[SOLVED] Lithium Ion low battery indicator circuit design.

Hi...

I am designed a boost regulator using switching regulator topology from webench design tool by TI.

IC name is LM3478

The circuit specifications are as follows:-

1. Vin :- 6.4v to 7.4v.
2. Vout:- 9v 1.5A
3. Expected efficiency by the report from webench is more than 90%.

I am using a battery Lithium ion 7.4v with 2000mAH capacity. I just want to make an 9V/1.5A backup for 5-10 minutes.

My question is how to implement a led which will glow when battery output is low. I have seen that 5v boost converter use LBI/LBO pins for achieving this state. But how to implement in my circuit?

Also, I have seen fuel gauge ic's, all are very expensive, Is there any way in low cost implementation? I want analog, just light up one led below threshold value.

Can I use a comparator and differentiate two input values and light up a led at the output? But if I do will it be near accurate in load? what about the hysteresis?

Thanks and Regards,
SaV

Hi,

It´s not as simple as it seems.
* safety: Are you sure your lithium battery is specified for a discharge current of more than 2A? (2000mAh is "charge" and says nothing about rated current)
Don´t overload your lithium battery, you risk overheat, explosion, fire...

The voltage limits (charge , discharge) are very tight ... so you need exact measurement.
And the relatively big discharge current combined with the internal series resistance makes it impossible to get a valid health status by reading the voltage.

Your controller doesn´t have LBI/LBO so it´s not worth to discuss..

A comparator to monitor the battery voltage is a solution (but as said above: not a good one). One input needs a fixed reference voltage. Maybe you already have one in your circuit, if not: there are ready to buy "voltage reference" ICs. The other input needs to be the battery voltage (usually divided by a resistive voltage divider and a smoothing cap).

..But lighting up a LED is - in my eyes - counterproductive, because it just consumes more current than before and it draws current from the battery until it is killed.
I recommend any undervoltage shutdown to safe battery from being killed by under-discharge.

Klaus

Most low power LED's will be nicely visible with as little as 1 mA of current through it. So if you can live with this additional current drain, you can try this circuit. The LED can be replaced with an opto to provide a shutdown signal. You can replace R2/3 with a 50 k trimpot for fine adjustment.

Hi... Thanks for your valuable time and so kind of you that you made the schematic along with simulation. I made this one and it is working, I made slight modification by changing the TL431 changing ATL431 and saved extra 500uA.