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bq24050 Charge Current

Al Walters
Prodigy 190 points
Other Parts Discussed in Thread: BQ24050, BQ25040

Hi all,

I've been looking at an issue that came up where customers have been noticing very long charge times in a few instances.  I've noticed now a couple times after looking further into this problem that the charge current (using a wall adapter capable of 1000mA) seems to stay at 50mA during the entire charge cycle or at least until the timer shuts off the charging.  I thought maybe the bq24050 might have been in a pre-charge mode at first but 50mA is far lower than the 2*(10% of max limit) or about 160mA.  In our case, we have used a 680 ohm ISET resistor for about an 800mA charge current limit.  In most cases, I do see the charge current reaching up to about 780mA as expected.  But in the anomaly case, it stays stuck at 50mA.  I found that when I disconnect and reconnect the wall charger, the charge current hits the 780mA as we wanted.  Has anyone seen a simliar issue like this?  Its not easy to reproduce but I've seen it more than once now.

Our design uses a USB wall adapter which the bq25040 senses through the DP/DM lines.  The wall adapter we're using has the DP/DM lines shorted which will indicate wall adapter to the charger IC and not a host.  My PRE_CHG input is set to a 2K ohm resistor and my ISET resistor at 680 ohms.

Thanks.

Al

  • 0
    TI__Mastermind 31120 points

    Does not sound familiar.  Please capture waveforms of each pin of the IC when in this condition.

    Only thing that could cause a 50mA charge is entering input voltage DPM mode.  What is your input voltage during this low charge rate time?

  • 0 in reply to Charles Mauney
    Prodigy 190 points

    Hi Charles,

    I was able to get the bq24050 back into that 50mA charge mode.  Here are the pin measurements that I was able to find but I dont have waveforms yet:

    VIN = +5.08V

    VOUT = +3.82V

    TS = +536mV

    ISET = +171.4mV

    PRETERM = +151.3mV

    ISET2 = +928.8mV

    DP = +3.34V

    DN = +3.34V

    If the wall charge adapter does encounter a spike in current, say from an in-rush condition, but recovers, shouldnt the charger IC also restore normal charging operation rather than stay in a DPM mode?  I dont think our design is doing this but I wanted to get an understanding of how the charger would respond in this condition.  Our design has never pulled more than 800mA at any one instance and we dont have a huge amount of bulk capacitance on the board.  But I'm getting setup to observe the input voltage on a scope.

    Al

  • 0 in reply to Al Walters
    Prodigy 190 points

    Hi Charles,

    I was able to disconnect another load on the battery charger IC output and looks like we are in the default 100mA USB charge mode.  The battery charger IC did not seem to detect the short across the DP and DM lines of the USB wall adapter for some reason.  I've taken a quick measurement and got about 1.28 ohms across the DP and DM lines of the wall adapter which falls within the <200 ohm dedicated charge port spec for USB.  I'm going to check with our software team, but could it be possible for software to control the ISET2 pin such that it changes the limit to 100mA?

    Update:  I did speak to our software team and we default the ISET2 pin as floating.  After the minimum recommended 600ms, our software will attempt USB enumeration.  I believe what may be happening is that the source is not being detected as a dedicated charger and assuming a host.  Since the ISET2 pin is floating, this would default the current limit to 100mA.

    Al

  • 0 in reply to Al Walters
    TI__Mastermind 31120 points

    The ISET pin voltage suggests the output current is 0.171V*0.8A/1.5V = 91.2mA.  You are most likely in USB 100mA mode.

    My guess is that you have something hanging on the ISET2 pin that toggled this pin after power up and since it is near the float voltage, the charging was changed to 100mA mode.

    If you fix the ISET2 pin it probably will work properly.

  • 0 in reply to Charles Mauney
    Prodigy 190 points

    Hi Charles,

    I found my problem.  The noise on that ISET2 line was much more than anticipated, especially if the microcontroller let that pin float.  There was nothing to really bias the voltage and it could easily change states.  I was under the impression we would drive it low to always use our limiting resistor on the ISET pin but this is not always the case when connected to a host that may be limited to 100mA.  So we need the capability to float the pin and have it properly biased.  I biased the pin using a voltage divider and put a 0.1uF cap on the line which made a tremendous difference.  Thanks.

    Update:  Charles, I noticed that the part supposedly has an internal biasing circuit for the ISET2 pin for around 0.9V.  However, this bias doesnt turn on until after VIN has been applied.  When I plug in the USB wall adapter, I do see the bias come up from 0V to the 0.9V but I still dont get the right current limit (its still doing 100mA rather than upwards of around 770mA based on the 680 ohm I have for the ISET pin).  However, when I pre-bias the ISET2 pin using an external circuit to around 1.0V and then apply VBUS to the VIN of the charger IC, I get the higher current limit.  Is this expected?  I was more or less relying on the internal biasing but didnt realize it was not active until VIN was applied.

    Al

  • 0 in reply to Al Walters
    TI__Mastermind 31120 points

    Yes the part does not burn power from the battery to bias the ISET2 pin, it comes up after input power is applied.

    A low on the ISET2 programs the use of the ISET1 resistor, float (0.9V) is USB100mA mode and High is USB500mA mode.

    You must pull the ISET2 low to program the current using the ISET1 pin.