Battery power back-uped circuit

[flotulopex]

Thanks Sayser,

Unfortunately, 3.2V will be too low - anyway, thanks for the Li-FePo4 advice


BTW, going back to AN1149 and the choice of the P-Channel MOSFET, as I don't know if a FDN360P will do the job?

Any clue?

[sayzer]

Consider this one: DMP3099L-7

I have my own charging circuit controlled by PIC with FVR.
Measure batt voltage by adc read on FVR to calculate the batt voltage.
If over 4.25V then cut the charge; if lower than 2.75V then suspend all operations.
All done within the pic.

You need an ppnTR-P-ch charging circuit; very simple.

[Ioannis]

FVR is not at all accurate, so that 4.25... I doubt you are safe.

Ioannis

[sayzer]

FVR is not at accurate, so that 4.25... I doubt you are safe.

Ioannis

Why is not not accurate?
I get 1.024V with 1x output.
And if the Vdd is 2,5V I still have 1.024V at my FVR output. I never encountered an accuracy issue.

But, if you have a high freq circuity on the same pcb, or even if your PWM module is running, then I agree, FVR can be disrupted.
Also, here is a simple circuit diagram for NPN-PCH charging. Assuming you are not charging the batt with high amps, LM317 would do the job. If not, change LM317 to a switching circuit to have higher efficiency and lower heat.

[Ioannis]

Because according to Electrical Specs, the FVR may be from -8% to +6% for example on 16F1827.

I would not trust this to charge a LiPo battery at all!

Ioannis

[Ioannis]

Roger,

The important specs is dependent on the application.

For example, if you drive the MosFet from a logic level chip, say a MCU, then you want the Vgs to be as low as possible for the MosFet to be fully closed and have minimum Rdson

Other spec like the circuit on the above Note, is the Rds itself, so you don't have thermal losses.

Also the Vds max should be more than the worst case on your circuit voltage levels.

Ioannis

[flotulopex]

Roger,
The important specs is dependent on the application.....
Ioannis

Thanks Ioannis.

I'll give it a try with a few different MOSFETs and see what happens.

For any reason, until I haven't tested it with my "hands" (and the voltmeter ), it doesn't get inside my head that easy.

Maybe I'm getting too old to play with all these fantastic toys

[Ioannis]

Roger, the minus Vgth is correct. Consider it a different visual angle!

In P-channel or PNP transistor the voltages are the opposite of the N-channel or NPN transistors. Means that the voltage og the gate in respect to the source is negative. The key to understand here is the term "in respect".

Ioannis

[flotulopex]

If I may ask further,

The "most important" value to take care off is the "Gate Threshold Voltage"; right?

It has to be as low as possible...but how low and why?



I thought RDSon value is very important too. But in this case, where the current is somehow small, is this less important in choosing the "right" MOSFET?