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LM51561: LM51561

robin gangopadhya18
Mastermind 6296 points
Part Number: LM51561

Tool/software:

Hello:

The 2nd prototype board using LM51561 in flyback mode is not even starting up.

Does  the oscillator not free run if pin 12 is above UVLO level? In this circuit, it is connected to input BIAS pin which is 5V.

What else could go wrong?

Unlikely that the exposed pad is not connected to GND plane.

Power level is a few watts: used mainly for generating floating gate drive bias voltages for high bandwidth power switches.

Any comment on how to test/force it to run the oscillator will be highly appreciated.

Only the relevant part of the ckt is attached.LM51561ckt_for_TI.pdf

-r

  • 0
    Mastermind 6296 points

    Pl see a part of the LM51561 PCB layout attached LM51561LAYOUT_FOR_TI.pdf

  • 0 in reply to robin gangopadhya18
    TI__Genius 15995 points

    Hi robin,

    Thanks for reaching out.

    For the schematic review it would be good if you could provide the complete schematic.

    Hoe does the device bahave? Do you have some measurements? I would like to see Vin, VCC, UVLO and SS pin.

    What i can see from the schematic is, that the sofstart capacitor is very low and the VCC cap very big. Can you try to use several uF for softstart and 1-4uF for VCC please?

    Best regards

    Moritz

  • 0 in reply to Moritz
    Mastermind 6296 points

    Hello Moritz:

    We had the SS cap and the VCC cap changed per your suggestion.

    The circuit did not work.

    The oscillator does not start.

    I checked Vcc output: 4.5v ( Bias is 5v only) , SS is also at 4.4 v.

    Pin 3 is low level.

    I can take pix of these...but "nothing" is going on!

    If I disconnect the transformer, you would still get the oscillator running, right?

    Our first line of debug should be to get the part free run the oscillator. This requires very few parts at: SS, RT & of course Bias, GND.

    Could it be that the chip is not soldered at RT or SS pins? What impact would you have if the exposed pad is not soldered?

    Appreciate your reviews on this .

    r

  • 0 in reply to robin gangopadhya18
    Mastermind 6296 points

    Hello Moritz:

    On second thought, SS &RT cannot be without solder because  each has some DC voltage!

    so it must be something else.

    On the 1st, proto, it "worked" but Vdrain was negative ..I figured, the xmfr was connected wrongly.

    I damaged that board because we have a mistake at pin 12 which required 2 cuts & 1 jumper...possible that the jumper does not make contact with pin 12 after rework.

    So started with the 2nd proto.

    Which does not even start!

    r

  • 0 in reply to robin gangopadhya18
    Mastermind 6296 points

    Hello Moritz:

    Good news: I removed the xmfr..then noticed pin 3 has gate output...may be it was there all along: the reason it was not driving the MOSFET gate is that R6 is OPEN!

    On the other hand, pin 3 has significant "dither"...which should not be. Or may be it is very noisy. Now , this is without any xmfr.

    Maybe pin 7 is not connected to GND..

    going to check.

    But this status does give us what to work with--and get to make it work fully...

  • 0 in reply to robin gangopadhya18
    Mastermind 6296 points

    Hello Moritz:

    Ugh: a terrible mistake I have done in not reading very carefully the DTHOFF node operation.

    I have it at AGND.

    So that makes DITHERING ON!

    Now I have to have the chip removed, cut pin 7 to AND. Jumper it to BIAS( which is 5vin node ). Reinstall a new chip.

    Dithering ON makes the driver signal dithering.

  • 0 in reply to robin gangopadhya18
    Mastermind 6296 points

    Hello Moritz:

    On second thought, I might as well leave DITHEROFF pin at AGND- at first I was a bit puzzled that I was not aware of this choice of AGND for pin 7.

    But let us proceed with xmfr connection & see how the secondaries behave plus the primary side feedback control.

    Will let you know asap.

    r