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BQ24725A: BQ24725A sense resistor and register setting

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Replies: 3

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Part Number: BQ24725A

Hi TI,

I am trying to use the BQ24725A in a lower cost battery charger design. Since our charging current is low(190mA), we used a relatively large sensing resister(0.15) at the output. Now it seems the output voltage(1.32V) is lower what we set(8.4V) through I2C, could you help to check?

The register setting is listed below:

0x12                0x0030 (option register)

0x14                0x0c00(charge current)

0x15                0x20c0(charge voltage)

0x3f                 0x1000(input current)

BTW, I have jumped the pin CMSRC to VCC input(9V25), but I don’t know why I need to do that, because in the block diagram, this pin provides the base power for the voltage pumper of ACDRV.

 

Please check the schematic below:

And I have tried to switch the sense resistor back to 10m. I can see some voltage output there. But it seems that the charge current I set through I2C is not accurate. Please check the voltage I measured across the load resistor(36ohm) at the output:

charge voltage register setting: 0x20d0

charge current register setting output voltage actual current
0xc0 4.98V 138mA
0x100 6.86V 190mA
0x140 8.31V 231mA
0x180 8.36V 232mA

This is a bit confusing.

Thanks so much!

Best Regards,

  • Hello Fan,

    Your register settings look okay. What is the battery voltage at when you are trying to charge it?

    Here is your info with expected current vs actual
    0xc0 = 192 mA, actual = 138 mA
    0x100 = 256 mA, actual = 190 mA
    0x140 = 320 mA, actual = 231 mA
    0x180 = 384 mA, actual = 232 mA

    So the scale is definitely off. The reason that you are not seeing it go above 231 mA is because at 8.36V you are hitting your charge termination of 8.384 V and entering constant-voltage mode. So any increase in the current past that amount will not result in a change to the system because it is already in CV mode.

    There seems to be a large impedance between the point you are measuring and the battery. If
    4.98 - VBATT = 0.138 * R
    6.86 - VBATT = 0.190 * R

    This solves to VBATT = 0V, R = 36 Ohms. If that is representative of your system, then the device will hit CV mode at 232 mA, which is what you are seeing.

    The first two results of 138 mA for a programmed 192 mA and 190 mA for a programmed 256 mA are not out of line from expected. At very low currents, 25-30% error is common. The regulation improves as the currents get higher, and at 4A the regulation is better than 1%.

    So the real question is where this 36 Ohm impedance is coming in. I would suspect Q205, but the backgate diode should conduct even if it is turned off. Can you measure voltage at all available nodes between the switcher and battery to try and identify where you see a large voltage drop?

    Assuming that my calculation of 36 Ohm from charger to battery is correct, the device is operating as expected. I know the numbers aren't what you would expect, but it is being operated at much lower current than the typical application that it is targeted for.

    Regards,
    STeve
  • In reply to Steve Preissig:

    Hi Steve,

    Thanks for your reply.

    I think your calculation met our test setup very well. I used a 36ohm resistor as the load(not a battery, the charger worked with a battery connected as the load, the resistor I used here was for the mass production test) and this was only to evaluate the circuit.

    My current problem is that when I started the charger, the output was not correct(1.32V). But I unplugged and  plugged the 36 ohm resistor, the output seemed to become normal. And also I tried to change the sense resistor back to 10m ohm, the charger was also able to output 8.4V with 36ohm connected.

    Look forward to you reply.

    Thanks!

    Fan

  • In reply to Fan WANG:

    Hello Fan,

    I am not sure that a resistor is sufficient to simulate battery behavior for your board testing.  The "getting started" procedure outlined in the EVM user guide is closely derived from the test procedure we give to our manufacturer to test the EVMs:

    http://www.ti.com/tool/bq24725aevm-710

    You can see that in this procedure we use an e-load in constant voltage mode to test the battery charging.  Your CM should have this equipment, and it will provide a more representative test than a resistive load.

    Also, some power supplies have the ability to sink current in CV mode (for instance, we use the Keithley 2420 sourcemeter, which has this capability.)  This or a similar device can be used instead.  But a resistor does not have the correct behavior to test the circuit in place of a battery. 

    Regards,

    Steve

     

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