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LMZ31707: Problem with voltage tracking

Prodigy 70 points

Replies: 12

Views: 112

Part Number: LMZ31707

I use two LMZ31707. One is set to 3.3V and the second to 5V.

During the startup the 3.3V Regulator hat to track the 5V Regulator.

I calculated the resistors about the equations in the datasheet, but the circuit is not working correctly.

I left the STEEL pin at the 3.3V open how described in the datasheet.  But from  my point of view, the 3.3V regulator is not in the tracking mode.

Is there a known issue or make i something wrong?

And the voltages look like this:

Ch1: is the 5V supply

Ch2: is the 3V3 suppy (this voltage should track the 5V supply)

Ch3: Is the voltage at the at the SS/TR pin of U19

BR:

Thomas

  • Hi Thomas,

    Your resistor divider to the U19 SS/TR pin looks correct.

    How are you enabling the devices, is it just with VIN_PROT supply rising? The INH pin dividers are slightly different, so I was also curious if you would be able to show waveforms for the two INH pins and two outputs in one scope capture?

  • In reply to Kris F:

    Hello Kris

    .
    You are right, the two voltage divider are slightly different. The reason is that the 3V3 regulator enables slightly earlier than the 5V regulator.

    I have also tried to remove the voltage divider at the 3V3 regulator (R122 and R123) and placed R135 in stead. So that the two INH Pins art connected to the same voltage divider.

    The result is nearly the same. The only difference is, that the two regulators start at the same time. But the Voltage is also not tracked correctly.

    The following picture show the two voltages and the corresponding INH signals.

  • In reply to thomas korherr:

    CH1: 5V

    CH2: 3V3

    CH3: INH of U20 (5V regulator)

    CH4: INH of U19 (3V3 Regulator)

  • In reply to thomas korherr:

    Can you please share a scope capture when you have the INH tied together? You mentioned that it still does not track correctly, but I would like to see what the waveform looks like.

    If it is easy to do on your board, another good experiment would be to swap R123 and R300 so that the INH of U20 (5V) comes up first before the U19 (3V3) and see if that has any impact on the tracking.

    Are you seeing this on multiple boards or just one?

  • In reply to Kris F:

    Thomas, any luck on this?

    In addition to my requests/questions in the last reply, have you double checked the SS/TR divider resistors (R294 to R297) that they measure as the right values you show on your schematic? Just want to confirm that none of those resistors are accidentally swapped, as that could lead to incorrect tracking behavior.

  • In reply to Kris F:

    Hello Kris,

    There are the requested pictures.

    he first picture shows voltages with the two INH pins connected together. R135 placed, that means the two voltage dividers are connected in parallel.

    The second picture shows the voltages with the two resistors (R300 and R123) are swapped.

    The scope channels are connected to the same nets than in the picture before.

    BR

    Thomas

  • In reply to Kris F:

    Hello Kris,

    There are the requested pictures.

    he first picture shows voltages with the two INH pins connected together. R135 placed, that means the two voltage dividers are connected in parallel.

    The second picture shows the voltages with the two resistors (R300 and R123) are swapped.

    The scope channels are connected to the same nets than in the picture before.

    BR

    Thomas

  • In reply to Kris F:

    hello kris,

    Yes.
    The check of the correct resistors at the voltage divider was one of the fist steps.


    BR
    Thomas

  • In reply to thomas korherr:

    Thomas, thanks for sending the waveforms.

    One debug step you can try is to remove the parallel top resistors in the SS/TR divider, R294 and R295.

    The U19 converter 3.3V output should not come up since SS/TR will effectively be pulled low.

    It would be good to confirm this is the case. When you do so, please have a scope channel on 5Vout, 3.3Vout and the U19 SS/TR pin.

    Reviewing your scope capture from your first post, the Ch3 Green SS/TR waveform is not what I would expect given the resistor divider. The SS/TR signal ramps up higher than it should based on the divider and the ramping 5V rail. Then, after both 3.3V and 5V rails have ramped, it is strange how the SS/TR waveform goes down but the 3.3V output does not go down.

    Hopefully the above test (removing the top resistors in the SS/TR divider) will reveal if there is any other unexpected path pulling up the SS/TR pin.

  • In reply to Kris F:

    Thomas, did you try the suggestion above?