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LM2852YMXA-1.0 데이터시트(PDF) 8 Page - National Semiconductor (TI) |
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LM2852YMXA-1.0 데이터시트(HTML) 8 Page - National Semiconductor (TI) |
8 / 13 page Block Diagram 20127012 Applications Information The LM2852 is a DC-DC buck converter belonging to Na- tional Semiconductor’s SIMPLE SYNCHRONOUS® family. Integration of the PWM controller, power switches and com- pensation network greatly reduces the component count required to implement a switching power supply. A typical application requires only four components: an input capaci- tor, a soft-start capacitor, an output filter capacitor and an output filter inductor. INPUT CAPACITOR (C IN) Fast switching of large currents in the buck converter places a heavy demand on the voltage source supplying PVIN. The input capacitor, C IN, supplies extra charge when the switcher needs to draw a burst of current from the supply. The RMS current rating and the voltage rating of the C IN capacitor are therefore important in the selection of C IN. The RMS current specification can be approximated to be the load current times the square root of the duty cycle: where D is the duty cycle, V OUT/VIN.CIN also provides filtering of the supply. Trace resistance and inductance de- grade the benefits of the input capacitor, so C IN should be placed very close to PVIN in the layout. A 22 µF or 47 µF ceramic capacitor is typically sufficient for C IN. In parallel with the large input capacitance a smaller capacitor may be added such as a 1µF ceramic for higher frequency filtering. SOFT-START CAPACITOR (C SS) The DAC that sets the reference voltage of the error amp sources a current through a resistor to set the reference voltage. The reference voltage is one half of the output voltage of the switcher due to the 200k Ω divider connected to the SNS pin. Upon start-up, the output voltage of the switcher tracks the reference voltage with a two to one ratio as the DAC current charges the capacitance connected to the reference voltage node. Internal capacitance of 20pF is permanently attached to the reference voltage node which is also connected to the soft-start pin, SS. Adding a soft-start capacitor externally increases the time it takes for the output voltage to reach its final level. The charging time required for the reference voltage can be estimated using the RC time constant of the DAC resistor and the capacitance connected to the SS pin. Three RC time constant periods are needed for the reference voltage to reach 95% of its final value. The actual start-up time will vary with differences in the DAC resistance and higher-order effects. If little or no soft-start capacitance is connected, then the start-up time may be determined by the time required for the current limit current to charge the output filter capacitance. The capacitor charging equationI=C ∆V/∆t can be used to estimate the start-up time in this case. For example, a part with a 3V output, a 100 µF output capacitance and a 3A current limit threshold would require a time of 100 µs: Since it is undesirable for the power supply to start up in current limit, a soft-start capacitor must be chosen to force the LM2852 to start up in a more controlled fashion based on the charging of the soft-start capacitance. In this example, supposea3ms start time is desired. Three time constants are required for charging the soft-start capacitor to 95% of the final reference voltage. So in this case RC=1ms. The DAC resistor, R, is 400 k Ω so C can be calculated to be 2.5nF. A 2.7nF ceramic capacitor can be chosen to yield approximately a 3ms start-up time. www.national.com 8 |
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