LT1640L/LT1640H
APPLICATIONS INFORMATION
Under some conditions, a short circuit at the output can
cause the input supply to dip below the UV threshold,
resetting the circuit breaker immediately.
The LT1640 then cycles on and off repeatedly until the
short is removed. This can be minimized by adding a
deglitching delay to the UV pin with a capacitor from UV to
V
EE
. This capacitor forms an RC time constant with the
resistors at UV, allowing the input supply to recover before
the UV pin resets the circuit breaker.
A circuit that automatically resets the circuit breaker after
a current fault is shown in Figure 9.
Transistors Q2 and Q3 along with R7, R8, C4 and D1 form
a programmable one-shot circuit. Before a short occurs,
GND
R7
1M
5%
C4
1µF
100V
R4
562k
1%
R5
9.09k
1%
R6
10k
1%
8
V
DD
3
UV
LT1640L
2
OV
V
EE
4
SENSE
5
C1
0.033µF
24V
GATE
6
R2
R3
10Ω 10k C2
5% 5% 3.3nF
100V
1640 F09a
2
Q2
2N2222
3
Q3
ZVN3310
R8
510k
5%
D1
1N4148
– 48V
Figure 9. Automatic Restart After Current Fault
U
W
U
U
the GATE pin is pulled high and Q3 is turned on, pulling
node 2 to V
EE
. Resistor R8 turns off Q2. When a short
occurs, the GATE pin is pulled low and Q3 turns off. Node
2 starts to charge C4 and Q2 turns on, pulling the UV pin
low and resetting the circuit breaker. As soon as C4 is fully
charged, R8 turns off Q2, UV goes high and the GATE
starts to ramp up. Q3 turns back on and quickly pulls node
2 back to V
EE
. Diode D1 clamps node 3 one diode drop
below V
EE
. The duty cycle is set to 10% to prevent Q1 from
overheating.
Undervoltage and Overvoltage Detection
The UV (Pin 3) and OV (Pin 2) pins can be used to detect
undervoltage and overvoltage conditions at the power
PWRGD
1
+
DRAIN
7
C3
100µF
100V
R1
0.02Ω
5%
Q1
IRF530
1640 F09b
9
LINER [ LINEAR TECHNOLOGY ]
