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AnsweredHot!MCP73833 MSOP-10: Can't get 1A charging current

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f.ch
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2019/06/13 14:43:41 (permalink)
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MCP73833 MSOP-10: Can't get 1A charging current

Hi,
 
I'm trying to charge a 1200 mAh Li-Polymer battery at 1A, using a MCP73833. I set the PROG resistor to 1k, and the THERM resistor to 10k (to disable thermal regulation). I have attached my circuit.
 
The problem is that the current is initially at 1000 mA, but quickly drops to 550 mA and stays there. Any idea what might be the issue?
 
Here is a picture of what happens at the sense resistor (200 mOhm) between -VBAT and GND: https://imgur.com/a/7a9Okf9
 
Thank you,

Fred
post edited by f.ch - 2019/06/13 15:01:21

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#1
ric
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/13 15:41:42 (permalink)
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What is the voltage across the cell when this happens?
My guess is that the charger has quickly moved from "constant current" mode to "constant voltage" mode.
In that case, it's working as intended.
 

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Bob White
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/13 17:30:58 (permalink)
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Do you have a switch mode power converter somewhere in the vicinity of your charger?
 
The charger is linear so the what you are seeing in the waveform appears to me to be switching noise coming from a switch mode converter nearby.

The photo shows waveforms at 5 µs/div with a period of about 28-29 µs (Fswitch at 33-34 kHz). The waveforms appear to show a noise spike at the beginning of the on-time, a big noise spike as the converter transitions from the on-time to the off-time, and then ringing during an idle time. Such waveforms could be coming from a switch mode converter (such as a flyback) operating in discontinuous conduction mode (DCM).
 
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f.ch
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/14 06:24:39 (permalink)
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Thanks to both of you for answering.
 
@Ric, the voltage varies from 3.2V to 4.2V, so not in constant voltage mode.
 
@Bob, I have an LDO that generates the 5V0 from a 120AC/6VDC power supply. I changed the power supply and the signal at the charger is a lot cleaner. However, I still get 600mA instead of 1A.
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f.ch
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/18 10:46:12 (permalink)
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Hi Bob,
 
I replaced the MCP73833 to see if it had been damaged by the noisy external power supply, but I still get the same behavior. The charging current is initially at 1A, then drops to 600 mA in 2-3 seconds and stays there.
 
Do you have other debugging steps I could do in mind?
 
Fred
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Bob White
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/18 12:01:21 (permalink) ☼ Best Answerby f.ch 2019/06/26 14:27:49
5 (1)
Take a look at Figure 2-8 of the data sheet.  It appears as if you are going into current limiting based on the die temperature.
 
The thermal impedance of the MSOP-10 is given as 113 °C/W with a four layer board.  This is important.  How do you have your part mounted?  What kind of PCB?  Copper weight?  Copper area?  Thermal vias to the other side of the board.
 
With 5 V in, 4 V out, and a current of 550 mA the device is dissipating 550 mW.  If the device is limiting at 500-600 mA then it appears that the internal die temperature is 100-115 °C.  If you ambient temperature is 25 °C then the temperature rise is about 90 °C which implies a thermal impedance of 163 °C/W.  If you do not have good heat sinking I could see the part getting this hot.
 
Can you measure the case temperature when the device is current limiting?  What is that temperature?
 
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f.ch
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/18 13:18:37 (permalink)
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That makes a lot of sense, Bob. The chip is quite hot on touch. I need to order some resistors to drop the LDO voltage to 4.5V, test it, and I will get back to you.
 
Thank you,
Fred
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Bob White
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/18 14:12:47 (permalink) ☄ Helpfulby Jim Nickerson 2019/06/19 07:38:44
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If you have some cold/freeze spray you try cooling the device while it is running and check to see if the current increases back to 1 A.  If so, then it is pretty certain that thermal limiting is the problem.
 
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f.ch
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/26 14:27:40 (permalink)
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Hi Bob,
 
I changed the LDO voltage to 4.5V and now the current starts at 1A, then stabilizes at 850mA. Still not perfect, but definitely an improvement.
 
It does look like I need better heat dissipation as you suggested, so this thread can be marked as answered.
 
Thank you,
Fred
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Bob White
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/06/26 15:01:01 (permalink)
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You're welcome.
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garageeks
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Re: MCP73833 MSOP-10: Can't get 1A charging current 2019/08/27 02:23:06 (permalink)
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Hi to all, I was dealing with MCP73833 in MSOP-10 package and experienced the same issue.
I would expect Microchip to place a mandatory heatsink warning in the datasheet because it is clear it won't dissipate enough heat through its pins, since it lacks a thermal pad.
 
Do you agree that the DFN package is the better one, and no heatsink is required to attain 1 A charge current?
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