LM2842: LM2842：the output can‘t turn-off when the SHDN PIN is LOW

Amelie Zheng

Part Number: LM2842
Other Parts Discussed in Thread: LM2742

Hi Team，

My customer use PNP to give PWR EN signal to LM2842 SHDN PIN. The SCH is following. However, When PNP is turn-off, the voltage of PWR_EN is 0V, but LM2842's output is 3V not 0V.

However, when disconnect the PNP and just let the PWR_EN connect to GND with R89, the output of LM2742 is nearly 0V .(several mV).

Could you please give some advice about that? Thanks a lot!

over 5 years ago

0 Ambreesh Tripathi over 5 years ago

TI__Expert 6860 points

Amelie ,

Try reducing R 89 to lower value and see if the device shutdown happens . The device cannot have floating SHDN pin .

--Ambreesh

0 Amelie Zheng over 5 years ago in reply to Ambreesh Tripathi

TI__Genius 10865 points

Hi Ambreesh，

Yes! It helps a lot! It can turn-off when use lower value of R89 (120Ω) when using LM2842 12V to 3.3V.

But when using LM2842 12V-5V, it still can't turn-off the output (1.8V not 0V) with the same PWR_EN control circuit with 12V to 3.3V. And at the time the voltage of PWR_EN is still 0V but output is 1.8V.

I am very confuse about PWR_EN is still 0V but output is 1.8V. I don't think when PNP turn-off, the state of PWR_EN is floating.

Could you please give some advice about this? How about change PNP control to NPN?

0 Ambreesh Tripathi over 5 years ago in reply to Amelie Zheng

TI__Expert 6860 points

Amelie ,

Is the device Switching for 5V output ? May be the output cap is charged and that is what you are seeing at the output .

---Ambreesh

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LM2842: LM2842：the output can‘t turn-off when the SHDN PIN is LOW