ha17384hps Renesas Electronics Corporation., ha17384hps Datasheet - Page 25

• Manufacturer Part Number: ha17384hps
• Description: High Speed Current Mode Pwm Control Ic For Switching Power Supply
• Manufacturer: Renesas Electronics Corporation.

Application Examples for Fuller Exploitation of Power Supply Functions
A number of application examples are briefly described below.
1. Soft start
Note: During a soft start, since PWM pulses are not output for a while after the IC starts operating, there is a lack of
Rev.3.00 Jun 15, 2005 page 25 of 28
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.
Operation: In this circuit, error amp output source current I
current detection level, using a voltage slope that charges soft start capacitance C
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:
External parts other than C
⎯ Diode D1
⎯ Diode D2
⎯ Resistance R
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.
T
ST
= (3.7 V/800 µA) × C
(C
ST
V
FB
unit: µF)
CU
IN
: Current detection level shift and current reverse-flow prevention.
: Together with diode D
: For C
7
2
hundred kΩ.)
IC internal circuit
(around error amp.)
2.5V
ST
ST
charge-up at end of soft start. (Use a high resistance of the order of several
operate as follows:
+
−
EA
ST
Figure 14 Circuit Diagram for Soft Start
≈ 4.62 C
I
800µA typ
O
2R
R
IN
ST
in the IC, C
(3V)
(ms)
(4.4V)
(1V)
1V
ST
To power supply
detection
comparator
O
Vref
5V
D
charge drawing when power supply falls.
(800 µA typ.) gradually raises the switch element
IN
8
1
V
COMP
External circuit
(only partially shown)
REF
ST
R
C
D1
CU
ST
. When the voltage at each node
(5V)
(3.7V)
D2