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TPS61088: SW pin overshoot and undershoot

Dimitar Ruzhev
Prodigy 10 points
Part Number: TPS61088


Hello,

I am working on implementing a boost converter using the TPS61088. May I ask for a bit of help in assessing the SW pin switching characteristics to see if they are within safe limits?

The boost converter has the following parameters:

- Vin = 5V, Vout = 12V, Iout ~1-1.2A resistive load

 - SW pin absolute ratings (as per the datasheet): -0.3V to 14.5V (I believe these are DC values), and short time transient AC voltage can be as high as 20V according to the forum.

The SW pin voltages are as follows:

- Spikes on the SW pin of 15.1V for about 10ns. This seems fine given the transient AC voltage rating of the chip

- When low, the SW pin maintains -0.7V. This is a lot more concerning

SW pin measurement

The measurements were taken using a 50Mhz oscilloscope with local ground connection on the PGND plane.

Please let me know if any additional information such as schematics or layout would be helpful.

  • 0
    TI__Genius 16795 points

    Hi Dimitar,

    thank you for reaching out.

    1. the -0.3V is also a DC value. Usually -3V within 10ns is safe for the device.

    2. if you are concern about the voltage, you can use RC snubber circuit that paralleled with the switch.

    Or you need to re-layout the PCB to minimize the switching loop, please pay attention to the output capacitor.

    Best Regards

    Fergus 

  • 0 in reply to Fergus He
    Prodigy 10 points

    Hi Fergus,

    Thank you for the prompt reply. 

    Gauging from what you said, the -0.7V on the SW pin (when the pin is low) would definitely be outside the safe operating region (due to it being present for the whole duration of the low state, although it seems to go to about -0.4V towards the end of the cycle). Could you help me troubleshoot why this negative voltage is occurring (I dont have a good explanation as to why it might be)? 

    I have attached the schematic and layout (note the front plane is an uninterrupted GND plane) of my design. VDC is the 5V input and the output is 12V. The inductor used is 2.2uH, 5.3mOhm, Imax= 6.3A

    All the best,

    Dimitar

  • +1 in reply to Dimitar Ruzhev
    TI__Genius 16795 points

    Hi Dimitar,

    Sorry, my former reply may not be so accuracy.

    Your schematic and PCB looks all right.

    I think you should first check the testing method.

    The test waveform may result from your probe or the test point.

    please do as the follows:

    1. calibrate your probe as the oscilloscope user guide.

    please make sure the probe keeps 0 when no voltage exists.

    2.  Use non-isolated probe alone and use ground ring like that in the picture.

    And please remove any other probes from the scope.

    the test point should be the SW net and GND net most close to the device.

    Using the two method you will get a correct waveform of your SW and I think it will be within spec.

    Best Regards

    Fergus

  • 0 in reply to Fergus He
    Prodigy 10 points

    Hi Fergus,

    Apologies for the late reply.

    As you suggested, upon taking a better measurement the SW pin voltage is within the safe limits. Thank you so much for the help!

    All the best,

    Dimitar