For my first attempt at Li-Po charging I chose the MCP73834-FCI/UN for availability, it has an output current of 1A. I set it up the example circuit in the datasheet and used an 6Ah battery to test it. I had it on my bench while working, monitoring the output every half hour or so and everything was looking good. After about 3 hours it just went up in smoke for no particular reason, the battery was at room temperature though not fully charged (4.04V).

The only thing I can think is that these units weren't designed to operate for such a long time. I disabled the charge timer on purpose since it would take a fairly long time to charge a 6Ah battery. Should I be taking a different approach when working with batteries of this capacity?

What input voltage are you using? -> 5V

Are you using a heat sink? -> 2 Layer FR4

What temperature did it reach? -> Battery: 25C, MCP73834: Wasn't monitoring but the charge rate was slower than expected so probably spend a lot of time in thermal shutdown.

What charge current are you using? -> 1A

Are you observing all data sheet requirements? -> Guessing No...

  • \$\begingroup\$ What was your input voltage? And your resistance between PROG and Vss? \$\endgroup\$ – Telaclavo May 19 '12 at 17:10
  • \$\begingroup\$ 1k which gave me 1A output (though I didn't measure this) \$\endgroup\$ – s3c May 19 '12 at 17:59
  • \$\begingroup\$ Also, exactly, which part number (including all letters at the end) are you using? See in page 25 of the datasheet that the part number determines the regulation voltage. \$\endgroup\$ – Telaclavo May 19 '12 at 18:05
  • \$\begingroup\$ MCP73834-FCI/UN \$\endgroup\$ – s3c May 19 '12 at 18:07

"No particular reason" = = = dissipated excessive energy OR exceeded Vinmax.
The latter is easy and probably fatal.

What input voltage are you using?
Are you using a heat sink?
What temperature did it reach?
What charge current are you using?
Are you observing all data sheet requirements? (= No :-) ).

If your Vsupply exceeds 6V the device may fry.
You load will reduce as battery charge increases and Vsupply may rise as Iin reduces = magic smoke.

The device can charge at max spec only with great care.

The table at te bottom of page 5 says:
" ... 41 C/W with ... 4-Layer JC51-7 Standard Board, Natural Convection"

Assume Vin = 6V.
Assume Iin max = 1A.
When battery is low = 3V then Power dissipated = (6v-3V) x 1A = 3W.
Heatsinked as above gives temp rise of 41 C/W x 3W ~= 120 C Fig 2-7 and Fig 2-8 show shutdown occurs from Tj = 95C so at above power ambient = 95C - 120C = -25C.
If you are not running this in a freezer with a big heatsink it will not work.

Solutions are lower Vin and/or better heatsinking and/or ...?

Excess Vin is a good first thing to check for. Then see as above.

Please supply information as per questions above.

You can use this controller to the 1A limit with due care.
Maximise cooling.
Add a heatsink if necessary.
Forced air / fan may be easy enough and useful.
Even a plastic pkg not notionally intended for heatsinking can have one added and you can place small radiators on PCB just outside "legs".

It would not take much effort to make Vin dynamic. Say Vin was 1V above vBattery at all times it would range from 4V at empty battery to 5V2 at full battery. A variable regulator feeding Vin and a simple monitor of Vin-Vout can adjust Vin to suit. A single transistor can act as monitor.
NPN. Emitter to0 Battery. Base to Vin via a say 10k. When Vin-Vbattery > 0.6V transistor turns on. Use transistor collector to pull base/gate of a PNP / PChannel series pass transistor low.
I have done similar to this to hold an LDO input at 0.6V above Vout & controlled a smps to feed it. Olin reports having done similar

Regulator max dissipation now becomes V_regulator_drop x Imax = 1V x 1A = 1 Watt if 1 volt headroom is needed and 0.5 Watt if 0.5V headroom can be tolerated.
At 1 Watt and 113 C/W it's still marginal but at 0.5W and 113 C/W the temperature rise = 0.5 x 113 ~= 55C. At Tambient = say 35C that's Tj = 55 + 35 = 100C and IC is just managing with heatsink PCB as specified in data sheet. It will be quite easy to get Rth_ja below 113 C/W.

I have heatsunk 1A power diodes in the past by soldering brass shim radiator tabs to the leads immediately next to the diodes. Just clamping a semi random piece of Al against the top of the MSOP pkg would work winders.

The MSOP-10 pkg is 3mm x 3mm. Gluing a 3mm square ended L or U of Al to the pkg top with conductive epoxy would work wonders.

Ugly diagram - you get the idea:

enter image description here

  • \$\begingroup\$ Thanks for the response Russel. I just assumed that at 6V input max it would run at 1A continuously with no problems and didn't give it any thought. Am I correct when I say this unit will survive at 0.5A driven from 5V when mounted on a two layer FR4? Is it common practice to state an output current that isn't easily attainable? \$\endgroup\$ – s3c May 19 '12 at 17:53
  • \$\begingroup\$ @s3c - Be sure that it's not the voltage that kills it. Vmaxoptg is 6V and Vmagicsmoke = 7V. 5V in will drop 2V when battery is flat and 0.8V at charged plateau voltage. At 0.5A that's 1 Watt down to 0.4W. Trise at 41C/W @ 1W =~ 40 C = OK . 2 layer Fr4 is hotter - maybe 60 C/W so Tcase almost boiling temp and thermal shutdown start. How hot does it get AND can you please answer remaining questions above. \$\endgroup\$ – Russell McMahon May 19 '12 at 18:28
  • \$\begingroup\$ The voltage seemed fairly stable though I wasn't watching it at the time, only smelled it. I've answered the other questions in the question description up top. \$\endgroup\$ – s3c May 19 '12 at 18:34
  • \$\begingroup\$ Just noticed that the 41 C/W is for the DFN. I'm using the MSOP10 so it's up to 113 C/W. \$\endgroup\$ – s3c May 19 '12 at 18:39
  • \$\begingroup\$ So basically from 5V I can't get more than 250mA (and even that is pushing it) meaning my 6Ah battery will take more than 24h to charge. Do other charge controllers get around this limitation or is this the norm? \$\endgroup\$ – s3c May 19 '12 at 18:46

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