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BQ24171: Output inestable

Daniel Blanch
Prodigy 75 points
Part Number: BQ24171

Hi, 

I'm working in a BQ24171 2 battery cell charger, based on the schematics on the evaluation board. Here is the schematic:

An it is change as follows:

But I have found a different behavior in the output. 

This application is for an input of 12 volts and charges 2 serial battery cells, but the issue appears when a battery is not present

I have compared the evaluation board voltages with the "new device" voltages

The voltages in The evaluation and the device are the following:

OVPSET: (evaluation=1.09V, device = 1.12V)

AVCC: (evaluation=11.5V, device = 11.6V)

TS: (evaluation=2.13V, device = 2.17V)

TTC: (evaluation=0V, device = 0V)

VREF: (evaluation=3V3, device = 3V3)

VREF: (evaluation=3V3, device = 3V3)

ISET: (evaluation=0.45V, device = 0.321V (modified to have less current that corresponds to 0.84Amps))

ACSET: (evaluation=0.825V, device = 0.321V (modified to have less current that corresponds to 0.84Amps))

ACDRV: (evaluation=17V, device = Oscilation)

CMDRV: (evaluation=11V7, device = Oscilation)

/BATDRV: (evaluation=11V9, device = Oscilation) 

The oscillation (for example in the Vbat signal is an 11.9 down to 9.41 volts) is a 25Hz saw signal.

Can you help me? (it could be related with the capacity in the Gate of the n-MOSFET Transistor?

Thanks in advance, 

Daniel

 

 

  • 0
    TI__Guru**** 166676 points
    Daniel,

    The oscillation at BAT is likely battery detection has explained in datasheet section 9.3.15. If you pull ISET to ground, does the oscillation go away?
  • 0 in reply to Jeff F
    Prodigy 75 points

    Hi Jeff,

    I have tried and it doesn't work, but I have found a significant point: 

    When I'm incrementing the Vin it works as the evaluation board, but, when I rise higher than 10.V in my board stop switching, but in the evaluation continuing working normally.

    I have checked the OVPSET and in both are near 1 volt if 10V in the Vin.

    And the other point is that the Battery detect doesn't work. (the output capacitance is 20uF).

    Thank you in advance,

    Daniel

  • 0 in reply to Daniel Blanch
    TI__Guru**** 166676 points
    Daniel,
    I have reviewed your schematic in detail using the design spreadsheet at www.ti.com/.../sluc244 and found some issues.

    1. The pull up and pull down resistors on BATDRV are incorrect. R103 should not be populated and R110 needs to be 1Megohm.
    2. The resistors for ISET and ACSET are sized oddly. ISET sets the charge current for 4A but input current limit is set for only 2A, since R109 the AC sense resistor is 20mohm. Was that intentional? The IC should run normally in input current limit (IDPM), clamping input current to 2A but you will never get 4A input current.
    3. Although not a big deal, R113 isn't needed.
  • 0 in reply to Jeff F
    Prodigy 75 points


    Hi Jeff,

    Really thank you! I have changed the BATDRV resistors and the system has started to work. The (non-battery) sense, the charge of the battery. Everything is fine.

    I will review the ISET and ACSET resistors to have a 3 Amp that is my first intention. But for doing some testing I was trying with less current.

    Thanks for your support, 

    Daniel