LTC1696

9

APPLICATIO S I FOR ATIO

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

kV

V

V

k

2

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

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.

CV

I

D

INT

CHG

=

1•

external timer capacitor is 10

µA for small feedback

transients and increases to 12

µA for large feedback

transients (greater than 100mV) from one feedback

input. The charging current increases to 18

µA 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

2.5V

1.8 V

5

4

1696 F02

1

2

3

LTC1696

RESET

Q1

2N7002

FB1

GND

FB2

OUT

TIMER/

RESET

R1

44.2k

1%

3.3V

R2

54.9k

1%

Q1

2N3904

SCR

2N6507

R5

470

Ω

5%

R6

22

Ω

5%

R3

44.2k

1%

R4

93.1k

1%

C2

0.1

µF

POWER

SUPPLY

Figure 2. External SCR with NPN Emitter Follower with Low Voltage Supplies