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RT8101PSP Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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RT8101PSP Datasheet(HTML) 11 Page - Richtek Technology Corporation |
11 / 15 page 11 RT8101/A Preliminary DS8101/A-01 March 2007 www.richtek.com Figure 7 shows the DC-DC converter's gain vs. frequency. The compensation gain uses external impedance networks ZC and ZF to provide a stable, high bandwidth loop. High crossover frequency is desirable for fast transient response, but it often jeopardizes the system stability. In order to cancel one of the LC filter poles, place the zero before the LC filter resonant frequency. In the experience, place the zero at 75% LC filter resonant frequency. Crossover frequency should be higher than the ESR zero but less than 1/5 of the switching frequency. The second pole is placed at half of the switching frequency. Figure 7. Bode Plot Component Selection 1) Inductor Selection The selection of output inductor is based on the considerations of efficiency, output power and operating frequency. Low inductance value has smaller size, but results in low efficiency, large ripple current and high output ripple voltage. Generally, an inductor that limits the ripple current ( ΔIL) between 20% and 50% of the output current is appropriate. Figure 8 shows the typical topology of synchronous step-down converter and its related waveforms. S1 S2 V IN i S1 i S2 I OUT V OUT + - R L r C C OUT i C V OR + - V OC + - V L + - L I L Frequency 10Hz 100Hz 1.0KHz 10KHz 100KHz 1.0MHz vdb(vo) vdb(comp2) vdb(lo) -40 0 40 80 -60 10 100 1k 10k 100k 1M 80 40 0 20 60 -20 -40 -60 Loop Gain Compensation Gain Modulator Gain Frequency (Hz) Figure 8. The waveforms of synchronous step-down converter According to Figure 8 the ripple current of inductor can be calculated as follows : (1) Where : VIN = Maximum input voltage VOUT = Output Voltage Δt = S1 turn on time ΔIL = Inductor current ripple fS = Switching frequency D = Duty Cycle rC = Equivalent series resistor of output capacitor L IN OUT OUT IN IN OUT L OUT IN I fs V V ) V (V L V V D ; fs D t ; t I L V V Δ × × × − = = = Δ Δ Δ = − VL VIN - VOUT - VOUT IL IL = IOUT ΔIL IS1 IS2 TS TON TOFF Vg1 Vg2 |
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