LM25011: PGD don't work

Hi Sir,

Left picture and right picture is both output of 3.3V PGD pin?

Thanks

Colin

Hi Colin,

Yes, both pictures are 3.3V PGD.

PGD is a negative voltage on the abnormal side.

Best Regards,

Hi Sir,

In abnormal picture, we can see that  the PGD has been negative when FET on. I am afraid PGD is pulled to negative voltage by other circuit. Could you share the schematic to check further?

Thanks 

Colin

Hi Sir,

Actually the PGD pin is internally connected to the drain of an N-channel MOSFET switch. When there is no output. the PGD pin is pull down to GND.

Did you have a test about PGD is negative voltage that LM25011 working alone?

Thanks 

Colin

Hi Colin,

Yes, PGD is negative voltage even when it working alone, but this phenomenon is observed our customer board.

When RT is bigger and On-time is longer in the abnormal product, the phenomenon become to disappear.

Conversely, when RT is decreased in the normal product, PGD become to have a negative potential.

Can you think of any reason?

Bes Regards,

Hi Sir,

Could you conduct a experiment about connecting PGD pin to outside 3.3V with a pull up resistor 10k and see whether there is negative voltage on PGD? Other thing is that keeping GND of probe is definitely a 0V, because I see there is lots of noise on PGD pin in abnormal picture.

Thanks

Colin

Hi Colin,

The requested test has already been done, but it was same result.
Since the voltage range is narrow, you will feel the noise is large.

Is there any reason for the negative voltage to be output from the internal circuit of the IC?

Best Regards,

Hi Colin,

This is the waveform when using FET.
When the FET is ON, PGD is 0V, and when the FET is OFF, PGD becomes negative.
Normal and abnormal were confirmed by changing Ron with the same IC.

Hi Colin,

I will send the schematic and layout by private message.

Hi Sir,

Got it, I will update when I review it.

Thanks

Colin

Hi Colin,

I have sent it to you in private message so please check it.

Hi Hirotsugu,

Please put a 10k resistor between SS pin and PGD pin.

Also I want to confirm that whether test point of V_PGD is PGND or AGNG, I would suggest  test point is AGND.

Another thing I want to confirm is that when PGN is negative voltage, whether output of U41 is normal(5.5V).

Thanks

Colin

Hi Colin,

Please do not publish schematics.

I would like to exchange private messages.

Hi Hirotsugu,

Ok, I got it.

Colin 

Hi Colin,

Thanks for your immediate response.

I will try 10kΩ.
I would check the GND used for the test points.
U41 on the 5V side does not work when PGD is negative.
If I cut the connection with PGD, U41 works and outputs 5V.

Best Regards,