SD46520 400KHZ 2A STEP-DOWN DC-DC CONVERTER DESCRIPTION The SD46520 is a step-down PWM control converter with a built-in internal power MOSFET. It achieves 2A continuous output current over a wide input supply range with excellent load and line regulation. Current mode operation provides fast transient response and eases loop stabilization. It provides cycle-by-cycle current limiting and thermal shutdown. In SOP-8-225-1.27 shutdown mode, the current is only 23μA. FEATURES * 2A output current * 0.2Ω internal power MOSFET * Stable with low ESR output ceramic capacitor ORDERING INFORMATION * Up to 95% conversion efficiency * shutdown mode * 400kHz fixed frequency * Thermal shutdown * Cycle-by-cycle over current protection Device SD46520 Package Seal SOP-8-225-1.27 SD46520 APPLICATIONS * 4.75－18V input voltage range * Distributed power system * 1.22－16V output voltage range * Battery charger * Programmable under voltage control * TFT LCD Monitors * Portable DVD * Set-Top Box HANGZHOU SILAN MICROELECTRONICS CO.,LTD Http://www.silan.com.cn REV:1.0 2009.04.07 Page 1 of 8 SD46520 BLOCK DIAGRAM 2 IN Internal 3.3V Regulators + ∑ + Current Sense Amplifier Oscillator 50/400kHz 7 Frequency Foldback Comparator Q R Q SW Current Comparator 1uA 2.30/2.53V S Zero current control UVLO 0.7V 1 1.8V Shutdown comparator EN BS 5V CLK 1.22V FB GND 3 4 Error Amplifier COMP 6 5 ABSOLUTE MAXIMUM RATINGS Characteristics Symbol Ratings Unit Input Voltage VIN 20 V Switch Voltage VSW -1~VIN+1 V Boost Voltage VBS VSW+6 V Feedback Voltage VFB -0.3~6 V Enable Voltage VEN -0.3~6 V Comp Voltage VCOMP -0.3~6 V Junction Temperature Tj 150 °C Lead Temperature TL 260 °C Tstg -65~150 °C Storage Temperature HANGZHOU SILAN MICROELECTRONICS CO.,LTD Http://www.silan.com.cn REV:1.0 2009.04.07 Page 2 of 8 SD46520 ELECTRICAL CHARACTERISTICS (Unless otherwise specified, VIN=12V, Tamb=25°C) Characteristics Symbol Test Condition 4.75≤VIN≤18V Min. Typ. Max. Unit 1.220 1.244 1.268 V 18 V Feedback Voltage VFB Input Voltage VIN 4.75 RONH -- 0.2 -- Ω RONL -- 10 -- Ω -- 0 10 μA ILIM 2.4 2.85 3.3 A GCS -- 1.95 -- A/V Av -- 400 -- V/V 500 770 1100 μΩ 360 400 440 KHz VFB=0V 46 58 70 KHz Upper Switch On Resistance Lower Switch On Resistance Upper Switch Leakage Current Limit ILEAK VCOMP<2V VEN=0V; VSW=0V Current Limit Gain. Output Current to Comp Pin Voltage Error Amplifier Voltage Gain Error Amplifier Transconductance Oscillator Frequency Short Circuit Frequency Gm ΔIC=±10μA fs fsshort Maximum Duty Cycle Dmax VFB=1.0V -- 90 -- % Minimum Duty Cycle Dmin VFB=1.5V -- -- 5 % Enable Threshold Voltage VEN ICC>100μA 0.7 1.0 1.3 V VEN=0V 1.5 2.5 3 μA VUVLO 2.37 2.495 2.62 V VUVLOH -- 210 -- mV 23 36 μA Enable Pull-up Current Under Voltage Lockout Threshold Voltage IENPUP Under Voltage Lockout Threshold Hysteresis Voltage IIN1 VEN≤0.4V -- Supply Current (Operating) IIN2 VEN≥2.6V;VFB=1.4V -- 3 3.5 mA Thermal Shutdown TOT -- 180 -- °C Supply Current (Quiescent) PIN CONFIGURATION HANGZHOU SILAN MICROELECTRONICS CO.,LTD Http://www.silan.com.cn REV:1.0 2009.04.07 Page 3 of 8 SD46520 PIN DESCRIPTIONS Pin No. Pin Name I/O Description 1 BS I/O 2 IN I 3 SW I/O 4 GND G Ground. 5 FB I Feedback voltage input pin. 6 COMP I/O 7 EN I Enable input pin. 8 NC I No connection. Bootstrap pin. Connect 10nF capacitor to SW pin. Input supply voltage. Switch pin. Connect with inductor. Compensation pin. FUNCTION DESCRIPTIONS SD46520 is a current mode DC-DC convertor with PWM control. The working process is as follows. At the beginning of a cycle, the switching MOSFET is off, the freewheeling MOSFET is on, SW is connected to the ground; the external bootstrap capacitor between BS and SW is charged by 5V voltage via internal Schottky diode. The bootstrap capacitor voltage is used as the power supply of the driver of switch MOSFET to make it work normally. When the bootstrap capacitor