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LM2853 Datasheet(PDF) 8 Page - Texas Instruments |
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LM2853 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 18 page Block Diagram 20201512 Applications Information The LM2853 is a DC-DC buck regulator belonging to Na- tional Semiconductor’s synchronous SIMPLE SWITCHER® family. Integration of the PWM controller, power switches and compensation network greatly reduces the component count required to implement a switching power supply. A typical application requires only four components: an input capacitor, 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 by: 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 should be added such asa1µF ceramic for higher frequency filtering. Ceramic capacitors with high quality dielectrics such as X5R or X7R should be used to provide a constant capaci- tance across temperature and line variations. For improved load regulation and transient performance, the use of a small 1 µF ceramic capacitor is also recommended as a local bypass for the AVIN pin. SOFT-START CAPACITOR (C SS) The DAC that sets the reference voltage of the error ampli- fier 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 200 k 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 20 pF 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 5A current limit threshold would require a time of 60 µs: www.national.com 7 |
Similar Part No. - LM2853_15 |
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Similar Description - LM2853_15 |
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