP 2 C (4) 1 .1 × 10 − 6 ≥ Min ,0.012 • VOUT COUT G EA IOUT CCOMP 2 = The first zero Z1 is due to RCOMP and CCOMP: 1 IC: VOUT 2.5V 3.3V 5V 2.5V 3.3V 5V 2.5V 3.3V 5V (8) 2πRCOMP CCOMP 2 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 πVOUT COUT fSW 10 G EAG COMP • 1.3V = 1.7 × 10 8 VOUT COUT (Ω ) (9) (13) RCOMP COUT RCOMP 22μF Ceramic 8.2kΩ 22μF Ceramic 12kΩ 22μF Ceramic 15kΩ 47μF SP Cap 15kΩ 47μF SP Cap 15kΩ 47μF SP Cap 15kΩ 470μF/6.3V/30mΩ 15kΩ 470μF/6.3V/30mΩ 15kΩ 470μF/10V/30mΩ 15kΩ 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 Figure 5 shows a sample ACT4012 application circuit generating 2.5V/2A output. but limit RCOMP to 15kΩ maximum. Active-Semi, Inc. COUT RESRCOUT Table 2. Typical Compensation for Different Output Voltages and Output Capacitors And finally, the third pole is due to RCOMP and CCOMP2 (if CCOMP2 is used): 1 (12) Table 2 shows some calculated results based on the compensation method above. (7) 2πRCOMP CCOMP (Ω ) 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. (6) 2πVOUT COUT And the proper value for CCOMP2 is: (5) 2 πAVEA CCOMP The second pole P2 is the output pole: fP 3 = (11) R ESRCOUT 1.3V AVEAGCOMP IOUT The dominant pole P1 is due to CCOMP: fZ 1 = (F ) 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: 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: fP 2 = (10) CCOMP = 1.2 × 10 − 5 VOUT COUT Figure 4. Stability Compensation fP1 = (F ) If RCOMP is limited to 15kΩ, then the actual cross over frequency is 3.4 / (VOUTCOUT). Therefore: *CCOMP2 is needed only for high ESR output capacitor AVDC = 1 .8 × 10 − 5 RCOMP CCOMP = -6- www.active-semi.com ACT4012 C3 10nF 7 to 20V BS VIN IN ENABLE EN IC1* ACT4012 2.5V/2A R1 12 K FB G + L1 10 μH/3A SW C1 10μF/25V COMP C2 2.2nF R3 8.2k R2 13k D1 C4 22μF/10V Ceramic C5 (OPTIONAL) *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 SYMBOL DIMENSION IN MILLIMETERS DIMENSION IN INCHES A A1 A2 B C D E E1 e L θ 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° 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° 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 [email protected] 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