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BQ51050B: Noise on CHG Pin

Michael Parry
Prodigy 20 points
Part Number: BQ51050B

Tool/software:

Hello, 

I’m hoping that someone could shine some light on the issue I’m having. I have a product that has the BQ51050B Chip in I have pulled up the CHG pin through a 100K resistor to BAT and interfaces this through a MOSFET to an MCU so it knows when we’re charging. After the design i realised that I can’t detect that it’s in a charger and full but ignore that for now.

Once the device is fully charged after some time (randomly) my MCU boots up from the interrupt of the CHG pin. I’ve probed the CHG pin and it appears to have some interference where it drops low for about 300us. I'm struggling to believe it is interference and its actually some condition on the CHG pin I’m not aware of. The BAT Net has 1.1uF of Capacitance.  I've attached a picture of my schematic and layout for reference. Theres a solid ground plane directly below and the CHG net length is 27.376mm

I could add a Cap to the gate of the Mosfet but I’m looking to understand what’s going on

Any insight would be appreciated?

  

  • 0
    TI__Guru 70006 points

    Hello

    This may not be noise.

    The 80mS time sounds like the Digital Ping Time from the Transmitter.  The digital ping will power the receiver, it will check if battery needs to recharge.  If it does not the receiver sends charge complete, and transmitter goes back to sleep.

  • 0 in reply to Bill Johns
    Prodigy 20 points

    Hi Bill, 

    Thank you for your reply. I did a little more digging today and the duration is shorter than I thought. It's around 300us but I think it looks too clean to be noise. I also think its from the ping becuase it occures every 2000ms

    the IC isnt getting warm and to be sure i used the 10K pulldown to disable it. But its still happening

    As I understand the TX will communicate to the RX to see if it needs charging, if it dont then nothing should appear on the CHG line so im not sure why im seeing this?!

  • 0 in reply to Michael Parry
    TI__Guru 70006 points

    Hello

    Thanks for the scope capture.  Suspect this is the digital ping from TX checking if the RX needs more power. 

    The dip could be start up behavior of the device, I will double check in the lab with your setting.

    Could you compare RECT pin with CHG ping?

  • 0 in reply to Bill Johns
    Prodigy 20 points

    Hi.

    Thanks for your help. If you could double check that would be amazing,

    Comparing against the Rect Pin is a great shout, unfortunatly I have only just seen this message and already left the office so I will have to do that tomorrow

  • 0 in reply to Bill Johns
    Prodigy 20 points

    Morning, The blue trace below is the RECT Pin, 

  • 0 in reply to Michael Parry
    TI__Guru 70006 points

    Hello 

    I cannot duplicate this in the lab, will work on it more tomorrow.

    It looks like start up behavior.

    How may units do you see this on?

  • 0 in reply to Bill Johns
    Prodigy 20 points

    Hi,

    I also think its start up behaviour but I cant put my finger on why or where in the proccess.

    I have 30+ units all doing it. Im using a charger from another design, Could it be to do with the transmitter profile? 

    The reason I ask is becuase i was playing around with it yesterday and was able to change at which point it happened.

    My input power to the tx is 12V and the Rx is a lipo battery so 3.7V nom. Therefore I use half bridge mode.

    Heres a few things I've discovered yesterday:

    • - The CHG pin only does that when the battery is fully charged.
    • - When I set the Ping to half bridge, I get that behaviour on the CHG pin when resting on charge,
    • - when I set the ping in full brige, the behaviour only happens when lifing the product in and out of the charger (not resting)

    Tx Config Settings:

    • Bridge Mode: half bridge 
    • Tx Max duty cycle: 50%
    • Tx Min duty cycle: 30%
    • Tx Freq Max: 149kHz
    • Tx Freq Min: 120kHz
    • Ping Frequenc: 174.99kHz
    • Tx DC Ping Duty Cycle: 50%
    • Ping Half Bridge: No

    Thanks for your help

    Mike

  • 0 in reply to Michael Parry
    TI__Guru 70006 points

    Hi Mike

    The CHG pulse may be caused by the RECT rate of rise.  I am seeing it in the lab with some transmitters that have a slow rate of rise.  Units with fast rate of rise do not show it.  The pulse occurs when RECT voltage crossed ~0.7V.

    The internal housekeeping voltage is derived from RECT. 

    Couple of things to try:

    1.) Do you have a 5V input single coil transmitter to test with?

    2.) Reduce output capacitor on the RECT pin.

    3.) Do you have another RX coil to test with?

  • +1 in reply to Bill Johns
    Prodigy 20 points

    Hi,

    When I power my transmitter board from 5V I also get the same result (same coil and caps). I also don’t have another rx coil to test with.

    I have just removed the 1uf Capacitor so there’s only a 100nf cap on there, I know its extreme but i also get a somewhat similar result. I have attached the image.   I did notice on the design notes that the capacitor values on the Rect pin is between 4.7uf and 22uf. I only had 1.1uF so am I supposed to go the other way and add capacitance 

  • 0 in reply to Michael Parry
    Prodigy 20 points

    Ive added 10uf on, so it has a total of 20.1uF and that looks to have sorted it. Does this tie up with your logic?

  • 0 in reply to Michael Parry
    TI__Guru 70006 points

    Hi Mike

    Increasing capacitance on RECT to 20.1uF has resolved the problem?

    That would not have reduced the rate of rise but it would add bulk filtering.  

    How many units have you tested?

  • 0 in reply to Bill Johns
    Prodigy 20 points

    Hi,

    I've only tried it on 1 unit.

    I was able to replicate the blip consistenly, I then added the capacitor and it dissapeared.Then to confirm I removed the capacitor and it came back.

    Then double checked by adding the capacitor back in, and it appeared

  • 0 in reply to Michael Parry
    TI__Guru 70006 points

    Hi Mike

    This an acceptable solution of you?