Questions about MCP73831 Charging Circuit

I am working on a project that I would like to include a LiPo charging circuit for use with your jelly bean LiPo batter packs, probably a 400mah. To keep cost down, I decided to go with a jelly bean part, the MCP73831 controller.

Now I have read the datasheet, and I pretty well understand what it is doing. However, there is one thing that seems like it should be obvious, but it’s not explicitly implied in the datasheet. And that is how the supply lines should be tied together.

From my understanding of the datasheet, the VBat pin of the controller will charge the battery, and will also supply the current needed to run the circuit (provided it is within the specifications of the controller). But the only thing I see on the datasheet is a general schematic, not a practical one showing how the load is tied into the circuit.

Now I just assume that it outputs whatever it outputs at VBat, and that would run the circuit, and the excess would be fed into the battery. And there is no other logic that would be needed. But this seems like it could be problematic. I could always use diodes between the +5v and VBat to create a voltage dependent switching supply gate that supplies the load. But then I would fall below what I would like to keep my voltage at once the battery’s voltage began to drop. (I’d really like to keep this in the 3.3v range since I’d like to incorporate some analog sensors).

I did see some mention on this Blog in which he uses further circuitry to switch between 5v and 3.3v supplies. However, due to size requirements, I’m barely going to have the space on the board for the charging circuit in it’s basic forum. That, and I have also experience reverse leakage on Mosfets when in their closed state…but thats another story.

In any case, I’ve attached a picture of the circuit that I’m planning on using, is essentially the same as in the datasheet with a slight modification to use a pin on the AVR rather than an LED. I’m mostly wanting to make sure I’m not going to have any issues with the LiPo venting when I plug in the USB.

MCP73831 Circuit.PNG

Is your question thus:

Can I run the load and charge the battery at the same time from the same supply lines?

In which case, Yes, you can (think of how your laptop works). If there is sufficient power to run your load and charge the battery the battery will be charged (if it needs to be, that's what all the protection circuitry is for, to prevent overcharging / overheating).

Other than that I think you're good to go.

jdgabbard: I am working on a project that I would like to include a LiPo charging circuit for use with your jelly bean LiPo batter packs, probably a 400mah. To keep cost down, I decided to go with a jelly bean part, the MCP73831 controller.

Now I have read the datasheet, and I pretty well understand what it is doing. However, there is one thing that seems like it should be obvious, but it's not explicitly implied in the datasheet. And that is how the supply lines should be tied together.

From my understanding of the datasheet, the VBat pin of the controller will charge the battery, and will also supply the current needed to run the circuit (provided it is within the specifications of the controller). But the only thing I see on the datasheet is a general schematic, not a practical one showing how the load is tied into the circuit.

Now I just assume that it outputs whatever it outputs at VBat, and that would run the circuit, and the excess would be fed into the battery. And there is no other logic that would be needed. But this seems like it could be problematic. I could always use diodes between the +5v and VBat to create a voltage dependent switching supply gate that supplies the load. But then I would fall below what I would like to keep my voltage at once the battery's voltage began to drop. (I'd really like to keep this in the 3.3v range since I'd like to incorporate some analog sensors).

I did see some mention on this Blog in which he uses further circuitry to switch between 5v and 3.3v supplies. However, due to size requirements, I'm barely going to have the space on the board for the charging circuit in it's basic forum. That, and I have also experience reverse leakage on Mosfets when in their closed state....but thats another story.

In any case, I've attached a picture of the circuit that I'm planning on using, is essentially the same as in the datasheet with a slight modification to use a pin on the AVR rather than an LED. I'm mostly wanting to make sure I'm not going to have any issues with the LiPo venting when I plug in the USB.

Based on the battery charge algorithms described, (constant current, voltage peak). I would separate your 'Circuit' and 'Vbat' nets. I would add 2 schottky diodes. one from +5V to Circuit power, the other from Vbat to Circuit power. By adding these two dioes, your circuit runs off of +5V if it is available or from Vbat is the charge power is not available. A good schottky diode (I use DB2W31900LCT-ND from DigiKey, 3A 30V with 490mV drop) will only drop your +5v down to 4.6v or more. The Vbat voltage will drop from 4.2V to 3.8v, Now, a 'dead' lipo 3.0V would drop down to 2.6V. What is acceptable for your circuit?

+5V anode -|>|- cathode to Circuit +- Vbat anode -|>|- cathode to Circuit |

Chuck.