LTC1642 Linear Technology, LTC1642 Datasheet - Page 8

• Manufacturer Part Number: LTC1642
• Description: Hot Swap Controller
• Manufacturer: Linear Technology

LTC1642
APPLICATIO S I FOR ATIO
Overvoltage Protection
The LTC1642 can protect a load from overvoltages by
turning off the pass transistor if the supply voltage ex-
ceeds a programmable limit, and by triggering a crowbar
SCR if the overvoltage lasts longer than a programmable
time. The part can also be configured to automatically
restart when the overvoltage clears.
The overvoltage protection circuitry is shown in Figure 8.
The external components comprise a resistor divider
driving the OV pin, timing capacitor C5, NPN emitter
follower Q2, and crowbar SCR Q3. Because the MCR12DC
is not a sensitive-gate device, the optional resistor shunt-
ing the SCR gate to ground is omitted. The internal
components comprise a comparator, 1.23V bandgap ref-
erence, two current sources, and a timer at the CRWBR
pin. When V
output is high and internal logic pulls the GATE down and
starts the timer. This timer has a 0.410V threshold and
uses the CRWBR pin; when CRWBR reaches 0.410V the
timer comparator trips, and the current sourced from V
increases to 1.5mA. Emitter follower Q2 boosts this cur-
rent to trigger crowbar SCR Q3. The ramp time t needed
to trip the comparator is : t(ms) = 9.1•C5( F).
Once the CRWBR timer trips the LTC1642 latches off: after
the overvoltage clears GATE and FAULT remain at ground
and CRWBR continues sourcing 1.5mA. To restart the part
after the overvoltage clears, hold the ON pin low for at least
2 s and then bring it high. The GATE voltage will begin
ramping up one system timing cycle later. The part will
restart itself if FAULT and ON are connected: GATE begins
ramping up one system timing cycle after the overvoltage
clears.
Figure 9 shows typical waveforms when the divider is
driven from V
causing the chip to pull the GATE pin to ground and start
the CRWBR timer. At time 2, before the timer’s compara-
tor trips, OV falls below its threshold; the timer resets and
GATE begins charging one system timing cycle later at
time 3. Another overvoltage begins at time 4, and at time
5 the CRWBR timer trips; FAULT goes low and the CRWBR
pin begins sourcing 1.5mA. Even after OV falls below
1.23V at time 6, GATE and FAULT stay low, and CRWBR
continues to source 1.5mA. FAULT goes high when ON
8
CC
CC
exceeds (1+R1/R5)•1.23V the comparator’s
. The OV comparator goes high at time 1,
U
U
W
U
CC
2.5A
V
OV
GATE
V
CRWBR
RST TMR
ON
FAULT
12V
ALL RESISTORS 5% UNLESS NOTED
OV COMPARATOR TRIPS AT V
RESET TIME = 200ms
CROWBAR DELAY TIME = 90 s
V
CC
OUT
IN
0.1 F
1
C7
Figure 9. Overvoltage Timing (High Side)
Figure 8. Overvoltage Protection Circuitry
2
R1
127k
1%
R5
12.4k
1%
9
4
6
OV
ON
FAULT
3
RST TMR
V
IN
CC
= 13.85V
16
LTC1642
3
0.010
R2
C1
0.33 F
SENSE
CRWBR
0V
GATE
GND
15
4
8
14
1
FDR9410A
Q1
2N2222
R3
100
R4
330
C5
0.01 F
C2
0.047 F
Q2
5
6 7
+
Q3
MCR12DC
8
1642 F08
C
0V
1.23V
0V
V
0.41V
1.23V
LOAD
CC
1642 F09
V
OUT