LM2586: Component Burnt

Raphael Leite

Part Number: LM2586

I have two boards that failed using the DC/DC switch regulator LM2586S-12 (input of 30VDC and output of 12V). I'm facing the following scenario:

-LM2586 ICs were severely burned on both failures, mostly on area of pins 5,6 and 7 (as per attached pictures).

-Board's design did not consider the use of the Schottky Diode SA51A shown on Figure 57 of the TI "SNVS121D-MAY 1996-REVISED APRIL 2013" data sheet.

Could you please send me any information that could help us on our investigation?

0 Frank De Stasi over 8 years ago

Please provide your schematic and a snapshot of your layout.

Also, any switch voltage wave forms that you have of a working unit.

The schematic on page 57 shows both a Zener and a Schottky diode.

I would say that the Zener diode is most important to clamp the voltage

on the SW pin to safe levels.

0 Raphael Leite over 8 years ago in reply to Frank De Stasi

Hi Frank, thanks for your answer.

This is the schematic:

And here you can see a picture of the board and the damaged component in detail.

0 Frank De Stasi over 8 years ago in reply to Raphael Leite

It would be helpful to see the pcb layers around the device.

It is hard to see, however be sure that there is enough copper area for heatsink and it is connected to ground.

Be sure that the input voltage is not overshooting the device limit.

Does the failure occur at star-up or after a while running?

It would be very helpful to see some scope shots of the switch pin during operation.

0 Raphael Leite over 8 years ago in reply to Frank De Stasi

Hi Frank,

The failure occurs while running. See picture below.

And this are the layers around the device, as you asked:

Is it ok?

Regards,

Raphael

0 Frank De Stasi over 8 years ago in reply to Raphael Leite

If it is damaged after a while it may be overheating. You may need more heat sink.

You could try to temporaritly use the T package with a heat sink and see if it survives.

The only other thing could be a spike on the SW pin. Can you use a fast scope and look at the SW pin ?

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