I think that connecting load directly to VBAT is not great, because the battery will be connected to the load while charging, which, I think, will not sink the full charging current into the battery, because part of that current will flow to the load. Yes/no/maybe?
You should add a P-FET between the battery and the load, and have Vin turn off the gate, and add a diode from Vin to load. See Microchip's application note: http://ww1.microchip.com/downloads/en/D ... 51746a.pdf Page 5. P-FET plus diode available in a single package: FDFMA2P857.
EDIT: Actually, I would use a discrete diode on a large footprint, so that users can replace it with desired current rating. The one in FDFMA2P857 is 1A only, which is not much at all.
I guess I'll have to add this if I am to buy this board. Perhaps, something to consider for next revision. Thanks.
Suggestion for MCP73833 lipo charger
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- adafruit_support_bill
- Posts: 88153
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Re: Suggestion for MCP73833 lipo charger
Thanks for the suggestion. I will forward it to the design team.
With these simple chargers, the battery will charge more slowly when connected in parallel with the load. But this is an acceptable compromise in many applications.
We do have more sophisticated chargers that incorporate load-sharing circuits, such as the solar charger and the PowerBoost 1000C.
https://www.adafruit.com/product/390
https://www.adafruit.com/product/2465
With these simple chargers, the battery will charge more slowly when connected in parallel with the load. But this is an acceptable compromise in many applications.
We do have more sophisticated chargers that incorporate load-sharing circuits, such as the solar charger and the PowerBoost 1000C.
https://www.adafruit.com/product/390
https://www.adafruit.com/product/2465
- adafruit2
- Posts: 22200
- Joined: Fri Mar 11, 2005 7:36 pm
Re: Suggestion for MCP73833 lipo charger
yeah at some point you just end up 'upgrading' to a smart load chip, which this is definitely not :) we go with it for size & price!
- acolin
- Posts: 7
- Joined: Thu Jun 06, 2013 11:15 pm
Re: Suggestion for MCP73833 lipo charger
Thanks. The other products that do include load sharing within the charger IC do not have sufficient current capacity for my application (I need at least 4A to power an Odroid XU4 with some USB peripherals). MCP73871 is too low current for this.
Even BQ24250 (with load sharing) is not sufficient: it supports 4A discharge (good) but only 2A incoming, so can't support my load, much less the load along with charging...); but maybe with BQ24250 I could get away without adding an external P-FET but only adding two diodes: Vin -> Vload, and Vbq_out -> Vload (I think I need two diodes, because having Vload = Vbq_out does not seem right, because then I think the buck won't be able to buck, so the battery will be effectively connected to Vin via a diode -- not good, but perhaps I'm wrong on that). Maybe I'll try it out with BQ24250: https://www.tindie.com/products/amberga ... g-charger/
I ended up trying my suggestion to modify the MCP73833 charger from Adafruit. I hacked on a P-FET (DMP4010SK3Q), a 10A diode, but this turned out to not be enough. Driving the P-FET with Vin is not going to work: the P-FET won't turn on soon enough when unplugging power, because Vin will have to drop way lower than Vload_min before P-FET turns on. Not sure why the app note even suggested this. So, I had to add a voltage supervisor (open collector, active high: NCP301HSN) to turn on the P-FET as soon as Vin begins to droop. And, then, also, I needed an auxiliary P-FET to drive the main P-FET's gate (which apparently was too much for the supervisor's output). It works now: Odroid powered at near 100% utilization of 4 big cores; while battery is charging at full 1A.
So, after all it takes more than just one P-FET and one diode, but P-FET+diode+supervisor+p-fet+2resistors, but you might still want to consider adding this *external* load sharing to that product, because it would make it uniquely more versatile than anything else you have for sale: ability to power loads up to ~5A while charging battery with full charge current of 1A. I would certainly buy this from you if it existed.
