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LMZ31506: SS/TR and PWRGD

Shane Augustine
Prodigy 140 points
Part Number: LMZ31506
Other Parts Discussed in Thread: LMZ31503, TPS54320

Greetings, we are attempting to track a 1.8V turn on (LMZ31506) to a 3.3V turn on (LMZ31503). The resistors we've calculated are R1 = 37.8K and R2 = 18.9K. Unfortunately this results in ~ 1.1V at the SS/TR input which result in the PWRGD signal to be pulled down since it's below 1.4V.  The PWRGD signal is required to enable another regulator in the sequence. Any thoughts on an alternate method to track the two devices and enable PWRGD from the 1.8V device?  Thank you

  • 0
    TI__Mastermind 47760 points

    Hi Shane,

    Is tracking working ok?  Or is just the PWRGD of 3.3V pulling low after start up? Could you send me the schematic?

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    Prodigy 140 points

    Thank you for the quick reply. Yes, tracking is working as the 1.8V "follows" the 3.3V. The issue we have is with the PWRGD on the 1.8V regulator. Unfortunately providing a schematic would be a bit of a challenge but we're pretty much following the schematic depicted in Figure 38 on page 23 of the April 2018 revised datasheet. Our schematic has Vout1 = 3.3V, Vout2 = 1.8V, R1 = 37.4K (vs calculated 37.8K), R2 = 18.7K (vs. calculated 18.9K), and Vin = 5V on both. One thing to note is that 3.3V is being regulated using an LMZ31503 while 1.8V is being regulated using an LMZ31506.  The only other difference from figure 38 is that we have STSEL grounded on the LMZ31506 but the datasheet shows this with a no connect.

    Thanks again!

  • 0 in reply to Shane Augustine
    TI__Mastermind 47760 points

    Hi Shane,

    Thanks for your information. Understand.

    1. Vout1=3.3V, So R1 and R2 connect from 3.3V(Vout1).  The STSEL pin of Vout1 (.3.3V) LMZ31503 should be connected to GND and STSEL pin of Vout2(1.8V) LMZ31506 should be open to enable tracking feature.

    2. It seems you did a wrong calculation for R1 and R2. for Vout1=3.3.V, Vout2=1.8V, R1 should be 28.7k, R2 should be 22.6k. Please double check. So the voltage on SS/TR is higher than 1.45V, The PWRGD should be ok.

    Thanks,

    Nancy

  • 0 in reply to Nancy Zhang
    Prodigy 140 points

    So in addition to the leaving STSEL open on the LMZ31506, the equations from the LMZ31503 datasheets should be used to calculate R1 and R2, and not the ones from LM31506, correct?

  • 0 in reply to Shane Augustine
    TI__Mastermind 47760 points

    Hi Shane,

    Sorry, R1 and R2 should follow the equations from LMZ31506. I think LMZ31506 and LMZ31503 have same VREF 0.8V. But they are different. Let me think about if we can solve this issue.

    Thanks,

    Nancy

  • +1 in reply to Nancy Zhang
    TI__Mastermind 47760 points

    Hi Shane,

    Please see the TPS54320 datasheet (page 16) in the link as below. LMZ31503 used TPS54320 as its converter inside module. I am not supporting LMZ31506 and am not sure what is inside. But should be similar. If you make Vout2 is slightly greater than Vout1 during tracking. For example, deltaV=-0.4V, use equation 5 and 6, you can have R1=29.4k, R2=22.k, then the voltage on SS/TD will be higher than 1.4V, PWRGD should be ok. Please try it and let me know if it works. Leave STSEL pin of LMZ31506 open first.

    If it is not working, what is output current for 1.8V? Is it possible change LMZ31506 to LMZ31503? We recommend track feature with same converter. Not different ones.

    Hope this helps.

    Thanks,

    Nancy 

    https://www.ti.com/product/TPS54320?keyMatch=TPS54320&tisearch=search-everything&usecase=GPN

  • 0 in reply to Nancy Zhang
    Prodigy 140 points

    I believe using the recommended resistor values above provided an adequate solution. It did add an additional ~1 ms delay between the 3.3V reaching regulation and the 1.8V reaching regulation but this should be sufficient. The PWRGD was also asserted which was our main goal. I reviewed the TPS54320 datasheet and was wondering if you could quickly explain  the deltaV, Vout1, and Vout2 parameters. Vout2 and Vout1 don't appear to be the voltage outputs of 3.3V and 1.8V as that would result in a deltaV of 1.5V so I don't completely follow this equation. Also you chose a deltaV of -0.4 but could a deltaV of -0.1V been chosen to get the two ramps even closer to each other? On another note, whether we kept STSEL open or grounded it didn't appear to have an affect on the waveforms.

  • 0 in reply to Shane Augustine
    TI__Mastermind 47760 points

    Hi Shane,

    It is good news. Vout1 is 3.3V of first output voltage. Vout2 is 1.8V of 2nd tracking output voltage. DeltaV is the difference of both tracking ramps during start up. You are right. DeltaV of -0.1V should get two ramps closer to each other. But -0.1V may not enough to make PWRGD asserted. In the datasheet TPS54320, Figure 25 is positive deltaV. Figure 26 is negative deltaV. Figure 27 is 0V deltaV. The equation for R1/R2 calculation on datasheet of LMZ31506/31503 are for 0V deltaV.

    Hope this helps,

    Thanks,

    Nancy