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BQ2973: Battery charging does not start

Narendra Suthar
Prodigy 20 points

Part Number: BQ2973

Hello,

Would you please help us to fix the issue mentioned in below link,

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1619954/bq2973-when-the-battery-voltage-drops-below-a-certain-level-charging-does-not-start/6256135

Battery is not charging event when battery voltage is 3.5V. What we observe is COUT pin set low when we plug-in the charger. We are using 5V/1.6A USB charger. 

Regards,

Narendra

  • 0
    TI__Mastermind 22725 points

    Hello Narendra,

    The COUT pin will turn OFF when a CHG protection has been triggered. For this protector, it has OVP and OCC. 

    If you are using the BQ29737, this device has an OVP of 4.25V. If you measured the voltage at the BAT pin, and saw 3.5V, I would not think OVP is the likely cause for COUT to go low. 

    This then leads to potentially OCC. The BQ29737 has an OCC threshold of -0.05V. I recommend reading Section 8.4.5 Charge Overcurrent Status and Section 8.4.2 Overcharge Status from the datasheet to confirm if those recovery methods work to confirm that OCC is being triggered here when the charger is attached. Another way to check if if you could probe the V- pin when the charger is attached to see if the threshold is crossed for the programmed delay. 

    Best Regards,
    Alexis

  • 0 in reply to Alexis_H
    Prodigy 20 points

    Hello Alexis,

    Thanks for the input. 

    I think it is charge overcurrent which is causing the COUT pin to pull low. Our finding is as below,

    The BQ29737 senses charge current as the voltage developed across the two series protection MOSFETs (U3A + U3B). When V- falls below VSS by more than the OCC threshold(-0.05V) for longer than the OCC delay time(16 ms), COUT is pulled LOW and charging is disabled. 

    We have used SI3932DV-T1-GE3 FETs. The typical Rds(on) at Vgs ≈ 3.5–3.7 V is about 50–60 mΩ per FET. Two in series gives ~100–120 mΩ. At 1A charge current creates a voltage drop across the FETs that exceeds the OCC threshold. Vds (V− − VSS) = I_charge × Rds(on)_total ≈ 1 A × 100 mΩ = −100 mV. This cause the COUT pin to pull low and disable the charge. 

    To verify above, we have reduce the charge current from 1A to 300mA and charging is started. COUT pin set high. 

    So, it is the MOSFET that causing the issue. We need to replace MOSFETs with lower Rds. 

    Would you please confirm our finding is correct?

    Thanks,
    Narendra

  • 0 in reply to Narendra Suthar
    TI__Mastermind 22725 points

    Hello Narendra,

    Yes, that sounds likely to be the cause based on what you shared. In Section 9.2.2. Detailed Design Procedure of the datasheet, we share detailed steps on how to select the FET including calculating the FET's Rds ON. I recommend using that as a reference to help if any further guidance is needed there.

    Best Regards,
    Alexis