voltage is higher than the switching MOSFET’s threshold voltage, the control loop circuit starts to work. The rising edge of the 400kHz clock signal sets the RS Flip-Flop. Its output turns on the switching MOSFET, the input supply is connecting to the output capacitor via the inductor, and the inductor current is increasing linearly and charges the output capacitor. The inductor current is sensed and amplified by the current sense amplifier. Ramp compensation is summed to current sense amplifier’s output and compared to the error amplifier output by the current comparator. When the sum of current sense amplifier’s output plus slope compensation signal exceeds the comp pin voltage, the RS Flip-Flop is reset. If the sum of current sense amplifier’s output plus slope compensation signal does not exceed the comp voltage during one cycle, the falling edge of the CLK resets the Flip-Flop. After RS Flip-flop is reset, the switching MOSFET is off, the freewheeling MOSFET is on, and the external Schottky diode is on at the same time which carries most of the inductor current, the inductor current decreases linearly. When the next rising edge of the clock arrives, sets the RS Flip-flop again, the switching MOSFET is on, which is cycled and the output capacitor is charged to the output voltage setting. The voltage of COMP pin is the integral of the voltage difference between FB feedback voltage and 1.22V reference voltage. It is proportional to the peak inductor current. If COMP voltage increases, the inductor current and the output current also increase. When COMP voltage is rising to the high clamp voltage 2.3V, the output current is up to the limited value. Enable control Enable pin EN has the enable and under voltage lock two functions. When EN voltage is lower than 1V, the chip is off; when EN voltage is higher than 1V but lower than 2.495V, the chip is enable, while COMP is pulled down to the ground and the switching MOSFET is not working, so this is under voltage lock state; When EN voltage is HANGZHOU SILAN MICROELECTRONICS CO.,LTD Http://www.silan.com.cn REV:1.0 2009.04.07 Page 4 of 8 SD46520 higher than 2.495V, the chip works normally. Short protection When the output voltage is short connected to the ground, FB feedback voltage is pulled down to the ground, and the oscillator frequency will decrease to 50kHz from normal 400kHz, which also decreases the inductor current average value, that is the output current is decreased. APPLICATION NOTE Output voltage setting As right figure shows, the output voltage is determined by the feedback resistors ratio of R1 and R2. Generally, R2＝10KΩ, R1 is decided by: R1=R2.(VO/1.22-1). Inductor selection When output current is large, the regulator is working under continuous current mode that is inductor current is continuous and will not decrease to zero. The inductor value will affect the ripple of inductor current, and the relationship between them is as follows: L= VO ⋅ (VI − VO ) . VI ⋅ fS ⋅ ΔI Where, VI is input voltage, VO is output voltage, fs is on-off frequency, ΔI is the peak-peak value of inductor current ripple. In general, ΔI is no larger than 30% of the maximum output current, thus to decide the inductor value. At the same time, the peak inductor current should be less than 2.4A, the value is decided by: ILMAX = IO + 1 1 V ⋅ (V − VO ) . ⋅ ΔIL = IO + ⋅ O I 2 2 VI ⋅ fS ⋅ L Input capacitor selection The input current of step-down DC-DC is not successive, an additional input capacitor CI is needed to keep input voltage stable. The input capacitor should be low ESR, and ceramic capacitor is the best choice, tantalum capacitor or low ESR electrolytic capacitor is all right. The input capacitor value should be larger than 10μF. Output capacitor selection The output capacitor CO is decided by the ripple requirement of output voltage. The output voltage ripple is decided by: ΔVO = ΔIL (ESR + ⋅1 ). 