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LMZM33602: PGOOD, VOUT unstable

Connor Shapiro
Prodigy 10 points
Part Number: LMZM33602

Our PCB contractor has had some quality issues and we are trying to debug a problem that seems to center around the LMZM33602 on our boards.

(note that the PGOOD signal is pulled up to 3V3 with a 10k ohm resistor elsewhere on the schematic)

 
On several of these boards, the issue manifests as a high pitched (I assume 5.1-5.6 kHz, for reasons detailed below) whine coming from the chip along with non-stable output voltage.

The waveform below shows PGOOD (CH1) and VOUT (CH2)

Any ideas on what could be the issue? Our problem boards have this behavior, although for some the frequency reaches as high as 5.6 kHz and VOUT actually gets as high as desired (12V) though it still experiences rising and falling in sync with the PGOOD (which always looks identical to the above waveform).

  • 0
    TI__Guru 50120 points

    Hi Connor,

    It looks like the schematic follows the required component values detailed in Table 1. Do you also have the PCB layout for this? I'd like to do a thorough review before ruling out the schematic/PCB. I've never experienced this issue myself but I'd imagine if you hear some high pitch frequency coming out of the device it may perhaps indicate damage in the internal inductor.   

    I have a few troubleshooting questions that would help me understand the system better:

    1. Is this issue observed during time zero startup?
    2. How well regulated is the input power supply? 
    3. Can you probe SW pin so I can see what the waveform looks like?
    4. Can you check if the output capacitors are soldered in place? I've seen issues where customers had cracked output capacitors which would result in a lower effective output capacitance than expected and cause the device to be well below the minimum required output capacitance for proper operation of the device

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Guru 50120 points
    Hi Connor,
    I haven’t heard back from you. Can you give me an update on this? I’m assuming you were able to resolve your issue. If not, just post a reply below so this thread can reopen and we can continue the troubleshooting.

    Regards,
    Jimmy