7805 works fine, LM2940CT5 not!

[DanPBP]

Hi All,

I have a PIC application (16f88 and 2 leds) that is working fine with a 7805 (from National) and a 6 volts 1 amper battery.

Yesterday, I replaced the 7805 with an LM2940CT5 (from National too) and my application didn't work. I tried with another LM2940CT5 and got the same result. I measure the input and the output and seemed ok. Just in case, tried another one, nothing happened.

I switched back to the 7805 and everything worked again!!!

I thought that the LM2940CT5 was better than the 7805, but is not.

Any ideas why is not working with the LM2940CT5? I cannot find out why is not working.

Thanks!

Daniel.

[BobP]

Hi danielhr77,

Make sure you have enough capacitance fitted both before and after the regulator. If too little or none they can oscillate which may not show on your meter?

Check datasheet for correct smoothing and decoupling capacitance values.

Bob

[Bruce]

Most +5V non LDO regulators require at least 7.5VDC minimum input to maintain a regulated +5V output.
I couldn't find a datasheet for the LM2940CT5, but, some LDO types require specific capacitor values on
the output or they will oscillate, and that causes major problems.

Edit: I found a data sheet on the LM2940, and it looks like you need at least 22uF on the output, and
drop-out will be around 0.5V, so you don't have much overhead.

[BobP]

Hi Daniel,

Just checked the datasheet
http://www.national.com/ds/LM/LM2940.pdf#page=13

and it clearly states that the mimimum capacitance on the 5volt side is 22uF (Pg13).

Let us know if this solves the problem?

Bob

Edit oops Bruce has just giving this info

[Luciano]

Hi,

Datasheet:
http://www.national.com/ds/LM/LM2940.pdf

* * *

Application Note 1148 Linear Regulators:
Theory of Operation and Compensation

See page 7, Output Capacitor Selection:
http://www.national.com/an/AN/AN-1148.pdf

* * *

Source of the above PDF links:
http://www.national.com/mpf/LM/LM2940C.html

Best regards,

Luciano