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BQ24073: Output voltage drops to 3.4V when no battery is connected and charging is disabled.

Pawel Sochon
Prodigy 10 points
Part Number: BQ24073
Other Parts Discussed in Thread: BQ24076

Hi, 

I've just noticed that when there is no battery connected to BQ24073 and CE pin is low (charging enabled) then output voltage is around expected 4.4V. But as soon as I pull the CE pin high (charging disabled), the output voltage drops to around 3.4V, which is enough to power my LDOs but a bit low to operate cellular module. I was expecting BQ24073 to continue to regulate output voltage to 4.4V. Am I missing something? Please can anybody point me to the relevant part of the datasheet that would describe this behaviour? 

It's not caused by current limit because I measure single milliamps when this happens. 

Thanks

  • 0
    TI__Mastermind 23220 points

    Hi Pawel,

    Welcome to E2E!

    You're correct, I do not expect this type of behavior to occur either. The /CE pin should only control the current going from the OUT pin to the BAT pin which would normally be for charging. 

    Can you provide the following:

    • Voltage waveforms showing OUT voltage change from 4.4 V to 3.4 V when /CE goes from low to high?
    • Schematic
    • Test setup 
      • Are you using a board you have made or an EVM
      • Is OUT connected to anything of simply floating
      • What kind of input are you using?
    • Does the same behavior occur if you do have a battery?

     Best Regards,

    Anthony Pham

  • 0 in reply to Anthony P Pham
    Prodigy 10 points

    Thank you Anthony for confirmation. After writing the whole below reply I realised that a wrong chip was used in production. It's BQ24076 which follows the battery voltage, not BQ24073. But I'm posting this anyway for anybody who would like to see how BQ24076 operates:

    Below is a sequence of /CE pin going high, then low, and again high (Yellow = 5V rail, Blue = OUT of BW24073, Pink = /CE pin):

    Zoomed in /CE going high:

    Zoomed in /CE going low:

    With battery connected I can see that the output voltage drops a bit when /CE goes high (Green = Vbat):

  • 0 in reply to Pawel Sochon
    TI__Mastermind 23220 points

    Hi Pawel,

    Yes, this is the correct behavior for the BQ24076 device. When the voltage at the BAT pin is below 3.2 V, the OUT voltage gets clamped to 3.4 V. This is mentioned in the datasheet. 

    Pawel Sochon said:
    With battery connected I can see that the output voltage drops a bit when /CE goes high (Green = Vbat):

    This is normal behavior as you will normally see an effective voltage from the battery when charging due to an internal resistance (creating a slightly elevated voltage). 

    I'm glad you were able to debug the part! I will mark this as resolved. 

    Best Regards,

    Anthony Pham