LMV7231 datasheet(12/18 Pages) NSC

12 / 18 page

Application Information

3 RESISTOR VOLTAGE DIVIDER SELECTION

The LMV7231 trip points can be set by external resistor di-

viders as shown in Figure 1

30114953

FIGURE 1. External Resistor Dividers

Each trip point, over-voltage, V

OV, and under-voltage, VUV,

can be optimized for a falling supply, V

THF, or a rising supply,

THR. Therefore there are 2

2 = 4 different optimization cases.

Exiting the voltage detection window (Figure 2), entering the

window (Figure 3), rising into and out of the window (Figure

4), falling into and out of the window (Figure 5). Note that for

each case each trip point can be optimized for either a rising

or falling signal, not both. The governing equations make it

such that if the same resistor, R3, and over/under-voltage ra-

tio, V

OV/VUV, is used across the channels the same nominal

current will travel through the resistor ladder. As a result R2

will also be the same across channels and only R1 needs to

change to set voltage detection window maximizing reuse of

resistor values and minimizing design complexity. Select the

R3 resistor value to be below 100k

Ω so the current through

the divider ladder is much greater than the LMV7231 bias

current. If the current traveling through the resistor divider is

on the same magnitude of the LMV7231 I

BIAS, the IBIAS current

will create error in your circuit and cause trip voltage shifts.

Keep in mind the greatest error due to I

BIAS will be caused

when that current passes through the greatest equivalent re-

sistance, REQ = R1‖(R2+R3), which will be seen by the

positive input of the window comparator, +IN.

LMV7231 bảng dữ liệu(PDF) 12 Page - National Semiconductor (TI)