LTC1642ACGN Linear Technology, LTC1642ACGN Datasheet - Page 12

• Manufacturer Part Number: LTC1642ACGN
• Description: IC,Electronic Circuit Breaker,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

Specifications of LTC1642ACGN

• Linear Misc Type: Positive Low Voltage
• Family Name: LTC1642A
• Package Type: SSOP N
• Operating Supply Voltage (min): 2.97V
• Operating Supply Voltage (max): 16.5V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Product Depth (mm): 3.99mm
• Product Height (mm): 1.5mm
• Mounting: Surface Mount
• Pin Count: 16
• Lead Free Status / Rohs Status: Not Compliant

2.5A
APPLICATIO S I FOR ATIO
Overvoltage Protection
The LTC1642A can protect a load from overvoltages by
turning off the pass transistor if the supply voltage ex-
ceeds an adjustable limit, and by triggering a crowbar SCR
if the overvoltage lasts longer than an adjustable 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, 1.22V bandgap ref-
erence, two current sources, and a timer at the CRWBR
pin. When V
output goes high and internal logic turns off Q1 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
creases to 1.5mA. Emitter follower Q2 boosts this current
to trigger crowbar SCR Q3. The ramp time ∆t needed to
trip the comparator is:
12V
LTC1642A
12
V
ALL RESISTORS ±5% UNLESS NOTED
OV COMPARATOR TRIPS AT V
RESET TIME = 200ms
CROWBAR DELAY TIME = 90µs
IN
t
CRWBR
R6
127k
1%
R5
12.4k
1%
= 9.1(ms/µF) C5
Figure 8. Overvoltage Protection Circuitry
9
4
6
CC
OV
ON
FAULT
RST TMR
exceeds (1+R6/R5) • 1.22V the comparator’s
V
CC
16
LTC1642A
3
0.015Ω
R2
C1
0.33µF
U
IN
SENSE
CRWBR
= 13.85V
GATE
GND
15
8
U
14
1
FDS6630A
Q1
2N2222
C5
0.01µF
R3
100Ω
R4
330Ω
C2
0.047µF
Q2
W
* ADD 220Ω RESISTOR IF
USING A SENSITIVE-GATE SCR
+
Q3
MCR12DC
D1
1N4705
18V
C
U
LOAD
V
OUT
CC
1642a F08
in-
Once the CRWBR timer trips the LTC1642A 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 2µs 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.
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
1.22V at time 6, GATE and FAULT stay low, and CRWBR
continues to source 1.5mA. FAULT goes high when ON
goes low at time 7, and GATE begins charging at time 8,
one RST TMR cycle after FAULT goes high.
Figure 10 shows typical waveforms when the OV divider is
driven from the N-Channel’s output side. Because the
voltage driving the divider collapses after the OV compara-
tor trips, FAULT stays high and CRWBR stays near ground,
which prevents the pin from triggering an SCR. The GATE
voltage begins ramping up after a RST TMR timing cycle.
To disable overvoltage protection completely, tie the OV
and CRWBR pins to GND. For overvoltage protection at the
GATE pin, but without latch off or a crowbar SCR such as
Q3 in Figure 1, tie CRWBR to GND.
RST TMR
CRWBR
FAULT
GATE
OUT
OV
ON
IN
Figure 9. Overvoltage Timing (Input Side)
t
1
CC
t
2
. The OV comparator goes high at time 1,
t
3
t
100ms/DIV
4
t
5
t
6
t
7
t
8
1642a F09
20V/DIV
2V/DIV
50V/DIV
20V/DIV
1V/DIV
2V/DIV
20V/DIV
20V/DIV
1642af