SparkFun Forums

[bmarvo]

First here's my schematic:



Its a max1555 powered from usb with the Li-Po battery connected to a two stage regulator. First an mc34063 to boost it to 5v and then a 3.3v linear reg (not a LDO).

The battery is: http://www.sparkfun.com/products/339

The problem is I don't think the battery is charging properly. Unloaded I measure 4.18v. As soon as I switched the power on it drops down to about 900 mV and drops after that by about 50 mV per second and my circuit is only pulling less than 100 mA. After I built the circuit I plugged it in to charge and the 'chg' led indicator turned on. About an hour later it turned off but the battery obviously cannot be fully charged.

If I power my circuit on while the charger is plugged in then the battery drops to about 1.1v and then keeps dropping by 50 mV per second from there.

Ive checked pretty extensively for shorts and found nothing.
Since the indicator isnt on there really isnt a way for me to tell if my battery is getting charged at all...

Anyone help is much appreciated

[Liencouer]

First, to get your #CHG indicator to work, reverse the LED. The #CHG pin is an active low open drain pin. (See datasheet, the pin description table).

Where did you measure that your circuit only draws 100mA? Is that calculated, or actually measured? Is the switch open while you're charging?

[bmarvo]

sorry forgot to mention that the led direction is wrong in the schematic. I have it the correct way in the circuit. The current is calculated

[Liencouer]

I would suggest actually measuring the load current. I suspect that the actual current is not what you calculated.

[bmarvo]

It seems as though it jumps up to over 2 amps for a split second on startup which then seems to cut off the battery.

If I measure the current at the input of the 3.3v linear regulator (bypassing the boost stage) it jumps up to about 300 mA then cuts off.

Any idea why its doing this? Is there anything I can do about it?

[Liencouer]

Do you have a monster cap that you're charging somewhere in the rest of the circuit? The current that you measure has to go somewhere. If you want to post the rest of the schematic, maybe we can point you in the right direction. Otherwise, just poke around. It's all there, you just have to find it. That's half the fun anyways...

Additionally, I just noticed that you're running your circuit while attempting to charge the battery. Sometimes this wont work well, particularly as the current draw of the load approaches the max charging current - there is no current left to charge the battery.

[bmarvo]

sure heres the full schematic:



as you can see I have an atmega128L, an 8x8 led matrix, a usb to uart bridge, and a small dataflash chip

The AVR hasn't been programmed yet so the led matrix doesn't draw any current. The only chips drawing current should be the ft232 and the AVR as well as the power indicator led.

If I bypass the voltage regulators and connect the battery directly to vcc (completely safe) Im only reading 11 mA of current draw (sweet!) but I need the voltage to be regulated as it is in the schematic.

As I mentioned before If I bypass the 5v boost converter and connect the battery directly to the input of the 3.3v linear regulator I read 300 or so mA current (its likely going higher) for a split second then it cuts out.

[waltr]

C16 is 100uF. Is this cap still in the circuit when you by-pass the boost regulator?
Try the same power applied test with just a spare 100uF cap. Does the current spike to 300mA?

[bmarvo]

The cap is still in the circuit when I try the test. Youre suggesting I try it without the cap? you think the cap charging has something to do with it?

[waltr]

I'm suggesting to use just the 100uF cap and measure the current draw
when charging that cap (do ensure the cap is discharged before the test).
This will tell you if you have an inrush problem due to charging that cap.

Remember- divide the problem then conquering is easy.

[bmarvo]

What I did was cut the trace (I have it laid out on a pcb) connected to c16 removing it from the circuit. Now I can power the circuit with the battery connected to the input of the 3.3v linear regulator. Here I measure 5 mA of current. If I bypass the regulator I see 11 mA of current still drawn.

Since the battery doesnt shut down with cap removed (ill still need a filter cap there) I guess its safe to assume that this is the problem? Also the battery still shuts down if I try to power the circuit with both stages of the regulator connected, could this be because of the 100uF cap at the input of the boost regulator? Ill try replacing it with something smaller and see what happens

edit: I just replaced the two 100uF caps with two smaller 10uF caps in parallel. The battery doesn't cut out now! but something is making a high pitched noise I really cant pinpoint the source of the noise. It could be either a.) the circuitry on the battery itself, b.) the the replacement caps (the high frequency, high ripple coming out of the boost converter can have something to do with it) or c.) im just hearing things.

edit2: I would also like to add that I'm getting the correct voltages from the dc/dc converters

edit5: scratch that I'm getting the high pitched noise still and can't pinpoint it

[bmarvo]

The sound is coming from the 180 pF capacitor. Ive mimized the sound by replacing the 100 uF caps with 5 and 10 uF caps. The problem now is that it can't supply near as much current as I originally designed it to. If I have more than one led on at a time on the matrix, it overloads and the battery continuously cuts out. The power indicator led connected directly to vcc doesn't stay at a constant brightness as my program runs.

Ill continue to experiment with different cap sizes. The 100uF capacitors were definitely the cause of the original problem though.

Kind of weird having that tiny (0805) cap emit that high pitch buzz. It was really quite loud (my roommate could hear it in the next room) to begin with but I got it down to where you can only hear it when its right up against your ear.

Anyone have tips to design around this sort of thing?