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Original question:

LM3880: Schematic review

LM3880: Issue with turning it on

Andrew Baek
Intellectual 905 points
Part Number: LM3880

awg_2v11 pg 22 to 26.pdf

U54 on the first page did not turn on.  I believe it follows the recommended circuit exactly (Figure 20).   The EN voltage is ~ ground.

Feedback would be appreciated.

  • 0
    TI__Guru 58080 points

    Please make sure following things:

    1. before VCC is above to 2.7V. the EN threshold is below its on threshold. ideally you do not want that EN reaches its on level before VCC reaches its working level.

    2. if above is true, check the EN threholsd to make sure it is above its on threshold to turn on the FLAGS output after VCC is at its working level.

    Regards

    Yihe

  • 0 in reply to yihe
    Intellectual 905 points
    HI Yihe,

    I measured the EN voltage and FLAG1/2/3 voltages on U27. They are all ~950 mV.
    I changed the C144, the EN capacitor to something small 22PF. The result was the same.

    On the other hand, U30 (another LM3880) works fine. The EN is driven by 1.8V in this case.

    Is Figure 20 of the datasheet still valid?

    Regards,

    Andrew
  • 0 in reply to Andrew Baek
    Intellectual 905 points
    I think the chip may be damaged.
    I tried Figure 20 with resistor divider and the direct enable.
    They all showed 950 mV on EN.

    I think the chip got damaged the first time I turned on the board.
    It may be due to the Cen, which is 1UF. Is this too large?
    Thanks,
    Andrew
  • 0 in reply to Andrew Baek
    TI__Guru 58080 points
    I do not think the size of capactor matter, the larger the size, the longer the delay. Figure 20 is valid.
    do you check the IC where the FLAGs are connected to. do they have any internal pull-up which may drive the FlAG over limits?
    Also the U27/U30 does not match the schematics you provided. what's the difference between U27 and U30 from the design point view?
    Regards
    Yihe
  • 0 in reply to yihe
    Intellectual 905 points

    Hi Yihe,

    We found the issue.

    Basically the Pins 2 and 3 were swapped on the schematic.

    I tested on eval board and LM3880 works as expected.

    Thank you,

    Andrew