LM2940T-8.0/NOPB output problem

" ... the output voltage varies with the input voltage and load resistor value ..."

No reason the LM2940T-8.0 should not regulate properly as it is a really simple, very reliable, 3-terminal regulator.

Note that the LM2940 maximum operating input voltage goes up to 26V, just short of your 28V.

The description "... output voltage varies with input voltage ... " suggests that the Pin2 to Ground connection is open, or the input and output connections are reversed, but that is too obvious.

Tell me about the output capacitor that you are using. Ceramic, Tantalum, or Aluminum? Value?

Do you have schematic, or a PCB layout, that you could share?

" ... is there any other part I can use? ... "

Without knowing your load current, which may influence your package choice, I can only make generic recommendations for parts similar to LM2940 with fixed 8.0V outputs. Note that the maximum operating voltage for these devices is in the 25V to 26V range:

   TL750M08
   UA7808
   UA78M08
   LM2937
   LM2930

Thanks for feedback. I did not test it with a PCB, but simply put it on a bread board and apply input voltage to Pin 1, ground to Pin 2, then use multi-meter to measure Pin 3 for output. I tried several of them (LM2940T-8.0) with input voltage from 10 V to 26 V. The problem is the same. I will check whether the break board has any problem tomorrow.

Regards,

Lihua

LM2940 should work OK on any breadboard.

You did not mention any input or output capacitors.

The LM2940 is PNP type LDO, and it requires a minimum 22uF output capacitor with 0.1 to 1.0 ohms of ESR, typically a Tantalum capacitor is used to meet this requirement.

Alternately, MLCC (ceramic) capacitor can be used for Cout IF a series resistor is added to simulate the required ESR, AND the ceramic capacitor value is selected to compensate for the capacitance de-rating due to the applied DC voltage (i.e. Vout). Using X5R/X7R dielectric is preferred.

Output capacitance AND ESR (or simulated ESR) is mandatory, not an option.

Donald,

Ok. I did not realize that the output cap is mandatory. After adding a 47 uf Tantalum cap, it works fine.

Thank you very much.

Lihua