Load Sharing Circuit w/ MCP73831 and LDO

[avc4eva]

Hi all,

I need a bit of guidance on a load sharing circuit to power my Cortex M0 PCB.

I am using a 110mAh 3.7V LiPo battery as my main source as this project is Wearable based, however i also wish to charge it from USB , i've done this succesfully using the MCP73831, however i want to be able to detect when USB is plugged in so it automatically charges and sources from USB and when it is not it simply uses the battery, I've had to manually use an SPDT switch to toggle this. I figured a MOSFET would be best for this, a circuit is shown below:



With the P-MOST turning off when 5V is applied this allows the battery to charge and the load to be sourced from USB.
With P-MOST on when 5V removed, the battery then sources to the load.

The issue i have with this circuit is that from 'load' i also have/need a 3.3V LDO to power my MCU, in worst case scenarios and with rdsON of the FET the voltage from 'load' could drop below 3.3 nullifying the circuit.

What would be the best way to have this load sharing that also allows me to use and LDO or similar to 3.3V?

Andrea

[JohnnyBerg]

There is a simulair thread here:

https://www.eevblog.com/forum/microcontrollers/rechargeable-battery-solution-for-mcu-projects/msg630455/#msg630455


note: I recognize the schematic, its comes from here:

http://blog.zakkemble.co.uk/a-lithium-battery-charger-with-load-sharing/

If my memory serves me correctly it seemed to have several flaws when I bread boarded it.

[avc4eva]

Thanks for the direction!

Ye i am familiar with the site you posted.

I'm still concerned about the drop out to the LDO though, which doesn't seem to have been covered.

[JohnnyBerg]

When the voltage on the input of the regulator comes below 3.5 volts, the output of the regulator starts falling with it. So the 3.3.V wil slowly drop, as the battery discharges.



That's why you cannot use the internal reference of the MCU anymore, because there is uncertainty over the supply voltage.

So, my NanoLipo has its own reference on board. As a extra, it is much more accurate then when using the supply voltage as a reference.

And, off coarse, the reference can be turned off, to save power.
We are on a battery 

[exapod]

Are you sure that in worst case scenarios you are dropping 0.4 V ?
If it is a wereable i assume the current consumption to be very low so 0.4V seems a lot, what is the rdson of your mosfet?
Most MCU and sensor will work below 3.3V so check if you can use lower voltages like 3V, 2.5V, 1.8V etc.

[JohnnyBerg]

@extrapod: to who are you referring?

Most LiPo batteries must not go below 3V, so I designed my circuit that the minimal CPU voltage is 2.7V, and then it goes to sleep. I do not use the brown out detection, but that would also be possible to do.

AO3401 has a Rds on of < 85mOhm for Vgs @ 2.5 V. no problem there 

[avc4eva]

@extrapod: to who are you referring?

Most LiPo batteries must not go below 3V, so I designed my circuit that the minimal CPU voltage is 2.7V, and then it goes to sleep. I do not use the brown out detection, but that would also be possible to do.

AO3401 has a Rds on of < 85mOhm for Vgs @ 2.5 V. no problem there 

Thanks for all your help, i'm trying to make my board with few components as it is small and limited space. What would you say i would need to get it to perform as minimally as possible?>

[exapod]

from the op:
 

The issue i have with this circuit is that from 'load' i also have/need a 3.3V LDO to power my MCU, in worst case scenarios and with rdsON of the FET the voltage from 'load' could drop below 3.3 nullifying the circuit.

Nominal voltage of a LiPo is 3.7V to go below 3.3V you are dropping 0.4V in the mosfet and i don't think is possible in a wereable device with a 110 mAh battery.
But if you want to use all the power from the lipo use a lower ldo regulator like : 3V, 2.5V or 1.8V if your circuit allows it.

[JohnnyBerg]

Thanks for all your help, i'm trying to make my board with few components as it is small and limited space. What would you say i would need to get it to perform as minimally as possible?

Define small, limited and  minimally

If you want I send you one of my nanolipo's (for the cost of shipping and the BOM) ?

[avc4eva]

from the op:
 

The issue i have with this circuit is that from 'load' i also have/need a 3.3V LDO to power my MCU, in worst case scenarios and with rdsON of the FET the voltage from 'load' could drop below 3.3 nullifying the circuit.

Nominal voltage of a LiPo is 3.7V to go below 3.3V you are dropping 0.4V in the mosfet and i don't think is possible in a wereable device with a 110 mAh battery.
But if you want to use all the power from the lipo use a lower ldo regulator like : 3V, 2.5V or 1.8V if your circuit allows it.

It was a big guess it would never really be that bad but i was just unsure on the LDO part as the nominal voltage of the LiPo and LDO dropout are kinda close, tad worrying.

[avc4eva]

Thanks for all your help, i'm trying to make my board with few components as it is small and limited space. What would you say i would need to get it to perform as minimally as possible?

Define small, limited and  minimally

If you want I send you one of my nanolipo's (for the cost of shipping and the BOM) ?

Again thanks! I'm ok no need to send anything, would the first circuit i posted be viable for my worries do you think?

[exapod]

Thanks for all your help, i'm trying to make my board with few components as it is small and limited space. What would you say i would need to get it to perform as minimally as possible?

Some time ago i made a wirst watch based on a pic24 : http://dangerousprototypes.com/forum/download/file.php?id=10621&mode=view .
On the bottom left of the picture you can see a MCP73832 the p mosfet a diode and a 3V ldo all in qfn-like package and really small.
All the passive are 0603 but if you are "crazy" enough you can use 0402.

[JohnnyBerg]

would the first circuit i posted be viable for my worries do you think?

It is a while ago, but I remember bread boarding it. I encountered some problems, with the mosfet and the speed of switching between USB and LiPo. If I remember correctly.
You either have to breadboard it yourself and try, or take my circuit. My circuit definitely works

There are some problems with the MCP73831 too. It oscillates when no battery is connected, and because of the tri-state output of the LED, it is very impractical to get the LiPo charge status into the MCU 

[diogoferreira72]

avc4eva the first circuit works fine for you?
I am with a similiar project that need a battery Charge and a Voltage Regulator for a 3.3v microcontroller