Even BQ24250 (with load sharing) is not sufficient: it supports 4A discharge (good) but only 2A incoming, so can't support my load, much less the load along with charging...); but maybe with BQ24250 I could get away without adding an external P-FET but only adding two diodes: Vin -> Vload, and Vbq_out -> Vload (I think I need two diodes, because having Vload = Vbq_out does not seem right, because then I think the buck won't be able to buck, so the battery will be effectively connected to Vin via a diode -- not good, but perhaps I'm wrong on that). Maybe I'll try it out with BQ24250: https://www.tindie.com/products/amberga ... g-charger/
I ended up trying my suggestion to modify the MCP73833 charger from Adafruit. I hacked on a P-FET (DMP4010SK3Q), a 10A diode, but this turned out to not be enough. Driving the P-FET with Vin is not going to work: the P-FET won't turn on soon enough when unplugging power, because Vin will have to drop way lower than Vload_min before P-FET turns on. Not sure why the app note even suggested this. So, I had to add a voltage supervisor (open collector, active high: NCP301HSN) to turn on the P-FET as soon as Vin begins to droop. And, then, also, I needed an auxiliary P-FET to drive the main P-FET's gate (which apparently was too much for the supervisor's output). It works now: Odroid powered at near 100% utilization of 4 big cores; while battery is charging at full 1A.
So, after all it takes more than just one P-FET and one diode, but P-FET+diode+supervisor+p-fet+2resistors, but you might still want to consider adding this *external* load sharing to that product, because it would make it uniquely more versatile than anything else you have for sale: ability to power loads up to ~5A while charging battery with full charge current of 1A. I would certainly buy this from you if it existed.
- acolin
- Posts: 7
- Joined: Thu Jun 06, 2013 11:15 pm
Re: Suggestion for MCP73833 lipo charger
Btw, this document from TI explains the problems with connecting the load directly to a battery, and discusses potential solutions, including roughly what I ended up doing: http://www.tij.co.jp/jp/lit/an/slva166a/slva166a.pdf
This is the circuit that ends up working for me:
I'll also try BQ25895, which has a P-FET with Ron=11mOhm, and a 5A charging current. But, even if the basic discharge functionality works for my load, the question will remain whether the current capacity of the power-path functionality will be sufficient. TBD.
This is the circuit that ends up working for me:
That did not work, even the basic functionality (not power path, but power the load from the battery). The max discharge current for BQ25240 is 4A, but the P-FET Ron is ~20-40mOhm, which is too high for a 4A load, because Vbat-Vds<Vbooster_in_min. I've observed this issue in my circuit with a discrete external P-FET, when I used a P-FET with a high Ron (I had to replace the P-FET with one with lower Ron to get the above circuit to work). Nevertheless, in a modification to the above circuit, I've replaced the Adafruit charger with this BQ24250 charger to get ~2A charging current (so, ignoring its power path feature, and connecting the load to the battery through a P-FET, as shown).Maybe I'll try it out with BQ24250: https://www.tindie.com/products/amberga ... g-charger/
I'll also try BQ25895, which has a P-FET with Ron=11mOhm, and a 5A charging current. But, even if the basic discharge functionality works for my load, the question will remain whether the current capacity of the power-path functionality will be sufficient. TBD.
- acolin
- Posts: 7
- Joined: Thu Jun 06, 2013 11:15 pm
Re: Suggestion for MCP73833 lipo charger
Having explored the design space more, if I were to do this over, I think I would go with this:
based on LTC4412 and LT3651
More details and pointers from TI folks here: https://e2e.ti.com/support/power_manage ... 40#2419540
Another related thread here: viewtopic.php?f=19&t=89585&p=451614#p451614
I'm posting this to gather as much relevant info in one thread. I think the task of charging and powering the load is very common one, and this may be of interest to others.
based on LTC4412 and LT3651
More details and pointers from TI folks here: https://e2e.ti.com/support/power_manage ... 40#2419540
Another related thread here: viewtopic.php?f=19&t=89585&p=451614#p451614
I'm posting this to gather as much relevant info in one thread. I think the task of charging and powering the load is very common one, and this may be of interest to others.
Please be positive and constructive with your questions and comments.