SRP Hitachi Semiconductor, SRP Datasheet - Page 26

• Manufacturer Part Number: SRP
• Description: High Speed Current Mode PWM Control IC for Switching Power Supply
• Manufacturer: Hitachi Semiconductor

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:
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
Operation: In this circuit, error amp output source current I
element current detection level, using a voltage slope that charges soft start capacitance C
External parts other than C
Note: During a soft start, since PWM pulses are not output for a while after the IC starts operating, there
26
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.
Diode D1
Diode D2
Resistance R
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 F to 10 F.
T
ST
= (3.7 V/800 A) C
V
FB
IN
(C
CU
ST
7
2
IC internal circuit
(around error amp.)
2.5V
unit: F)
: Current detection level shift and current reverse-flow prevention.
: Together with diode D
: For C
hundred k .)
ST
ST
+
EA
charge-up at end of soft start. (Use a high resistance of the order of several
operate as follows:
Figure 14 Circuit Diagram for Soft Start
ST
4.62 C
I
800 A typ
O
2R
R
ST
IN
(3V)
(ms)
in the IC, C
(4.4V)
(1V)
1V
To power supply
detection
comparator
D
Vref
5V
ST
IN
charge drawing when power supply falls.
O
(800 A typ.) gradually raises the switch
8
1
V
COMP
External circuit
(only partially shown)
REF
R
C
D1
CU
ST
(5V)
(3.7V)
D2
ST
. When the