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

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?

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.

• So, what would be the state of the open drain output when Vin is greater than 20V. And I didn't quite get what INH and INL is...
– Deku
Nov 26, 2018 at 5:48
• High. You would re-label the output for your application voltage, of course (this particular one is configured for a 24V supply). INH sets the voltage it (nominally) switches at when detecting undervotlage, INH the voltage it nominally switches at when turning undervoltage detect off. Nov 26, 2018 at 5:49
• The reference is nominally 2.402V so the divider for INH and INL gives you two voltages. The actual detected voltage is the INH and INL multiplied by either 10x or 40x depending on the range. So the maximum voltage is 24.02V for the 10x and 96.08V for the 40x range. Refer to page 8 of the datasheet. Nov 26, 2018 at 5:52
• Also, what do they mean by 100V rated output? does it mean that the open drain can sink 100V or that it provides 100V as normal comparators do, if the Vin is 100V?
– Deku
Nov 26, 2018 at 5:54
• When you said that the state of the open drain output would be High, do you mean something like 5V or (Vin - 1V)?
– Deku
Nov 26, 2018 at 6:01