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TPS61379-Q1: TPS61379 breaking

Ville Keskilohko
Prodigy 210 points
Part Number: TPS61379-Q1
Other Parts Discussed in Thread: TPS61378-Q1, ESD401

Tool/software:

I have a design that uses TPS61379 to boost Li-Ion battery voltage up to 15V. Output supplies BLDC motor driver. Motor driver current limit can be higher (660mA) than what boost regulator can output (based on FET current limit). We have had some cases where TPS61379 has broken down. It has been confimed by measuring that the motor driver is ok and that there's short circuit from TPS61379 SW and VO pins to the ground, but not from VIN to ground. The switcher is protected against many faults such as output overcurrent. Any ideas what can cause the switcher to break down? We have had cases where switcher has failed even with motor running freely without any mechanical load. Attached is a part of schematics for reference. 1.5uH inductor is TFM252012ALMA1R5MTAA rated for 3.1A. The switcher compensation seems to be a bit off, can this cause it to fail catastrophically?

  • 0
    TI__Genius 16295 points

    Hi Ville,

    Thanks for reaching out.

    1. It seems a little strange that the current is not enough for TPS61379-Q1.

    The input current can be calculated that Iavg=15*0.66/0.9/4.2=2.6A, it is already larger than its peak current limit 2A.

    2. the failure of SW short to GND is mainly caused by overstress of low side MOS.

    It usually results from bad layout or Vin surge.

    Bad compensation causes unstable, resulting higher voltage or current, but it should be ok if with good layout.

    Additionally, the compensation looks ok.

    It is recommended to send out your layout for reviewing.

    Best Regards,

    Fergus

  • 0 in reply to Fergus He
    Prodigy 210 points

    Hi,

    Yes, we know now that the switcher is not capable of full output current. If you calculate the input current with lower Li-Ion battery voltage such as 3.8V, the input current is even higher. We have had fails even with lower motor current, which is why we are curious. We are planning to change it in next revision to TPS61378-Q1, which seems to be pin compatible, but with much higher current limit. The internal FET has current limit, which should protect it from overcurrent. I would expect the output voltage to droop if the current is too high or completely shut down for a while.

    The input comes directly from Li-Ion battery with ESD401 for ESD protection and a P-FET for reverse polarity protection so there shouldn't be any surges etc. exceeding the operation limits.

    Switcher part of the layout in the picture below. For clarity, I didn't add all the layers, but there's a ground plane on the next layer and also on the other side of the PCB. VIN comes to the switcher in the third layer. The layout guidelines from the datasheet has been followed where possible.

    We are going to upgrade to TPS61378, but I wouldn't want to have the same problems with that. Not all units have failed, we have seen only few of these, but it would be good to understand possible reasons as the switcher is protected against overcurrent, overtemperature etc.

  • 0 in reply to Ville Keskilohko
    TI__Genius 16295 points

    Hi Ville,

    Thanks for the quick reply.

    For the layout, it looks reasonable for most of the parts, 

    there are two points seems be potential failure reasons:

    1. The Vout back loop go through VCC's GND.

    as we known, the output switching loop is the most important loop and should be as small as possible.

    The capacitor in this loop absorbs the spike when switches on and off.

    when VCC cap is also involved, the VCC of the device may be influenced and causes unwanted mistakes. 

    the VCC's gnd is the reference of analogy signals like Comp and FB, it is best for the device to have an 'AGND' connect to VCC GND.

    In this layout, the loop is as below, from via to thermal pad GND then to VCC cap's GND.

    There is high frequency current in this loop and may influence the FB and Comp signal.

    it is recommended depart this switching loop with the VCC GND loop and add vias near the VCC cap's GND.

    2. the boot capacitor is a little far, please also place the boot cap as close as possible.

    Best Regards,

    Fergus