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SC4812STRT Datasheet(PDF) 7 Page - Semtech Corporation |
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SC4812STRT Datasheet(HTML) 7 Page - Semtech Corporation |
7 / 15 page 7 2007 Semtech Corp. www.semtech.com SC4812 POWER MANAGEMENT Application Information Introduction The SC4812 is a 8 pin peak current mode controlled PWM controller for isolated or non-isolated DC-DC switching power supplies. It features an internal startup regulator, programmable external soft start and programmable cycle -by-cycle current limit It operates in a fixed frequency at 260KHz. The SC4812 can be applied in a flyback or forward topology with the input voltage ranging from 36V to 75V. Startup Regulator The internal startup regulator of the SC4812 makes its initial startup without a lossy startup resistor or external startup circuitry to save cost and board space. As input voltage exceeds 12V, it provides regulated 7.4V to V CC which is with the maximum turn-on voltage 6.7V. Soft-Start/Shutdown and V CC Lockout During power startup, the output voltage has to ramp up in a controlled manner to avoid overshoot. The SC4812 internal non-inverting terminal to the error amplifier is connected to the soft-start pin, which forces the internal 2.5V reference to gradually ramp up during power startup. In a non-isolated application, as the internal error amplifier is used, output voltage of a converter ramps up with close loop startup fashion smoothly. In case of an isolated application in which the internal error amplifier may not be used, isolated voltage feedback is through an opto-coupler and connected to the COMP pin. During power startup, the COMP pin will follow the soft-start pin voltage through an internal diode. Soft-start operation begins when SS/SHDN ramps above 0.7V. When soft-start completed, SS/SHDN is regulated to 2.50V, the internal voltage reference. Pull SS/SHDN below 0.5V to disable the controller. Undervoltage lockout shuts down the controller when V CC is less than 5.5V. The internal startup regulator and the reference remain on during shutdown. Current-Sense The function of the CS pin is to limit the peak current through the MOSFET. Current is sensed at CS as a voltage across a sense resistor between the source of the MOSFET and GND. An RC filter is recommended to connect CS to the sense resistor to reduce effect of the MOSFET turn-on leading edge spike and noise. Select the current-sense resistor with the following equation: R S = 0.465V/(1.2xILIM_PRI) where , ILIM_PRI is the maximum peak primary-side current. When VCS > 465mV, the power MOSFET turns off after 70nS propagation delay from the switch current reaches the trip point to the driver. PWM Comparator and Slope Compensation The SC4812 runs fixed 260 KHz frequency by an internal oscillator. At the beginning of each switching cycle, the OUT pin switches the MOSFET on. the OUT pin switches off after the 80% maximum duty cycle has been reached, regardless of the feedback. Since an artificial slope is required for current-mode operation when duty cycle is larger than 50%, the SC4812 uses an internal ramp generator for slope compensation. The internal ramp signal is reset at the beginning of each cycle and slews at 26mV/µS. The PWM comparator compares the instantaneous primary peak current to the feedback error signal through the opto-coupler, the internal offset and slope compensation and determine when to turn off the MOSFET. In steady state operation, the MOSFET turns off when: I LIM_PRI · RS > VOPTO – VOFFSET - VSLOPE where I LIM_PRI is the current on the primary side through the MOSFET. V OFFSET is 1.5V internal offset voltage and V SLOPE is the artificial ramp starting at with slew rate 26mV/ µ S. When selecting an inductor in a forward-converter or magnetizing inductance in a flyback-converter, the following condition must be met to avoid control-loop sub- harmonic oscillations: S / mV 26 L V R K N N O S P S µ = × × × where K = 0.5 TO 1, AND N S and NP are the turns of the main transformer on secondary and primary side respectively. L is the output inductor on secondary side in a forward-converter or magnetizing inductance on secondary side in a flyback-converter. |
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