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BQ24095: Short Circuit Detection/Protection

Part Number: BQ24095
Other Parts Discussed in Thread: TPS61202

The BQ24095 has a number of short circuit protection features build in.  In the datasheet there is a mention that the IC does a 100mA short circuit detection.  What are the parameters for this test to return a short circuit result ending in the chip being shut down and needing to be power cycled in order to be unlatched?

The datasheet gives no indication of the conditions needed to warrant a short circuit.  

Any supporting material of documentation would be apprecitated if possilbe.

Many thanks,


Sean

  • Hey Sean,

    There is a section on Battery Short Protection in our Electrical Characteristics table. We designate a short circuit at the output as occurring when the output voltage hits 0.8 V.

    Best Regards,

    Anthony Pham

  • Hi Anthony

    Many thanks for your reply.

    There are a few things that don't make sense to me.  For example, if I isolate the system and just place a resistive load on the output I get the following results:

    1) 220R - All good.  This is expected as 220R at 4.35V is 19.7mA.

    2) 180R - Latches.  This is expected as 180R at 4.35V is 24mA so fails the 15mA test.  However the voltage only goes upto the 0.8V that you mentioned which is 4.4mA.  So not sure if I am understanding the SC test properly.

    In the datasheet on page 20 they have a flow diagram that indicates that the current limit is set to 100mA before the short circuit tests are conducted, so where does this 100mA come from and is this a SC test limit?

    In my application, the BQ24095 is supplying a battery as well as the system.  When the battery goes below its lower threshold the BMS removes the battery from the circuit, thus leaving only the system connected to the BQ24095.  The system front end consists of a 5V SMPS IC, the TPS61202.  When this powers up (currently as soon as the BQ24095 output turns on) the BQ24095 latches up.  I am assuming due to the short ciruit test, but not sure which one and what the limit was that caused it. 

    I am delaying the start up of the TPS61202 to a point where the BQ24095 is operarional and has the internal clamping of 1.1A to 1.3A.  By the way the ISET is set to 1A and I have also set the PRE-TERM to 10k (1A precharge).  But this is not working either. 

    I am sure I am powering up the TPS61202 once the SC test routine is completed, beacuse I can see the output line dip for the battery check routine before the TPS switches in.  As soon as this switches in the BQ IC powers down and latches. 

    The TPS has a soft start up that limits the current to 400mA and the BQ should be in the region where it clamps the output to 1.1A to 1.3A and besides that I have the ISET programmed for 1A.

    So not sure what it is that I am missing here?

    Any ideas are more than welcome.

    Regards,


    Sean 

  • Hey Sean,

    However the voltage only goes upto the 0.8V that you mentioned which is 4.4mA.  So not sure if I am understanding the SC test properly.

    I'll reserve some time to look at our test procedure for this device and see if there's something that will help clear up the SC test.

    where does this 100mA come from and is this a SC test limit?

    The 100mA is an internally set value that is used for the short tests that's part of the power-up routine. It's the upper limit that the device will allow for a short circuit since a short circuit would allow for all the current to be pulled to ground. It's a safety measure to protect the input as well as the charger but to functionally check for any short circuits and check pins. 

    By the way the ISET is set to 1A and I have also set the PRE-TERM to 10k (1A precharge).  But this is not working either. 

    The pre-charge can only be set to up to 40% of the programmed charge current. Is there a particular reason for setting the pre-charge equal to the fast charge setting? By chance, do you have a schematic you can share? If it's just the charger and the TPS61202, that'd be good. 

    I am sure I am powering up the TPS61202 once the SC test routine is completed, beacuse I can see the output line dip for the battery check routine before the TPS switches in.  As soon as this switches in the BQ IC powers down and latches

    Correct me if I'm wrong, you're saying the BQ24095 does complete its power up routine but then once the TPS gets enabled, the BQ24095 latches and powers down.

    Are you able to provide a waveform at the OUT of the BQ24095 where the TPS61202 turns on? 

    Best Regards,

    Anthony Pham

  • Hi Anthony

    Thanks for the above explainations and offering to look into getting some info on the SC tests.

    To answer your enquires above please see below:

    1) In the datasheet, they say on page 5 that the PRE-TERM can be set from 5% - 50% of the ISET value.  Then there is mention that the PRE-CHARGE is twice the value of the PRE_TERM value.  My reason for doing this was only to see if I could get the max amount of current to prevent the BQ IC from going into shutdown mode.

    Here is a snippet of the BQ24095 IC and the TPS61202.

    Please note that I have removed the 47uF cap, as the BQ24095 spec makes mention that the output capacitance should not exceed 10uF.

    2) Here are a few traces of what is happening on the BQ24095:

    In the trace below, I have implemented the following mod:

    * The EN track going to the TPS61202 has been cut, and I have place a RC filter in series.  This now delays the powering up of the TPS61202 until the BQ24095 has done its battery check.  FYI there is no battery connected to the circuit.

    The traces are as follows:

    Blue - VBUS

    RED - BQ24095 Output

    Green - VAUX pin 

    Yellow - 5V output of the TPS61202 

    When the VBUS is plugged all is ok, until the TPS enables the output.  As can be seen the Yellow trace starts heading towards 5V but the BQ24095 switches its output of and latches off.

       

    The way I got around this issue was to place a diode from the USB VBUS line to the 5V output of the TPS61202 IC, with the RC filter still in place.  The traces for that are below:

    Hopefully I have given you enough information here.  Please let me know if you require anything more.

    Cheers,

    Sean

  • Hey Sean,

    Thanks for the info! I'll look and see if I can find some time in the lab for this. How much current is the TPS61202 expected to pull?

    Best Regards,

    Anthony Pham?

  • Hi Anthony

    I am not expecting the system to pull that much current at all from the 5V rail (TPS61202 output), as none of the peripherals are enabled until the microcontroller is enabled and the micro can't be enabled unitl the 5V rail is up and running.    

    I did some current measurements where I placed a 0.55 ohm resistor inline with the BQ24095 output rail and measures across it with a scope.  

    The scope traces are as follows:

    Blue - VUSB Output

    RED - 5V

    GRN - BQ24095 Output

    YELL - BQ24095 Output after 0.55R resistor

    BLK - Math of GRN - YELL trace

    In this setup I have the RC filter on the EN pin of the TPS61202 to delay the startup of the voltage regulator.  I don't have the diode from the VBUS to the 5V output, as this obscures what current the BQ24095 has to supply.

    The Blue trace is the insertion of the USB cable. 

    Then the standard 40ms delay (GRN & YELL trace) the BQ24095 output does it thing.  The diip on these traces is the battery check I believe.

    Just after a 150ms then the TPS61202 is enabled.  Here we can see the 5V (RED trace) is climbing up.  It also looks like the BQ24095 is limiting the current to 1A (0.52V across the 0.55R resistor) as is expected.

    What I don't expect is that when the 5V rail reaches 5V is appears to be stable for about 20ms, the BQ24095 current drops and then the BQ24095 output and the 5V output all drop to 0V.  The 5V rail obviously taking longer due to the capacitance on the rail.

    As mentioned in prevoius posts, I get around this by placing a diode from the VBUS line to the 5V rail.  Oh also worth mentioning there is no PRE-TERM resistor so the BQ24095 should default to its 10 - 20% PRE-TERM value.

    Regards,

    SQ 

  • Hey Sean,

    Apologies for the delay in response, I'm working on acquiring a similar enough set up to replicate what you're seeing. Just curious, does the behavior persist if you reduce the current set on the BQ24095?

    Best Regards,

    Anthony Pham