HA16158P Datasheet, PDF (12/23 Pages) Renesas Technology Corp – PFC & PWM Control IC

HA16158P/FP
2. Soft Start Circuit (for PWM Control)
This function gradually increases the pulse width of the PWM-OUT pin from a 0% duty cycle at start-up to
prevent a sudden increase in the pulse width that may cause problems such as transient stress on external
parts or overshoot of the secondary-side output voltage.
The soft start time can easily be set with a single external capacitance.
V_CT
(internal signal)
PWM-SS
comp. out
(internal signal)
3.2V
1.6V
V_PWM-SS
PWM-OUT
Figure 2
Soft start time tss-pwm is determined by PWM-SS pin connection capacitance Css-pwm and an internal
constant, and can be estimated using the equation shown below.
Soft start time tss-pwm is the time until the PWM-SS pin voltage reaches upper-end voltage 3.2 V of the
IC-internal CT voltage waveform after VREF starts up following UVLO release.
Soft start time tss-pwm when Css-pwm is 3.3 nF is given by the following equation.
tss-pwm =
Css-pwm × Vct-H
Iss-pwm
=
33 [nF] × 3.2 [V]
25 [µA]
≈ 4.2 [ms]
* Iss-pwm: PWM-SS pin source current, 25 µA typ.
Rev.0, Aug. 2002, page 12 of 23