# Voltage monitor MAX6457 problem

Discussion in 'Electronic Design' started by [email protected], Aug 15, 2007.

1. ### Guest

Dear group:

I am using the Li-Ion battery (7.4V nominal) to power my circuit. I
need to monitor the battery volgate and shut down the whole thing when
it drops below 6V.

I am using MAX6457 to monitor the voltage. The datasheet is here:
http://datasheets.maxim-ic.com/en/ds/MAX6457-MAX6460.pdf.

I have trouble selecting values for the resistors R1 and R2. According
to the equation on
page 8, you should fix R2 and than set R1 = R2 (Vtrip / Vth - 1). I am
really confused, what are the values of Vtrip and Vth in my
case... not very clear from the datasheet. I know I can select these
resistors by trial and error but I have to demostrate devising the
correct values according to the datasheet. Thanks in advance.

2. ### Fred BloggsGuest

Vtrip=6V (low Vbatt level) and Vth=Vth(-), the monitor will pull open
drain OUT low when Vbatt *falls* below 6V. Depending upon the grade of
6457 as designated in the suffix as A, B, or C, the Vth(-) will be 0.5,
5, or 8.3 % respectively below Vth nominal of 1.228V. This means you use
Vth=0.995x1.228, 0.95x1.228, or 0.917x1.228 for Vth in the equation. The
nominal Vth reference varies +/-2% over parts and temperature.

3. ### Guest

Thanks a lot! If it works, you saved my day