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LM2940-N: LM2940 has some problem.

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


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?