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BQ25302: IC getting switched while charging

Pallav Aggarwal
Prodigy 20 points
Part Number: BQ25302

Hi TI Team,

I have designed a charger circuit around BQ25302.

Schematic:

Observation:

The output voltage is set to 4.2V and a single-cell phone battery is connected

Input Voltage is kept at 5.4V DC

I am seeing charging getting stopped, input current becomes zero after a few minutes of charging.

Like it starts with 420mA current then it reaches 350 or so, then it stops, and after random seconds or minutes, it comes back again for a few seconds and then switches OFF.

When it becomes OFF, the output voltage is 4.079V

and when it starts again, it shows 4.193V @ roughly 353mA

What could be the reason for such a behavior?

Please review my PCB layout as well:

Thank you.

  • 0
    Prodigy 20 points

    when charging stops.

    I have checked RGEN output is 3.88V - 3.98V 

    EN pin: 1.2V-1.35

    STAT pin: 2.5-2.9V

    When charging is ON, RGEN  is 4.88V

    EN: 0.08V

    STAT: 0.07V

  • 0 in reply to Pallav Aggarwal
    TI__Mastermind 24075 points

    Hi,

       It looks like you are in CV phase when charge current is on, which causes charge current to gradually taper until it terminates. There is too much impedance somewhere in the path between where the charger IC senses the battery voltage and the actual battery. Tie BAT pin as close as possible to the battery through layout to avoid this.

  • 0 in reply to Kedar Manishankar
    Prodigy 20 points

    Thanks for your response.

    1. Please check if ~12mm track from the battery to battery sense pin could be an issue?

    2. Should I try connecting a short wire to connect directly and see if it improves?

    3. Can a battery connection with 1.5 inches wire (see below) be an issue?

    Thank you.

  • 0 in reply to Pallav Aggarwal
    TI__Mastermind 24075 points

    The issue is most likely the wires in between your battery and the board. The BAT sense trace is beneficial as it is a sense trace which could be tied as close to the battery as possible. Trace impedance would not be as much as those wires. Here we have to consider the wire impedance, trace impedance, internal impedance of battery and if there are CHG/DSCHG FET in between battery and board.

    What is happening is most likely a big iR drop which is enough to trigger the recharge threshold. Once charging stops, the IR drop is removed, and if this causes enough of a drop that the recharge threshold is hit, then you will observe the behavior you are seeing. 

  • 0 in reply to Kedar Manishankar
    Prodigy 20 points

    Thanks for your response.

    I will try with a much thicker wire and a much shorter wire tomorrow and let you know the result.

    Is path impedance an issue because the charger is a high-frequency DC_DC type?

    Could you help me with a critical review of my PCB layout, please?

    Thank you.

  • 0 in reply to Pallav Aggarwal
    Prodigy 20 points

    One more thing I noticed just now, That I have used 0.1uF Cap instead of 47nF cap for BTST. Could this be an issue too?

  • 0 in reply to Kedar Manishankar
    Prodigy 20 points

    I have tried shorter and thicker wires connected to the battery, the behavior is the same.

    If I don't put the battery, will LED blink, or will be OFF? It remains off for a couple of seconds and then starts blinking very fast much greater than 1Hz and then blink, comes, and goes.

    Could you please help me with some suggestions?

  • 0 in reply to Pallav Aggarwal
    TI__Mastermind 24075 points

    Hi Pallav,

       I would expect the STAT to flash quickly. This is because the charger is charging up the output caps (battery is absent). Caps are only 10uF so they charge and discharge quickly, so STAT will light up as it reaches full charge, terminates, and then falls down to recharge threshold. Once recharge threshold is hit, charge will be enabled again and the cycle repeats. Yes, you should use 47nF cap for BTST