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.

LM2936: Not Regulating

Part Number: LM2936

I purchased several fixed output LDO regulators, specifically LM2936Z-5.0/NOPB.

I connected this LDO with a 100nF capacitor on input and a 100uF cap on the output as the datasheet shows.  With a 12-20V input the output is at 5.0V until I place a load on the output.  Then the output voltage equals the input voltage and destroys the MCU.  At first I thought the MCU was pulling to much current, even though I had measured it at 7 mA beforehand.  So I set up a test circuit and simply placed a 475k resistor to ground as the load on a second regulator.  Same thing happened.  But there should have only been 10.5 uA of output current according to V=IR, and the LM2936Z-5.0 is rated for 50 mA of output current.  I then tried a 3.3M resistor and had the same problem with a third regulator.  After removing the load, the output voltage remains equal to the input voltage, almost as though the LM2936 has blown.  If I discharge the 100uF capacitor and start again, the output voltage slowly builds to 5V, but eventually, quickly jumps to the same value as the input voltage.

Any ideas?  I do not want to continue destroying the units if that is what is happening.  Thank you.

  • Hi Terry,

    What kind of 100 uF output capacitor are you using? LM2936 requires an output capacitor with ESR as shown in Figure 9 of the datasheet. You will need a tantalum, aluminum electrolytic, or a ceramic capacitor with a series resistor between OUT and GND for the LDO to be stable. See the External Capacitors section on page 15 of the datasheet for more details.
  • Thank you Eric for the quick reply. I missed this detail in the datasheet. I am using an electrolytic capacitor. I checked my info on the capcaitors I have and no ESR is given. I will test a few different series resistors in the 0.25 to 1 ohm range and see what the effect is. The datasheet suggests that a larger capacitor may help. I will try that as well. Again, thank you. -Terry
  • Eric, my original post contained a typo. I was using a 10uF capacitor on the output as shown on the datasheet, not a 100uF cap. The next larger cap I could find quickly was 47uF. Switching to the the 47uF cap seems to have solved the problem. And no need for a series resistor when testing with an output load of 1.5 uA, 10.5 uA, 10.5 mA and 50 mA. Thank you again for your help.