Voltage monitor MAX6457 problem

Aug 15, 2007 2 Replies
L

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:

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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.



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F

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.
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L

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

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