Power Management Gboard 800 + MCP73871

Hi Guys,

A quick problem. Trying to work out an optimum solution here.
Problem statement initially was: Designing a mini back-up system for my Itead Gboard 800.
Where the board should be powered from a 12VDC adaptor and on a power failure a battery should be able to supply the power.

My findings: Looked up in-line power management solutions i.e. battery chemistry, voltage, required current, and management IC.
Found the right battery for the job i.e. Li-Ion 18650 type 3.7V @ 2250mAh. The management IC worked out to be MCP73871.

Now the Problem:

Background:

The Gboard800 as per its electrical characteristics chart works 7-23VDC @ 30mA<Gboard<500mA i.e. there is a DC-DC converter on the board for various voltages.
The board consists of an MCU with arduino bootloader and SIM800 module. The MCU side of the board works at 3.3VDC and the GSM side works in around 3.7-4.1VDC region.

Voltage supply: There are two supply points on the board. First is the barrel plug type socket for the adaptor and the second is a battery socket.
If I use the adaptor socket all the voltages are regulated and smoothly delivered to the target devices through the DC-DC convertor.
However, if I use the Battery socket the MCU side gets regulated but the GSM module gets the actual supply voltage coming through the battery socket.

So the problem:

I'm using MCP73871DM development module to design the aforementioned circuit. The module works at
4.45-6VDC. When there is supply, it outputs the voltage straight on. When the supply is cut off 3.8VDC from the battery helps powering the module.

Now at 4.45V (regulated to 3.3V) the MCU on Gboard works fine. SIM800 module will start working but during its boot-up sequence it will shut down due to over voltage. Datasheet specified voltage for SIM800 module is 3.5 to 4.4VDC.
I'm missing out the window by few milli-volts.

How shall I tackle this problem?

The power management module won't work on anything less than 4.43V and anything above this will cause SIM800 module to shut down.

PS: Sorry for the long post. let me know for any specifics.
Thanks!

Bump....

I've managed to chop the extra volts by adding a Schottky diode (STPS10L25D) with a forward drop voltage of 350mV.
Now my specs are within the right range.

So if I now supply my battery management system 4.5VDC, delivered output is 4.15VDC or thereabouts which should be good for the GBoard.

What I'm now experiencing is a bit weird for me. I'm monitoring the output pin of the battery management module, when there is power present via the adaptor I see 4.15VDC. As soon as I power the GBoard and the GSM module starts to operate somehow the output voltage is pushed down to 3.65VDC i.e. it goes into battery supply mode even though the power in is still available.

Why is that? Only thing I could pick from MCP73871 datasheet is Under-Voltage Lockout (UVLO). How can I avoid this?

Replies would be appreciated.

Thanks!