Datasheet

10
LT1640L/LT1640H
1640lhfb
APPLICATIONS INFORMATION
WUU
U
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,
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.
V
EE
V
DD
LT1640L PWRGD
SENSE
C1
150nF
25V
C4
1µF
100V
C3
100µF
100V
Q1
IRF530
R2
10Ω
5%
R8
510k
5%
R3
18k
5%
C2
3.3nF
100V
R4
562k
1%
R7
1M
5%
R5
19.1k
1%
R9
10k
1%
R6
562k
1%
Q3
ZVN3310
Q2
2N2222
D1
1N4148
R1
0.02Ω
5%
4
3
2
OV
–48V
UV
56
8
7
1
GATE DRAIN
1640 F09a
3
2
+
GND
(SHORT PIN)
GND
*
* DIODES INC. SMAT70A
1N4148
43
21
Figure 9. Automatic Restart After Current Fault
1640 F09b
NODE 2
50V/DIV
GATE
2V/DIV
1s/DIV