Li-ion battery charger and boost circuits

[rrb011270]

Mabuhay!

I have a PIC18F452 system and I added a backup battery circuitry. I use the MAX1736 that will charge a single Li-Ion 3.3v battery and a boost converter using the MAX1675 that will boost the voltage level from 3.3v to 5.5v.

I can't operate the system at 3.3v coz I am using some chips that require 5v.

My concern is... I want the max1675 to be shutdown during normal operation and when their is a power failure it will be enabled. I check the documents included with the max1675 and I found-out their is a shutdown active low pin. How can I utilize this such that during normal operation the max1675 is shutdown and during power failure it boost the voltage coming from the 3.3v li-ion battery.

Any help, tips nad/or ideas will do...

Thanx

[neil]

Hello, I have had a look at the MAX1675 and remember doing something like this for a handheld unit a few years ago. I really can't remember how I did it and I don't have the documents anymore, but here is an idea along the same lines:

I understand that you have 2 supplies in your system, a normal running supply, and a battery backup.

You could use a single transistor (yes a discrete component - something not often mentioned on here!!) to invert the logic to the SHDN' pin. Eg. a pullup resistor to your 'system VCC' and the SHDN' pin to the junction between transistor collector and pullup. Emitter goes straight to ground. This would mean that with a signal to the transistor base coming from your normal operating supply (separate from 'system VCC' with a diode) would switch the transistor on and pull your SHDN' low, also pulling (small) current through the pullup res.

If your main supply fails, the base signal is removed, switching the transistor off. If you have enough capacitor charge in your reservoir cap. (eg a 10uF electrolytic on 'system VCC') This should hold for long enough to pull SHDN' high and give enough time for the MAX1675 to start up and supply the power, keeping the capacitor charged and the system running.

Looking at the datasheet, the startup time is about 750µs between SHDN' going high and having an output voltage of about 3.3V. It doesn't appear to specify the load current for this time though.

This method looks like a bit of a 'frig' but I think it will work. If my description is a bit vague, I can try and get a schematic to you. It may be worth building this on veroboard or something to see if it works.

Hope this helps,
Neil.

[rrb011270]

Hello Neil,

Here's the schematics on the Li-Ion charger and boost converter...

Please review it if it has some error in the design.. I myself not satisfied so I still keep searching if this will work.

I need help on this type circuits for my PIC system backup supply.

Here is the image:


Thnx

[neil]

Hello, I have had a look at your design and it looks like it should work fine as it is. The are a couple of points however. You don't appear to be using the low battery detection on the MAX1675. The datasheet states that if it isn't used, connect LBI and LBO to ground. This will at least save on 2 resistors!

What is the 3-pin device to the right of the picture? I can't see the number as the picture is too small. Is it a reset chip?

I have added my idea with the transistor to switch on the DC-DC when the power fails and created an OrCaD project, and also a DXF which can be viewed in Autocad or similar. Can you view either of these file types? I cannot successfully create a 'picture' version, as it comes out too small!

I will send it to you on your reply.
Regards,
Neil.

[rrb011270]

[neil]

Hello, I wouldn't use the 3.3V reset chip to enable the DC-DC converter, as the voltage would have to drop from 5.5 to 3.3 before the converter comes on. This would be a 'brownout' to your circuitry and would probably cause unwanted resets or data corruption. The PIC would probably be OK as they will work at lower voltages unless you have brownout reset enabled, but you mentioned having other devices which require 5V.

I would replace the MAX6462 with a 5V version and this should do the job better than my idea with the transistor.

I will email you the design shortly however.

Regards,
Neil.

[rrb011270]

Hello Neil,

Thank you for the help. I'll test the circuit tis coming Monday.