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AN557 数据表(PDF) 13 Page - STMicroelectronics |
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AN557 数据表(HTML) 13 Page - STMicroelectronics |
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13 / 52 page  13/52 AN557 APPLICATION NOTE Figure 9. Soft Start Circuit. Figure 10. Soft Start Waveforms. The PWM signal begins to be generated as soon as the output voltage of the error amplifier crosses the ramp; at this point the output stage begins to commutate, slowly increasing its ON time (see fig. 10). The charge of the Css capacitor, as already mentioned, begins each time the device is supplied with pow- er and after which an anomalous condition is created, as the intervention of thermal protection or of the undervoltage lockout. CALCULATING THE DUTY-CYCLE AND SOFT-START TIME Let us suppose that the discharge time of the oscillator capacitor, Cosc, is neglected. This is an approx. valid for switching frequencies up to 200KHz. Let us indicate with Vr the output voltage of the error ampli- fier, and with Vc the voltage of the oscillator ramp. Figure 11. Soft Sart Time Waveform. CLAMPED ERROR AMPLIFIER OUTPUT Vc OUTPUT CURRENT SOFT START TIME t D93IN011 t |
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