HA17384SPS Datasheet, PDF (25/29 Pages) Hitachi Semiconductor – High Speed Current Mode PWM Control IC for Switching Power Supply

HA17384SPS/SRP, HA17384HPS/HRP, HA17385HPS/HRP
Application Examples for Fuller Exploitation of Power Supply Functions
A number of application examples are briefly described below.
1. Soft start
A soft start is a start method in which the PWM pulse width is gradually increased when the power supply is
activated. This prevents the stress on the transformer and switch element caused by a rapid increase in the PWM
pulse width, and also prevents overshoot when the secondary-side output voltage rises. The circuit diagram is
shown in figure 14.
V_IN 7
FB 2
−
EA
+
2.5V
IC internal circuit
(around error amp.)
I_O
800µA typ
(4.4V)
(3V)
2R
(1V)
R
1V
D_IN
8 ^VREF
(5V)
Vref
5V
R_CU
D2
COMP
1
(3.7V)
D1
C_ST
To power supply
detection
comparator
External circuit
(only partially shown)
Figure 14 Circuit Diagram for Soft Start
Operation: In this circuit, error amp output source current I_O (800 µA typ.) gradually raises the switch element
current detection level, using a voltage slope that charges soft start capacitance C_ST. When the voltage at each node
is at the value shown in parentheses in the figure, the soft start ends. The soft start time is thus given by the
following formula:
T_ST = (3.7 V/800 µA) × C_ST ≈ 4.62 C_ST (ms)
(C_ST unit: µF)
External parts other than C_ST operate as follows:
⎯ Diode D1 : Current detection level shift and current reverse-flow prevention.
⎯ Diode D2 : Together with diode D_IN in the IC, C_ST charge drawing when power supply falls.
⎯ Resistance R_CU : For C_ST charge-up at end of soft start. (Use a high resistance of the order of several
hundred kΩ.)
Note: During a soft start, since PWM pulses are not output for a while after the IC starts operating, there is a lack of
energy during this time, and intermittent mode may be entered. In this case, the capacitance between Vref and
GND should be increased to around 4.7 µF to 10 µF.
Rev.3.00 Jun 15, 2005 page 25 of 28