LM2940-N: LM2940 has some problem.

user4815151

Prodigy 70 points

Part Number: LM2940-N
Other Parts Discussed in Thread: LM2990

HI，dear

I chose the chip LM2940IMPX-5.0 in my design, the requirement is shown as follows:

the input voltage is ±8.4V，and the output voltage is ±5V. when the switch is joined up, the circuit is working.

my design is shown as picture:

however, the problem comes out:

1. join the switch1 and switch2 together, and join the switch3 and switch4 together. the output is right.

2.break the switch1 and switch2, keep the switch3 and switch4 together,then join the switch1 and switch2 together, the output of +5 is wrong, the output is about 0.9V.

3.break the switch3 and switch4, keep the switch1 and switch2 together,then join the switch3 and switch4 together, the output of ±5V is OK.

conclusion: the chip LM2940 has some problem, can you tell me in step 2, why the output of 5V is wrong?

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi User,

looks like latch-up: When you have a load connected to the regulators which draws a current from the output of LM2940 to the output of LM2990, like an OPAmp for instance and you turn of the LM2940, then the current continues to flow and makes the output voltage of LM2940 fall under a level where latch-up occurs. Rising the input voltage of LM2940 again makes the output hang at a non nominal output voltage.

A diode protecting the output against reverse voltages can help. Connect the cathode of protection diode to the output pin of LM2940 and the anode to GND.

Kai

0 user4815151 over 6 years ago in reply to kai klaas69

Prodigy 70 points

Hi，dear
I knew what you explian so little, I tried the circiut as you told shown in picture. If we don't add the diobe, the output is -0.9V, add the diobe, the output is -0.7V. However, it's no use. In addition, tell me which diode should I choose？ Or maybe can you help me to choose another chip like LM2940?
Thank you very much.

0 kai klaas69 over 6 years ago in reply to user4815151

Guru 140200 points

Hi User,

we take a 1A or even 3A Schottky for this.

Kai

0 user4815151 over 6 years ago in reply to kai klaas69

Prodigy 70 points

HI，Kai
Could you tell me the series of Schottky diobe in detail?

0 kai klaas69 over 6 years ago in reply to user4815151

Guru 140200 points

Hi User,

for 1A regulators I usually take the 30BQ040 as latch-up protection diode.

Kai

0 Eric.Preiss over 6 years ago in reply to kai klaas69

TI__Expert 4350 points

Hi User,

Did Kai's suggestion work? Were you able to prevent the LM2940's OUT from going negative so that the regulator can startup up when you apply voltage on IN?

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LM2940-N: LM2940 has some problem.