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BQ24650: intermittent charging

Isaac Parsons
Intellectual 450 points
Part Number: BQ24650

Hi there, 

I'm working on a charger for one of our products and I'm having issues with the BQ24650. The input to this device is a 9V wall wart going into VIN, instead of a solar panel output. 

the mosfets I am using are NVTFS5C471NLWFTAG and the inductor is ETQ-P8M4R7JFA

The battery is a CAM72F (https://www.evwest.com/catalog/product_info.php?products_id=301)

For MPPSET I did not populate R7 and used a 10k resistor for R3, so MPPSET should be pulled up to 9V and thus will always be on, correct?

When I attempt to charge the battery, the device works intermittently. I.e. it charges for 5 seconds and then turns off for 5 seconds. When I put an oscilliscope at the input I notice that its very noisy and rises slowly until it turns off, so I assume that its reaching the overvoltage cut off value. I'm just not sure why this would be happening, so any help would be greatly appreciated

Also, I replaced the 10k NTC at R4 for a fixed resistor that sets the voltage within the range specified in the datasheet, so I dont believe its anything temperature related. 

Thanks

  • 0
    TI__Mastermind 24075 points

    Hi Isaac,

       MPPSET is the setpoint at which the charger will enter its input voltage regulation loop. When using solar panels you are meant to set it at 80% of OCV so that the input voltage gets regulated at the point, which usually corresponds to the maximum power point. Not meant to be set at your input voltage. 

  • 0 in reply to Kedar Manishankar
    Intellectual 450 points

    But since Im not using solar panels, does it matter? Do I still need to set it at 80% of the maximum input voltage?

  • 0 in reply to Isaac Parsons
    TI__Mastermind 24075 points

    When you enter the input voltage regulation loop, the charge current will be decreased to maintain the input voltage at that point. When you have input current it will cause a small voltage drop (cables from supply to board/EVM, trace impedance) before being sensed by the IC at VCC, which means that odds are you aren't sourcing exactly 9V on VCC of the IC. So if you have your setpoint at your input voltage, the charger will keep trying to regulate the input current to reduce the voltage drop so that the MPPSET voltage is maintained.

    The solar panel is an example of how MPPSET/input voltage regulation is meant to be used, and not that this feature is only meant to be used with solar panels. A sanity check would be to check STAT1 STAT2 status when you see that there is no charging. If STAT1 is lit, the just change the MPPSET threshold lower, as this is indicating charge in progress and that there is no fault, meaning you are most likely entering and exiting the MPPSET loop cyclically. If neither STAT1 or STAT2 are lit, then there is a fault condition and it will change my assumption.

  • 0 in reply to Kedar Manishankar
    Intellectual 450 points

    Just tried it and neither are lit

  • 0 in reply to Isaac Parsons
    TI__Mastermind 24075 points

    Please capture VCC, SRN, STAT1 and PH on a waveform and capture the intermittent charging. As we are entering a fault condition this will give us more information.