Datasheet

11
LTC1642
1642fb
APPLICATIO S I FOR ATIO
WUU
U
Automatic Restart After the Circuit Breaker Opens
The LTC1642 will automatically attempt to restart itself
after the circuit breaker opens if the FAULT output is tied
to the ON pin. The circuit is shown in Figure 5. Diode D1
blocks the weak FAULT pull-up current source from unbal-
ancing the R6-R5 divider.
During a continuous current limit such as a load short,
Q1’s duty cycle is equal to the circuit breaker timer period,
divided by the sum of the circuit breaker and system timer
periods:
Short - Circuit Duty Cycle =
+
C
CC
4
410 1•
The duty cycle is 9% for the Figure 5 circuit. Waveforms
during a load short are shown in Figure 6.
Undervoltage Lockout
An internal undervoltage lockout circuit holds the charge
pump off until V
CC
exceeds 2.73V. If V
CC
falls below 2.5V,
it turns off the charge pump and clears overvoltage and
current limit faults.
For higher lockout thresholds tie the ON pin to a resistor
divider driven from V
CC
, as shown in Figure 7. This
circuit keeps the charge pump off until V
CC
exceeds
(1+R6/R5) • 1.34V, and also turns it off if V
CC
falls below
(1+R6/R5) • 1.22V.
D1
1N4148
D2
1N4705
18V
GATEON
14
SENSE
15
4
FAULT
6
BRK TMR
2
V
CC
16
R2
0.015Ω
Q1
FDS6630A
GND
LTC1642
8
RST TMR
3
R5
60.4k
1%
R6
464k
1%
R10
30k
1642 F05
R3
100Ω
R4
330Ω
C2
0.047µF
+
C1
0.33µF
C4
0.33µF
ALL RESISTORS ±5% UNLESS NOTED
V
IN
12V
2.5A
V
OUT
C
LOAD
Figure 5. Automatic Restart Circuit
Figure 6. Automatic Retry Following a Load Short
V
GATE
20V/DIV
V
OUT
10V/DIV
V
BRKTMR
1V/DIV
V
RSTTMR
1V/DIV
40ms/DIV
1642 F06
D1
1N4705
18V
GATE
14
SENSE
15
ON
4
V
CC
16
R2
0.015Ω
Q1
FDS6630A
GND
LTC1642
8
RST TMR
3
R6
464k
1%
R5
60.4k
1%
V
IN
12V
2.5A
1642 F07
R3
100Ω
R4
330Ω
C2
0.047µF
+
C1
0.33µF
ALL RESISTORS ±5% UNLESS NOTED
V
OUT
UNDERVOLTAGE
LOCKOUT
THRESHOLD = 10.7V
C
LOAD
Figure 7. Setting a Higher Undervoltage Lockout