LTC1696

9

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For more information www.linear.com/LTC1696

APPLICATIONS INFORMATION

Feedback Inputs

The LTC1696 has two feedback inputs that allow monitor-

ing of two output voltages. The trip point of the internal

comparator is set by an internal reference of 0.88V with

an external resistor divider network (Figure 1). The resis-

tors R1 and R2 values are calculated with the typical trip

point of 0.88V.

R1

R1+R2

R2 =

0.88

As an example, let’s calculate values for R1 and R2 for a

3.3V supply in which an overvoltage indication is required

at +10% (3.63V). First, a value for R1 is chosen based on

the allowable resistor divider string current. This is de-

termined by power dissipation requirements and possible

sensitivity to noise coupling into the resistor divider. In

this exercise, assume the resistor divider current is 20µA.

R1 is calculated from:

R1=

=

0.88V

20µA

= 44k

The nearest 1% value for R1 is 44.2k. Now, calculating

for R2 yields:

R2 =

44.2k •(3.63v − 0.88V)

0.88V

= 138.1k

Choosing the nearest 1% value yields 137k.

FB1

R2

R1

LTC1696

+–

–

+

Figure 1

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 in-

ternal latch is reset by pulling the TIMER/RESET pin low

through an external N-channel MOSFET switch or pulling