A704
7. Start-up Circuitry
When the power is turned on, the input rectified voltage
charges the hold-up capacitor C5 and the
Vin,dc
output capacitor Cout via a start-up resistor R as shown in Fig. 4. As the voltage of VCC pin reaches the start
in
threshold voltageVTH (ON) , the A704 activates and drives the entire power supply to work.
−T
D
C
out
⋅(C // C
)
out
V
≅
(V − I
⋅ R )(1− e Rin
)
5
TH (ON)
in
DDST
in
C
+ C5
out
Where IDDST is the start-up current of A704
VDD
Gate
CS
RIN
C5
GND
VIN
IDDST
COUT
Fig.4. Start-up Circuitry
Since the start-up current IDDST is only 5uA typical, a large RIN such as 2M ohms can be used. Also with a C5 is
2.2uF/50V, and Cout is 22uF/50V, the Start-up delay TD_ON is less than 0.63 sec for 90Vac input.
For example, if the input voltage
is 310Vdc, output voltage Vo=12Vdc, and start-up resistor R is
in
Vin,dc
2MΩ，the power dissipation on the R is:
in
2
2
o
2
2
V
− 2V
310 − 2×12
in,dc
P
=
= 48mW
Rin
=
R
2M
IN
Contack Info:Samsun Zhang 13556850583 IC9898@163.com
Copyright © 2008 ADDtek Corp.
11
A704_V0.6 -- AUGUST 2008
ADDTEK [ ADDTEK CORP ]
