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BQ24725A: BQ24725A sense resistor and register setting
Part Number: BQ24725A
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
This is a bit confusing.
Thanks so much!
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In reply to Steve Preissig:
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.
In reply to Fan WANG:
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:
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.
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