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Hot!MCP73812 Battery Charger problem - Reverse-Blocking Protection

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ems23
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2019/07/21 12:11:12 (permalink)
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MCP73812 Battery Charger problem - Reverse-Blocking Protection

Hello I am using the MCP73812 to charge a small 130mAh battery.

I want to detect if the charger is plugged it so I use a simple circuit on the 5V rail that to detect if the voltage is present.
 
this doesnt work as there is always voltage present, even if the charger is not plugged in, despite having a 10k Ohm pull down on the 5V rail. If I pull the 5V down, i can measure a drain of 1.5mA, which is quite a lot and upping my pull down to draw so much current would have a very adverse effect on battery life of my device. [see Schmatic.jpg] 
 
 
 
I was under the assumption that this would not happen ever as the datasheet says:
Reverse-Blocking Protection
The MCP73811/2 provides protection from a faulted or shorted input. Without the protection, a faulted or shorted input would discharge the battery pack through the body diode of the internal pass transistor.
 
 




Is this something I am doing wrong or the datasheet doesn't specify a Reverse-Blocking Protection acceptable leakage current?? 
 
post edited by ems23 - 2019/07/21 12:16:27

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    PStechPaul
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    Re: MCP73812 Battery Charger problem - Reverse-Blocking Protection 2019/07/21 13:26:58 (permalink)
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    Does this happen only with your detection circuit connected? The schematic is incomplete and some components are not identified, such as "D1". Are you measuring just current into the charge controller? There may be some idle current drawn by the 3v3 regulator.


     
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    ems23
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    Re: MCP73812 Battery Charger problem - Reverse-Blocking Protection 2019/07/21 16:02:20 (permalink)
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    sorry I might not been clear. Please be assured that schematic is complete treat the Q1 as any standard N-fet.
     
    when usb charger is not present while VBAT is 3.7V one would not expect to measure any voltage in the indicated spot (safe side of F1). Substituting the 10k to 1k gate pulldown would fix the problem but will drain the 1.5mA making this sensing solution rather inefficient.
     
    this has nothing to do with any other parts of the system. Just MCP73812
     
    in reality there is voltage present causing the fet to be turned on and pulling ch_present low at all times even if charger is not present. 



     
     
    this should not happen if reverse blocking would work correctly! As this voltage leaks through from the VBAT side...
    post edited by ems23 - 2019/07/21 16:13:43
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    ric
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    Re: MCP73812 Battery Charger problem - Reverse-Blocking Protection 2019/07/21 16:45:51 (permalink)
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    Just a thought.
    In the condition you are testing, are USBD_P and USBD_N both low?
    If not, they would leak some current through U1 pin 5 (the USBLC6 VBUS pin) and raise the point you are measuring.
     

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    ems23
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    Re: MCP73812 Battery Charger problem - Reverse-Blocking Protection 2019/07/21 22:32:08 (permalink)
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    ric
    Just a thought.
    In the condition you are testing, are USBD_P and USBD_N both low?
    If not, they would leak some current through U1 pin 5 (the USBLC6 VBUS pin) and raise the point you are measuring.
     



    good suggestion, I thought about this also but my assumption was that the signals were active because the leakge happen the other way, 5V rail is being lifted to a higher potential than 0 (about ~2.7V) I have checked by removing U1 to be sure no change. 


    maybe I should get in touch with microchip support about this? What’s the best way?
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    ric
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    Re: MCP73812 Battery Charger problem - Reverse-Blocking Protection 2019/07/21 22:36:46 (permalink)
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    ems23
    maybe I should get in touch with microchip support about this? What’s the best way?

    http://microchip.com/support
    Lodge a "Support Ticket".
     

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