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5. Feedback control circuit
1
- F/B terminal of PIC1 determines output duty cycle.
- Duty definition : Refer to Vs1 in Fig4-10.(Duty up: Vs1 up, Duty down: Vs2 down)
- C-E(Collector-Emitter) of PIC2 and F/B potential of PIC1 are same.
- At F/B terminal open : SHUT OFF mode
3 Operation descriptions
a. Internal OP-Amp Ô+Õ base potential of PIC3 is 2.5V and external Ò-Ó input potential is connected with PR38
and PR39 to maintain Vout of 5.8V. (Vout = ((PR36 x PR39) / PR39) x 2.5V)
b. If load of 5.8 V terminal increases(or AC input voltage decreases) and Vout decreases below 5.8V, then :
PIC3 ÔPÕ potential down below 2.5V
PIC2 Diode current down --> PIC2 C-E current down --> PIC2 C-E voltage up --> PIC1 F/B voltage up -->
OUT Duty up --> Transformer 1st current up --> Transformer 1st power up --> Vout up --> Maintain
Vout 5.8V
c. If load of 5.8 V terminal decreases(or AC input voltage rises) and Vout rises above 5.8V, then :
Reverse sequence of the above description ® Duty down ® Vout down ® Maintain 5.8V
(i.e., the feedback to maintains 5.8V).
- PR35, PR36 : Reduce 5.8V overshoot
- PR37, PC43 : Prevent IC3 oscillation(for phase correction)
- PC18 : Adjust feedback response rate
Samsung Electronics
ON/OFF Control
PIC1
5
OUT
F/B
4
2
3
PIC2
PC18
GND
È F/B potential is set to 3.6V internally.
F/B potential decreases to 1-2V by C-E voltage of PIC2 ( controlled by normal
operation. )
È When F/B terminal is open, shut off occurs because 2.8V is exceeded.
(Refer to clause 4.)
--> PIC3 A-K of base current down --> PIC3 of A-K current down -->
Trans
PD35
PC38
PR35
C
A
E
K
PR36
PR37
K
-
+
PIC3
2.5V
A
Fig. 2-17
Reference Information
PL37
+ +
+ +
PC39
PR38
PC43
P
PR39
GND
5 .8V
PR40
100/2W
2-7
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