HA16158P Datasheet, PDF (13/23 Pages) Renesas Technology Corp – PFC & PWM Control IC

HA16158P/FP
3. Soft Start Circuit (for PFC Control)
This function gradually increases the pulse width of the PFC-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 PFC output voltage (B+ voltage).
The soft start time can easily be set with a single external capacitance.
V_ramp
(internal signal)
3.4V
V_PFC-SS
PFC-SS
comp. out
(internal signal)
PFC-OUT
Figure 3
Soft start time tss-pfc is determined by PFC-SS pin connection capacitance Css-pfc and an internal
constant, and can be estimated using the equation shown below.
Soft start time tss-pfc is the time until the PFC-SS pin voltage reaches lower-end voltage 0.65 V of the IC-
internal RAMP voltage waveform after VREF starts up following UVLO release.
Soft start time tss-pfc when Css-pfc is 3.3 nF is given by the following equation.
tss-pfc =
Css-pfc × (VREF – Vramp-L)
Iss-pwm
=
33 [nF] × (5 – 0.65)
25 [µA]
≈ 5.7 [ms]
* Iss-pfc: PFC-SS pin sink current, 25 µA typ.
In addition, when you do not use a soft start function, please ground this terminal.
Rev.0, Aug. 2002, page 13 of 23