• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 64 subscribers
  • Views 602 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UA78M: ua78M33 Oscillating

Niel Nielsen
Prodigy 10 points
Part Number: UA78M

I have a design that uses a 78M33 to reduce the voltage from a 12 volt power supply down to 3.3V for an embedded processor and an LED display driver.  The current draw on the 3.3v is about 40 ma.  I used a 4.5 uF tantalum cap on the input and a 10 uF cap on the output.  I know that these are much larger than the data sheet calls for, but I want a very quiet system.  There are also three ceramic .1 caps across the 3.3v line that are located next to the IC's using 3.3V.

The problem is that the output of the 78M33 oscillates with this capacitance on the output.  I had to remove the 10uF output cap and go with just the three .1 uF caps to get the problem to stop.  WHY?  The data sheet doesn't mention any limits on the output capacitor size, and with other 78 series regulators I have used in the past, the bigger the caps, the better and quieter the system.

The next plan is to add an ESP-12s to the design, which calls out the need for a 10uF cap next to the ESP12S.  I'm afraid that this is going to bring back the oscillation from the 78M33.  Does anyone have any idea what is going on  with this?  Is there a different regulator that I need to use?

Thanks,

Niel

  • 0
    TI__Guru 55940 points
    Hi Niel,

    In order to help debug, please provide a scope shot with Vin, Vout, and Iout showing the oscillations.

    Please note that UA78M was originally released in 1976. A more modern part will be able to provide better performance. Do you have any requirements that you are looking for outside of 12 Vin, fixed 3.3 Vout, and 40 mA?

    Very Respectfully,
    Ryan
  • 0
    Guru 140200 points
    Hi Niel,

    take a 47...100µF/25V aluminium electrolytic cap at input and a 10...22µF/25V cap at output of regulator.

    Remember that these regulators expect to have a big storage cap at input. And to have the bigger filter cap at input, not at output. So, your 4µ7 tantal might be too small.

    Adding a cap at output is very wise and I do it also, nearly allways. I would recommend a cap with enough ESR, which is the 10...22µF/25V aluminium electrolytic.

    Is the +12V at input of regulator fabricated by a switcher?

    Kai