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BQ24295: Input and Charge current limits

Gluonn R&I
Prodigy 235 points
Part Number: BQ24295

Hi,

II designed LiPo charging circuit using BQ24295. Circuit also can provide power to other circuits through USB.

I tried powering the circuit with a deeply discharged battery and load drawing 1A. I've 3 questions:

Question 1:

BQ is limiting input current to 1A always. If the load is drawing 1A, as expected, BQ is shutting down battery charging. So, it is evident that INLIM is set to 1A. I read back the register values, and this is what I got:

Input Source Control Register : 0b01011111
Power On Configuration Register : 0b00111011
Charge Current Control Register : 0b00100000
Pre Charge Termination Current Control Register : 0b00010001
Charge Voltage Control Register : 0b10110010
Charge Termination Control Register: 0b10011100
Boost Voltage Control Register: 0b10010011
Misc. Operation Control Register: 0b01001011
System Status Register: 0b10101100
Fault Register : 0b10000000

The resistor on ILIM pin is 150 Ohms. Can you please help me understand why BQ is behaving like this and what needs to be done to increase input current limit?

Voltage on ILIM pin is 0.3 VDC

Question 2:

I set the Fast charge current limit to 2A by writing 0x80 into REG02. It stays that way for a while (About 2 minutes) and will revert back to default value of 1A. What could be the reason for this behavior?

Question 3:


Boosted output is falling to 4.7V with no power from USB and load is drawing 1A. Is this because of the LiPo battery(1200mAh) itself?

 

I am attaching schematics for your reference.

Thanks

Teja

Schematic(09052017).pdf

  • 0
    TI__Mastermind 25800 points

    Hey Teja,

    In response to (1), based on your register read-out, the charger seems to be in VINDPM. You said you are using a USB source. What is the current capability of this source? If it is 1A rated, then you are over-loading the input source by pulling that much current, and our charger will attempt to regulate the input source voltage down to 4.76V (based on your setting). Another possibility is line impedance. If your input cable is too long, you may be inadvertently tripping the VINDPM threshold of 4.76V just by I*R drop alone. So please check the VBUS voltage as well.

    In response to (2), are you resetting the watchdog timer? It is currently set to 40s timeout, which will reset the charger current setting after it expires.

    In response to (3), where are you applying the 1A load current? Is this from SYS or from PMID? Also, what is the battery voltage you are boosting from? 

    Regards,

    Joel H 

  • 0 in reply to Joel Hernandez II
    Prodigy 235 points

    Hey Joel,

    Thank you for the response.

    1. The AC-DC adaptor rated for 2A @ 5VDC. I am using USB Charger doctor to measure voltage(VBus) and current(Ibus) draw from the same. I saw that voltage is always between 5.05V to 5.1V and did not fall below 5V at any point. The BQ went to DPM because I connected my iPhone, which is drawing about 0.8A and remaining 0.2A is going to battery.

    Regarding the cables, I used 2 cables (short and long) long one is from Belkin and is rated to carry upto 2.4 Amps. The input current never went beyond 1.05A. It oscillated between 0.5-1.05Amps all the time. So, I am pretty sure BQ is regulating it. If you used REG08 to say that device is in DPM, I suspect that the device may very well be IINDPM and is not in VINDPM.

    Further, DS(Page 5) says that BQ regulates maximum input current(3A max) by maintaining 1V on ILIM pin. But when I measured the voltage on the pin, it is at 0.32V which explains why BQ is taking 1A instead of 3(3A Max * 0.33  = 1 Amp). What do you think is going on here?

    2. Your explanation explains the behavior. WD is resetting the register. I will disable WDT by software.

    3. I connected the PD on PMID pin.

    Teja

  • 0 in reply to Gluonn R&I
    TI__Mastermind 25800 points
    Hey Teja,

    1) Where are you measuring the VBUS voltage? Is the Charger Doctor connected directly to the USB port? If so, I recommend using a meter to measure directly at the pin of charger IC. Measuring across C1 would be acceptable.

    Just as a sanity check, can you change the Input Voltage Limit (REG 0x00[6:3]) to a lower voltage setting, for example 4.36V (0b0110 or 0x37 for entire register) and see if you get more current out of the port?

    3) If you disable the watchdog timer, do you still see the drop in PMID voltage? What is your battery voltage when you test? What do REG 0x08 and REG 0x09 read when you are in boost mode?


    Regards,
    Joel H
  • 0 in reply to Joel Hernandez II
    Prodigy 235 points

    Hi Joel

    1. Your hypothesis that the BQ is in VINDPM is correct. When I measured the voltage at C1, it's about 4.7VDC, very close to default VINDPM threshold of 4.76 V. So, the device is entering DPM mode. On disabling WDT and reducing the VINDPM threshold, I can draw currents > 1A.

    3. Even after disabling WDT, I can't get solid 5V out of the PMID pin. I can only get 4.7V from the pin. Here are contents of registers:

    Input Source Control Register : 0b00110110
    Power On Configuration Register : 0b00111011
    Charge Current Control Register : 0b00100000
    Pre Charge Termination Current Control Register : 0b00010001
    Charge Voltage Control Register : 0b10110010
    Charge Termination Control Register : 0b10001100
    Boost Voltage Control Register : 0b10010011
    Misc. Operation Control Register : 0b01001011
    System Status Register : 0b10100100
    Fault Register : 0b00000000

    Thanks

    Teja

  • 0 in reply to Gluonn R&I
    TI__Mastermind 25800 points

    Hey Teja,

    Ok perfect.

    And just to confirm about 4.7V out of PMID, this is happening for the same reason. If you want 5.0V out of PMID, you need to set VINDPM to 5V by setting VINDPM to 0b1110 (Reg 0x00), which sets the VINDPM threshold to 5V. However, note that your charge current will be reduced significantly to regulate the input voltage. 

    The boost mode (OTG) only works when the input source is removed, and only the battery is present to power the charger. When OTG operates, PMID will be regulated to 5V. As of right now, system status is reading Adapter Source (Register 0x08[7:6] = 0b10 instead of 0b11).

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    Prodigy 235 points
    Hi Joel,

    Thank you so much for your explanation. I misunderstood some parts and you cleared all of them for me. I really appreciate your inputs. Everything is working as expected now. I am closing this thread.

    Again, thank you very much for your insights.

    Teja