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TPS74801: TPS74801 is working abnormally

user5911856
Prodigy 150 points
Part Number: TPS74801
Other Parts Discussed in Thread: TPS54620

Hello,

    The two tps74801 in the following figure are not working properly.  All output is about 1.0V. 

    3.3V is right. PG_1V2 is 1.8V.

   Why they are working abnormally? Thanks!

  • +1
    TI__Mastermind 46065 points

    Hi,

    The only thing I can see that may be an issue is that the BIAS headroom on the 1.8V device is only 1.5V, while the datasheet says that a separate VBIAS is required when VIN < VOUT + 1.62V. However, the 1.2V device should have no issue with the given configuration. 

    Can you capture 'scope shots showing the startup of both devices along with the input voltage? 

    Thanks,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 150 points

    Hi Nick,

     Please see pictures in below. How can I fix them? Thanks!

    1.  1.2V picture

    2. 1.8V picture

  • 0 in reply to user5911856
    Prodigy 150 points

    Hi Nick,

    By the way, I used the two desginment in other project before in this project. They are working very well, please see the SCH in below. They can output 1.2V and 1.8V normally.

  • +1 in reply to user5911856
    TI__Mastermind 46065 points

    Hi,

    What is happening on the 3.3V rail that is causing it to dip to ~1.3V each time the device restarts? I suspect this is the reason. I don't think this device has a UVLO circuit so I don't know what the minimum input voltage level it needs to not restart, but try adding more input cap to hold up the input during these dips. Also, where did you measure the 3.3V? Was it before or after the ferrite bead at the input? 

    Regards,

    Nick

  • +1 in reply to Nick Butts
    Prodigy 150 points

    Hi Nick,

    I'm not sure why this phenomenon occurs. I used TPS54620 to produce 3.3V power. The previous design had a 5V input and worked normally. The new design adopts a 12V input and is designed using webench.Please refer to the schematic diagram of the old and new designs below.

    Regards,

    Sunny

  • +1 in reply to user5911856
    TI__Mastermind 46065 points

    Hi Sunny,

    There could be differences in the loading condition between the old and new designs. I also see that there are ferrite beads all over the place, and so if there is a load transient on the buck regulator you would see a big dip on its output (the 3.3V input to the LDO) because of all of the inductance between the buck and the LDO. Can you try shorting out some of the series ferrite beads to see if it helps? I'm seeing that there are at least effectively 2 ferrite beads in series between the regulators (FB26 || FB27 and also FB29 and FB32); do you really need this many ferrite beads? They are likely killing the transient performance of the buck, and the ferrite beads at the output of the LDOs are probably negatively effecting the transient performance of those as well.

    Regards,

    Nick

  • +1 in reply to Nick Butts
    Prodigy 150 points

    Hi Nick,

     I have conducted many kinds of tests and found that there is a problem with the 1.8V circuit. When I remove FB32 which is input of 1.8V circuit. 3.3V and 1.2V is normal. When I connect FB32 and remove FB34 which is output of 1.8V circuit. 3.3V is abnormal. I compared it with the old design, and except for the different number of magnetic beads used for output, the other circuits are almost the same. However, the old design can stably output 1.8V, while the new design will affect 3.3V.

  • +1 in reply to Nick Butts
    Prodigy 150 points

    Hi Nick,

    Because I removed FB34, I think the load is very light. And I also connected 22uf in parallel at the input capacitor( C240 )，3.3V is still affected by the 1.8V circuits. please the pulse diagram in below. the period is about 29ms, and the pulse width is about 40us. I'm not sure if this can help you analyze. 

    Thanks!

  • +1 in reply to Nick Butts
    Prodigy 150 points

    Hi Nick,

    I am sorry for bother you. I think I found the root cause. Because I use PG_1V8 to switch the 3.3V by enable PMOS. Please see picture in below. But the 3V3 is shorten to GND. So I think big current make P3V3 drop down, causing abnormal output of 1.2V and 1.8V. Now, all power is ok. Thank you for patiently helping me analyze the reasons.

  • 0 in reply to user5911856
    TI__Mastermind 46065 points

    Hi,

    I'm glad you were able to figure it out. Did you just short Q1 then? What was the solution that fixed it?

    Regards,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 150 points

    Hi Nick,

    At the beginning, I test the P3V3, it is not shorten to GND. So I think 3V3 is ok. Then, I removed R224, found the output of P3V3 is normal. At this point. Q1 did not switch on.I used flying wire to make R223 pull up to P1V8, the output, not P3V3. In this case, the Q1 was damaged by bit current. So I see, 3V3 is shorten to GND. Then, When I solved the trouble of shorten, P3V3 is ok.