LTC1642A Datasheet, PDF (11/16 Pages) Linear Technology – Hot Swap Controller

LTC1642A
APPLICATIO S I FOR ATIO
Automatic Restart After the Circuit Breaker Opens
The LTC1642A 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 = ^C4
C4 + 10 • C1
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.
R2
Q1
V_IN
0.015Ω
FDS6630A
12V
2.5A
+
V_OUT
R6
464k
1%
16
15
V_CC SENSE
R3
100Ω
C_LOAD
4
ON
14
GATE
6
FAULT
R4
330Ω
D2
1N4705
D1
LTC1642A
18V
1N4148
R5
60.4k
1%
2
BRK TMR
R10
30k
RST TMR GND
C2
0.047µF
C4
0.33µF
3
8
C1
0.33µF
ALL RESISTORS ±5% UNLESS NOTED
1642a F05
Figure 5. Automatic Restart Circuit
V_GATE
20V/DIV
V_OUT
10V/DIV
V_BRKTMR
1V/DIV
V_RSTTMR
1V/DIV
40ms/DIV
1642a F06
Figure 6. Automatic Retry Following a Load Short
V_IN
12V
2.5A
UNDERVOLTAGE
LOCKOUT
THRESHOLD = 10.7V
R2
0.015Ω
Q1
FDS6630A
+
16
15
V_OUT
C_LOAD
R6
V_CC
464k
1%
SENSE
R3
100Ω
LTC1642A
4
ON
14
GATE
R5
60.4k
1%
RST TMR GND
3
8
C1
0.33µF
R4
330Ω
D1
1N4705
18V
C2
0.047µF
1642a F07
ALL RESISTORS ±5% UNLESS NOTED
Figure 7. Setting a Higher Undervoltage Lockout
1642af
11