AN557 datasheet(13/52 Pages) STMICROELECTRONICS

Numer części	AN557

Szczegółowy opis	EASY APPLICATION DESIGN WITH THE L4970A

PDF	52 Pages

Scroll/Zoom	100%

Producent	STMICROELECTRONICS [STMicroelectronics]

Strona internetowa	http://www.st.com

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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

CURRENT

SOFT START TIME

D93IN011

AN557 Arkusz danych(PDF) 13 Page - STMicroelectronics