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BQ25180: Battery not charging, but no fault is indicated

Scott Brenneman
Genius 3275 points
Part Number: BQ25180

We're trying to use a BQ25180 to charge a LiFePO4 cell, but it's not working.  The BQ25180 doesn't indicate any faults, though.  Here's a register dump:

0x0 (STAT0) -- 0x01 (should be 0x41 if it were charging)

0x1 (STAT1) -- 0x00

0x2 (FLAG0) -- 0x00

0x3 (VBAT_CTRL) -- 0x0A (3.60 V is the target)

0x4 (ICHG_CTRL) -- 0x2A (charging enabled, 150 mA fast-charge current)

0x5 (CHARGECTRL0) -- 0x1C

0x6 (CHARGECTRL1) -- 0xE7

0x7 (IC_CTRL) -- 0xC7

0x8 (TMR_ILIM) -- 0x4D

0x9 (SHIP_RST) -- 0x11

0xA (SYS_REG) -- 0xE9

0xB (TS_CONTROL) -- 0xC8

0xC (MASK_ID) -- 0xF0

Measured battery is 3.25 V, and TS is at 0.39 V, which looks to me to be safely in the normal range.  Input power is 5 VDC through a Schottky diode (not shown).  We are not using SYS.  We are not planning to use the Wake timers nor Ship Mode.

What are we missing?

Thanks,
Scott

  • 0
    TI__Expert 8245 points

    Hi Scott,

    Can you try bypassing the Schottky diode and tell me the behavior you observe under that condition? Could you also provide a VSYS measurement even though you are not using VSYS? Right now I am thinking that either the Schottky diode is limiting current for some reason or that the device is in DPPM.

    Thanks,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Genius 3275 points

    Hi Jackson,

    - Shorting the diode has no effect.  Also, if there were a current-limiting event, I'd see current being drawn from the bench supply, but I never see any current draw to speak of.  And, STAT0 is telling us that Input Current Limit, VDPPM and VINDPM are all not active.

    - SYS is tracking VIN, which puzzles me since we have selected "SYS disconnected and left floating" in SYS_REG, bits 3 and 2 (2b10).  For instance, even with the diode I'm seeing 4.8 V at VIN and VSYS, well above the UVLO level, and indeed STAT0 bit 0 is telling us the VIN power is good.

    - I also tried disabling TS (bit 7 of IC_CTRL, so 0x47 in that register instead of 0xC7), but that didn't help either.  I didn't expect it to, since the TS voltage looks correct.

    I also put a scope on VBAT with no battery to see how it compares to Figure 9-2, i.e. is VBAT pulsing, but VBAT is sitting at zero.

    Could the chip be in some sort of sleep mode, or that its registers are not reporting the status correctly?  I know that this is a new chip; could there be an errata list we haven't seen?

    Thanks,
    Scott

  • 0 in reply to Scott Brenneman
    Genius 3275 points

    Hi Jackson,

    An update: i just cycled VIN, which I know happens to reset the SYS_MODE bits to 00, and what do you know? It started charging.  I tried all four combinations of SYS_MODE, and the only one which causes charging is 00.

    Why does the setting for SYS affect charging?  it makes no sense to me.

    Thanks,

    Scott

  • 0 in reply to Scott Brenneman
    TI__Expert 8245 points

    Hey Scott,

    That is really interesting behavior. Not sure why the SYS_MODE setting is preventing charging here. I am going to test this on an EVM in the lab today to try and see if this behavior can be replicated on our end. I'll take a deep look at what is happening in each of the SYS_MODE settings and report back to you ASAP.

    Thanks for the detailed report of what you were able to observe. It really helps in the debug process.

    Best Regards,

    Jackson

  • +1 in reply to Jackson DiGiovanna
    TI__Expert 8245 points

    Hey Scott,

    I noticed the same behavior in the lab. I did a little digging and found the cause of this behavior.

    SYS_MODE = 00 is intended for normal operation of the device so charging is enabled.

    SYS_MODE = 01 is supposed to function as a USB suspend mode. During USB suspend it would make sense that the device would not be able to charge as the current to charge the battery comes from IN (USB connection).

    SYS_MODE = 10 forces SYS to be floating. This is achieved by opening the internal FET that connects IN to SYS. The current that travels through this IN FET is the same current that would normally travel through the BAT FET to charge. Because the IN FET is disconnected, the current never gets to the BAT FET and therefore charging can not occur.

    SYS_MODE = 11 pulls down SYS to GND. During this mode of operation it would make sense that charging can not happen as the battery will have a higher voltage than SYS (where the BAT FET connects internally). 

    For your case, I would recommend leaving the device in SYS_MODE = 00 unless there is some specific reason you need to utilize any of these other SYS modes so that the device will function normally and charging will be enabled.

    Best Regards,

    Jackson

  • 0 in reply to Jackson DiGiovanna
    Genius 3275 points

    Hi Jackson,

    Thank you for the explanation.  Obviously, an update to the datasheet with some of this would be appreciated by the next person.  My only concern with leaving SYS_MODE=00 is this: in battery mode will the quiescent current increase vs. floating SYS?  The datasheet doesn't appear to address this, unless I'm missing that.

    Thanks,
    Scott