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BQ25172: Issue when setting VSET to 1-Cell on the BQ25172 Linear NiMH Battery Charger

Robb Foote
Prodigy 80 points
Part Number: BQ25172

Tool/software:

We are testing the BQ25172 linear NiMH Battery Charger on our new product in development. From our findings, when we set up a 1-Cell NiMH battery as the device to be charged and the BQ25172 (VSET) is set to '1-Cell', the BQ25172 doesn't charge, but a fault condition is created. Below are the conditions that were set up on the BQ25172. All resistor tolerances used in this testing are 0.1 %. 

Conditions:

Vin: 5V (USB-A)

VSET: 62 kΩ = 1-Cell

ISET: 2 kΩ = 150 mA

TMR: 11 kΩ = 14 hours

NiMH Target Battery (V): 1.20 Volts

VOUT: 2.33 Volts

However, when we set up the (VSET) node to a 2-cell (36 kΩ), the NiMH battery chargers at 150 mA, which didn't charge at all before using the 1-cell selection. 

Vin: 5V (USB-A)

VSET: 32 kΩ = 2-Cell

ISET: 2 kΩ = 150 mA

TMR: 11 kΩ = 14 hours

NiMH Target Battery (V): 1.23 Volts

VOUT: 2.33 Volts

One fact to note is that when the VSET was set up for a 2-cell battery, the Vout voltage didn't change between 1-cell and 2-cell; the value output was still 2.33 Volts. However, when the VSET is set to a 4-cell battery, the Vout voltage changes to 4.6 Volts. So when VSET is switched back to a 1-cell battery, Vout is still 2.33 Volts. Unsure if that is the minimal output voltage for the BQ25172 or if there is a layout issue with our BQ25172 on the PCB.

Also, in the figure below, why don't the recommended operating conditions specify the minimum voltage for VOUT and the minimum current for IOUT?

We intend to use a 1-cell NiMH battery and not set up this BQ25172 IC for a 2-cell, so the charger will stop charging when the battery is fully charged. Using a 2-cell configuration on a 1-cell battery will impede the charger's logic from working correctly in our situation. 

Any help would be greatly appreciated. 

  • 0
    TI__Guru 69186 points

    Hello

    I will have an update on your question later today.

  • 0 in reply to Bill Johns
    TI__Guru 69186 points

    Hello

    The BQ25172 has a recharge voltage threshold of 1.33V pre cell and overvoltage threshold of 1.7V per cell.  Changing Vset change the multiple of the voltage, 1Cel is 1.33V recharge and 1.7V OVP, 2Cell is 2.66V and 3.4V.

    Recharge voltage - if the battery voltage is above this point charge will not start.  When voltage drop below this point charge will start.

    OVP - overvoltage protection, battery voltage above this point is a fault condition that will stop charging and STAT will blink at 1 Hz rate.

    The BQ25172 will regulate the charge current not battery voltage, the battery will determine what voltage.  

    If you are seeing 2.33V on your battery, this would be an overvoltage condition and set a fault in 1Cell setting.

    What battery are you using?

    Also can you seen a schematic of your circuit?

  • 0 in reply to Bill Johns
    Prodigy 80 points

    After testing the BQ25172, if you measure its output, it will output a separate voltage supply per VSET setting. So, on the evaluation board (B125172EVM), when a 1-cell battery is selected with VSET, it will output around 1.45 Volts. When a 2-cell battery is selected with VSET, it will output around 2.33-2.6 Volts. Then, when a 4-cell battery is selected with VSET, the charging IC will output 4.6 Volts.

    Our battery is a AAA NiMH rechargeable cell with 1.2V 800mAh.

    I've attached the schematic below.
    EPWR comes from a USB-C Connector without the adjustment from a Power Delivery IC, so in this scenario, it is 5 Volts. 

  • 0 in reply to Robb Foote
    TI__Guru 69186 points

    The schematic looks good do not see any problems with it.

    Test results

    1-cell = 1.45V,

    2-cell = 2.33-2.6V

    4-cell = 4.6V 

    I set this up in the lab with and cannot duplicate this behavior.  

    Couple of things to check:

    1.) If you have a current meter in output remove it, current can be monitored at ISET, or input current.

    2.) Remove any meters connected to VSET and TMR pins.  Ohm meters have caused problems.

    3.) Send photo of test set up