Figure 1: Simplified SMPS Using LR745
VIN
IIN
High Voltage
IAUX VAUX
D2
C2
LR7
VOUT
VCC
GND
C1
PWM IC
UC3844
LR745
Figure 1: LR745 Start-up Waveforms
Stage
I
16.0
Stage
II
Stage
III
PWM IC Start Threshold Voltage
VOUT 13.5
(Volts) 12.0
LR7 VOFF Trip Point
8.0
Auxiliary Supply Powers PWM IC
4.0
0.0
t
3.0
IIN
(mA)
2.0
1.0
0.0
12.0
IIN ≈ 0mA
t
VAUX 8.0
(Volts)
4.0
0.0
30.0
IAUX
(mA)
20.0
10.0
0.0
VAUX = 12V
t
IAUX = 20mA
t
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