HA16107P Datasheet, PDF (7/40 Pages) Hitachi Semiconductor – PWM Switching Regulator for High-performance Voltage Mode Control

HA16107P/FP, HA16108P/FP
3. Programming of Maximum ON-Duty (Du Max)
The preceding equations should be used to program the deadband or maximum ON-duty. The following table gives
a summary.
Condition
RT1 > RT2
RT1 = RT2
RT1 < RT2
Triangle
waveform
Du max
Less than 50%
50%
Greater than 50%*
Note: In a primary-control switching regulator, Du Max > 50% is dangerous because the transformer will saturate.
Soft Start and Quick Shutdown
One purpose of the soft-start function is to protect the switching controller and power MOS FET from surges at power-
up. Another purpose is to let the secondary-side DC voltage rise smoothly.
When power goes off, the quick-shutdown function rapidly discharges the capacitor in the soft-start circuit (and at the
same time switches the PWM output off) to prepare for the next power-on.
The soft-start function in these ICs lets the PWM output develop smoothly from zero to the designated pulse width at
power-up. The soft-start voltage is the 3.8 V voltage value of an internal Zener diode, so the PWM output is able to
start widening gradually as soon as the soft-start function starts operating. The soft-start function will start promptly
even if C_ST is large.
The soft-start and quick-shutdown modes are selected automatically in the IC, under control of the UVL signal.
Rev.4.00 Jun 15, 2005 page 7 of 39