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AN557 데이터시트(PDF) 13 Page - STMicroelectronics

부품명 AN557
상세설명  EASY APPLICATION DESIGN WITH THE L4970A
PDF  52 Pages
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제조업체  STMICROELECTRONICS [STMicroelectronics]
홈페이지  http://www.st.com
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AN557 데이터시트(HTML) 13 Page - STMicroelectronics

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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
Vc
OUTPUT
CURRENT
SOFT START TIME
t
D93IN011
t



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