# Establishing Safe Operation Range for Lipo Battery with Buck Boost Converter

I have a circuitry that I need to power up with lipo battery. I need to regulate battery to 3.3V with buck-boost converter, since LDO can not be able to provide required current output and causes heating problem etc. In order to safely work with lipo, I need to disconnect the load when battery drops to 3.1-3.2 V. Therefore I checked for the buck-boost converters with UVLO property but I could not be able find buck-boost converter with 3.1V minimum Input Voltage. Hence, I wonder how appropriate to plug a simple diode between input of the buck-boost converter and Positive terminal of the battery for establishing 0.7V Voltage drop in order to being used with 2.5V minimum Input Voltage Buck-Boost Converters?

• Build your own/use an external UVLO to control the EN pin? – winny Aug 3 '20 at 11:30

The regulator you listed may continue working down to 1.7 volts - that is its UVLO level so, adding a diode in series with the power rail to the device is not going to work AND, the diode will represent a big power loss in normal operation because it's conducting power supply feed current. This defeats the object of using a switching regulator in many ways.

Instead, use a comparator circuit and resistors to turn off the EN input when the voltage drops down to 3.1 volts. Here's the sort of circuit that would work to protect the battery: -

You don't have to use SW1 to directly turn the battery off to the TI chip because you can flip the inputs of the comparator and drive the enable pin directly. Or just use it as it stands with a P channel MOSFET.

Version with inputs flipped - note that R4 moves also: -

## Simulation

Note that R1 has changed to 4.9 Mohm to make this operate correctly around 3.1 volts: -

I'm varying the battery voltage between 2 volts and 4 volts (blue trace) with the comparator output (red trace) flipping accordingly around 3.1 volts.

• Thanks Andy it is exactly what I was looking for. I read the article and that circuitry seems like providing 3V UVLO. Just to clarify, connecting the load of that circuit to the ti chip Vin, EN, ILIM0, ILIM1 pins will provide me safe 3.0-4.2V battery operation right? – tronics Jul 31 '20 at 21:30
• Correct, 3 volts is the threshold where the circuit switches off. Does the article tell you what PMOS device to use? I only checked briefly. Your actual load current may dictate that the PMOS device is not the one in the article. – Andy aka Jul 31 '20 at 21:39
• There is nothing related with p-mos in the article. It functions as basic switch and 1m resistance is connected to the gate which basicly turn p-mos on in case of any output from LT1495 so I guess any pmos would do the work. If you have any suggestions about pmos device I would use it in the circuit. – tronics Jul 31 '20 at 22:29
• Find a PMOSFET with low $V_{GS(threshold)}$ - something less than 1.5 volts and choose one with low $R_{DS(ON)}$ so there isn't much volt drop or, go for my original thought of using a comparator circuit that directly drives the enable pin. If you don't want to swap the inputs over (to avoid using the PMOSFET) then use an inverter on the op-amp output to drive EN. – Andy aka Jul 31 '20 at 23:40
• Hey Andy I edited my question with your suggestions. If you can take a look it will be highly useful. Thanks again – tronics Aug 1 '20 at 9:15