Understanding Undervoltage in LTC2965 - 100V Micropower Single Voltage Monitor?

Question

The undervoltage specifications for this IC is a bit confusing to me as this is the first time I'm working with these kind of circuits.

Aim : I want to monitor the voltage of a battery pack (consisting of 8 x 14 V batteries) being used to power my house. The voltage monitor must be able to trigger a relay to switch to mains power when the battery pack are below a certain level (96 V in this case). I understand that there are voltage monitors designed for batteries in this case that would come with all the settings but I want to switch the battery over to Mains power supply much before the specified 92 V.

In my case the undervoltage shouldn't really matter as my interested voltage range is from 96 V to 112 V. But this circuit designed for 24 V level monitor specifies a term "24 V Undervoltage" which is puzzling me

The spec sheet for the IC specifies the Undervoltage lockout as 3V under Vin RISING condition. Infact the operating range for the IC is 3.5V to 100V which means to say that the IC will not operate below 3.5V.

What do they actually mean by these terms? Is this kind of circuit well suited to be adapted for my aim? Or would it be better for me measure the voltage of single battery and assume rest of the 7 batteries would be of the same voltage?

Answer 1

The "/24V UNDERVOLTAGE" output goes low when the 24V supply is undervoltage, in this particular case it is less than 18.2V or between 20V and 18.2V and on the way up from < 18.2V. It's an open drain output so it can be pulled up to whatever logic level you like (5V in this case).

The maximum INH level is 2.402V * 40 = 96.08V and INL should be less than that. To get that nominal setting you use the 40:1 range rather than the 10:1 range shown in the diagram.

The 3V supply lockout built into the chip is not much of a concern since your battery will be far above that under reasonable conditions (assuming it's actually connected). What that means is that the output is not guaranteed to be low if the supply voltage Vin is < 3V.

The absolute maximum voltage of the chip is 140V so you may wish to add some protection (eg. with a TVS).

If the 96V level (and perhaps 90V level, up to you to pick a level) for switching is acceptable you can connect INH directly to REF and add two resistor to determine the hysteresis. If you set the hysteresis too small the device may switch back and forth excessively, so your design has to take that into account.