Datasheet

LTC1696
9
APPLICATIO S I FOR ATIO
WUUU
R
V
I
V
A
k
FB
DIVIDER
1
088
20
44==
µ
=
.
The nearest 1% value for R1 is 44.2k. Now, calculating for
R2 yields:
R
kVV
V
k2
44 2 3 63 0 88
088
138 1=
()
=
.•. –.
.
.
Choosing the nearest 1% value yields 137k.
The chosen values for R1 and R2 yield an overvoltage
threshold of 3.608V (+9.3%). With worst-case tolerances
applied, the minimum overvoltage threshold is 3.481V
(+5.5%) and the maximum overvoltage threshold is 3.738V
(+13.3%).
Reset Function
In the event of an overvoltage condition, the OUT pin of the
LTC1696 is latched into an active high state. The internal
latch is reset by pulling the TIMER/RESET pin low through
an external N-channel MOSFET switch or pulling V
CC
voltage below the UVLO trip point of 1.94V.
Glitch Filter Timer
The LTC1696 has a programmable glitch filter to prevent
the output from entering its active high latched condition
if transients occur on the FB1 or FB2 pins. The filter time
delay is programmed externally by an external capacitor
C1 connected to the TIMER/RESET pin.
The time delay is given by:
t
CV
I
D
INT
CHG
=
1•
where V
INT
is the internal reference voltage of 1.185V and
I
CHG
is the internal current source charging the external
capacitor C1. The current source I
CHG
charging the
external timer capacitor is 10µA for small feedback
transients and increases to 12µA for large feedback
transients (greater than 100mV) from one feedback
input. The charging current increases to 18µA for large
feedback transients from both feedback inputs.
SCR Crowbar
The LTC1696 can deliver continuous output current typi-
cally 80mA at high supply voltage to trigger an external
SCR crowbar in the event of an overvoltage condition as
shown in the typical application on the front page of the
data sheet. The output current decreases when the supply
voltage reduces. It delivers 25mA at a supply voltage of 5V.
At a low supply voltage of 3.3V, the output current reduces
to 10mA and an external NPN emitter follower is needed to
boost the current in order to drive the SCR crowbar as
shown in Figure 2. The power dissipation due to the high
output current at high supply voltage can potentially
exceed the thermal limit of the package. This is avoided by
resetting the device rapidly when the external SCR crow-
bar has been triggered, so that the device is not kept in the
active high state for too long.
C1
1nF
6
V
OUT2
2.5V
V
OUT1
1.8 V
5
4
1696 F02
1
2
3
LTC1696
RESET
Q1
2N7002
FB1
GND
V
CC
FB2
OUT
TIMER/
RESET
R1
44.2k
1%
V
CC
3.3V
R2
54.9k
1%
Q1
2N3904
SCR
2N6507
R5
470Ω
5%
R6
22Ω
5%
R3
44.2k
1%
R4
93.1k
1%
C2
0.1µF
POWER
SUPPLY
Figure 2. External SCR with NPN Emitter Follower with Low Voltage Supplies