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TPS62420: output voltage is low (always around 0.3V)

Yilun Xu
Prodigy 80 points
Part Number: TPS62420
Other Parts Discussed in Thread: DAC39J84EVM,

Hi,

I was using the TPS62420DRCR to generate 2.5V and 0.9V DC with a 5.0V input. However, I always measured around 0.3V output voltage on both SW1 and SW2 outputs. The DEF_1 and ADJ2 voltages were measured to be 0.07V and 0.2V respectively, which made me think that the external resistor network was working (the voltage ratio matched the resistor ratio: 0.07/150*(150+475)=0.3V, 0.2/182*(182+95.3)=0.3V). But the DEF_1 and ADJ2 voltages were still much lower than the TPS62420DRCR reference voltage, which should be 0.6V. Attached are the schematic, BOM, and layout for my PCB. I followed the TPS62420DRCR design in the DAC39J84EVM schematic, except that I used an available part to replace the obsolete part for C95 and C105. Does anyone have any thoughts of why the output voltage is so low and the reference is not 0.6V?

To debug this issue, I tried the following test on the SW1 line. Removed L3, C103, C106, and C107. Applied and adjusted the DC voltage with a voltage source on U13 pin 4, then measured the response on pin 5 and pin 6 of U13 with an oscilloscope. Only when the 3.5V DC voltage was applied on pin 4, I could see the PWM transition and the feedback loop started to work. At this time, pin 5 and pin 6 was measured to be 0.6V and 3.5V.

Thanks and best,

Yilun

  • 0
    TI__Genius 15990 points

    Hi Yilun,

    That is a bit strange. Can you check the inductor currents of both converters to make sure there is no short or high load on the output. If the devices are in current limit, it can cause the DEF_1 and ADJ2 pins to be below reference.

    Best regards,

    Varun

  • 0 in reply to Varun John
    Prodigy 80 points

    Hi Varun,

    I have some test results to discuss with you.

    For the previous test, the inductors were heated too much during the soldering. Therefore, the inductance dropped from 2.2 uH to 100 nH.
    Then I replaced the IC (U13) and the inductors (L2, L3) with new ones. The 0.9V channel (SW2) works properly now. But the 2.5V channel (SW1) still generates low output voltage.
    I did some tests to debug the issue:
    1. Redo the test "I tried the following test on the SW1 line. Removed L3, C103, C106, and C107. Applied and adjusted the DC voltage with a voltage source on U13 pin 4, then measured the response on pin 5 and pin 6 of U13 with an oscilloscope." The new result is: when the 2.5V DC voltage is applied on pin 4, we can see the PWM transition and the feedback loop starts to work. At this time, pin 5 and pin 6 are measured to be 0.6V and 2.5V. This means the IC (U13) generates the correct output voltage without installing the inductor. This test also generates the 0.9V output correctly on the SW2 line.
    2. Install the L2 and L3 and measure the output voltage. The SW2 line generates the 0.9V DC successfully. The output voltage of the SW1 line is still low. Attached are the screenshots of pin 4, 5, 6 from the oscilloscope.
    a) pin4: has spikes (repetition rate is ~2.4kHz). Mean value is 0.85V. Max value is 1.31V.
    b) pin5: has spikes (repetition rate is ~2kHz). Mean value is 0.21V. Max value is 0.45V.
    c) pin6: we can see the switching. But the frequency is ~3.3MHz, which is out of the nominal range (2.0~2.5MHz) in the datasheet. Mean value is 0.92V. Max value is 5.6V.
    3. The IC device marking: BQF 08W Z967 (or BQF 0BW Z967? I cannot distinguish the "8" and "B" on the device). Can you help check this marking? Could it be a bad chip?
    4. We replaced the C105 capacitor with a 22uF low ESR capacitor, a 10uF capacitor, but they didn't help.
    Any thoughts on this?
    Thanks and best,
    Yilun
  • 0 in reply to Yilun Xu
    TI__Genius 15990 points

    Hi Yilun,

    Looking at the 3 scope shots for the 2nd converter, they seem to be running at 3.3MHz. To me it still looks like the device is in current limit - either due to improper soldering or a short at the output. Can you take 1 scope shot with all 4 signals - VIN, SW, Inductor current and VOUT?

    Best regards,

    Varun

  • 0 in reply to Varun John
    TI__Genius 15990 points

    Hi Yilun,

    Were you able to debug this further and get the scope shots?

    Best regards,

    Varun

  • 0 in reply to Varun John
    TI__Genius 15990 points

    Hi Yilun,

    I am closing this thread for now. In case you need further support you can comment here and reopen the thread.

    Best regards,

    Varun

  • 0 in reply to Varun John
    Prodigy 80 points

    Hi Varun,

    It turned out to be the bad IC issue. After I replaced the TPS62420, it worked.

    Thanks for your reply here.

    Best,

    Yilun