8fS ⋅ C O HANGZHOU SILAN MICROELECTRONICS CO.,LTD Http://www.silan.com.cn REV:1.0 2009.04.07 Page 5 of 8 SD46520 Where, ΔVO is output voltage ripple, ΔIL is inductor current ripple, ESR is the equivalent serial resistor of output capacitor. The freewheeling diode selection Select the schottky diode as freewheeling diode to reduce the loss caused by forward voltage drop of the diode. At the same time, the maximum current of schottky should be higher than 2.4A. Compensation network The resistor-capacitor compensation network connected to COMP pin make sure the stability of the control loop circuit. As the right figure: The DC gain of the control loop is: A VDC = VFB ⋅ A VEA ⋅ GCS ⋅ RL VO Where, VFB is feedback voltage, 1.22V; VO is setting output voltage; AVEA is the voltage gain of error amplifier, 400V/V; GCS is the trans-conductance of current sense(about the value of the output current divided by COMP pin voltage), 1.95A/V; RL is load resistor. The control loop has two poles of inportance. The first pole P1 is decided by the following formula: where, R OEA = A VEA GEA fP1 = 1 2π ⋅ R OEA ⋅ CC1 , , GEA is the trans-conductance of error amplifier, 770uA/V. The second pole P2 is decided by: fP2 = 1 。 2π⋅ RL ⋅ CO The control loop has a main zero Z1, which is decided by: fZ1 = 1 。 2π⋅ R C ⋅ CC1 When the output capacitor ESR is large, another zero Z2 is introduced which is decided by: fZ2 = 1 . 2π⋅ ESR ⋅ CO At this time, it needs to add another compensation capacitor Cc2 to introduce an additional pole P3 compensate this zero , and this pole is decided by: f = P3 1 2π⋅ R C ⋅ CC2 The system crossover frequency fC is very important. If it’s too high , it will cause the system unstable. If it’s too low, it will slower the system transient response. In general, fC is about 1/8~1/10 of the switch frequency. Use the following procedure to choose the value of the compensation components. First, according to the selected crossover crossover fC, choose the value of compensation resistor Rc: HANGZHOU SILAN MICROELECTRONICS CO.,LTD Http://www.silan.com.cn REV:1.0 2009.04.07 Page 6 of 8 SD46520 RC = 2π⋅ CO ⋅ VO ⋅ fC GEA ⋅ GCS ⋅ VFB . Second, select Z1 as about 1/5~1/4 of crossover frequency to confirm the compensation capacitor Cc1 value: CC1 = 1 . 2π⋅ R C ⋅ fZ1 When compensation capacitor Cc2 is needed, the value of it is decided by: CC2 = ESR ⋅ CO RC . TYPICAL APPLICATION CIRCUIT SD46520 HANGZHOU SILAN MICROELECTRONICS CO.,LTD Http://www.silan.com.cn REV:1.0 2009.04.07 Page 7 of 8 SD46520 PACKAGE OUTLINE SOP-8-225-1.27 UNIT: mm MOS DEVICES OPERATE NOTES: Electrostatic charges may exist in many things. Please take following preventive measures to prevent effectively the MOS electric circuit as a result of the damage which is caused by discharge: z The operator must put on wrist strap which should be earthed to against electrostatic. z Equipment cases should be earthed. z All tools used during assembly, including soldering tools and solder baths, must be earthed. z MOS devices should be packed in antistatic/conductive containers for transportation. Disclaimer: • Silan reserves the right to make changes to the information herein for the improvement of the design and performance without further notice! • All semiconductor products malfunction or fail with some probability under special conditions. When using Silan products in system design or complete machine manufacturing, it is the responsibility of the buyer to comply with the safety standards strictly and take essential measures to avoid situations in which a malfunction or failure of such Silan products • could cause loss of body injury or damage to property. Silan will supply the best possible product for customers! HANGZHOU SILAN MICROELECTRONICS CO.,LTD Http://www.silan.com.cn REV:1.0 2009.04.07 Page 8 of 8