LM25085A RT pin not working

Jamaal_Charles

Hello,

On page 11 of the LM5085A datasheet it shows that by pulling the RT pin to ground, the power supply should shutdown the output. We are using an nFET to pull RT down. The nFET is working and when I turn it on, the line pulls to ground, but the regulator output does not turn off. I measured a voltage of 0.0004V on the RT pin with the meter which is well below the 0.73V+/-50mV that is needed, yet the output remains on. Any idea why this would be? Was there a problem with this design for a certain batch of ICs, or could damage to the regulator have caused this issue? I have two boards running with this problem of not being able to turn off the regulator power.

Thank you

over 11 years ago

0 Vijay86929 over 11 years ago

TI__Genius 11490 points

Hi Charles,

Can you share the schematic? VIN/VOUT, and other details of the circuit.

0 Jamaal_Charles over 11 years ago in reply to Vijay86929

Genius 3245 points

Thanks Vijay,

Here is the schematic. Let me know if you have any questions.

0 LeonardEllis over 11 years ago

TI__Guru 71526 points

just curious - what happens if you hard-wire the RT pin to GND? Same behavior, or operates correctly???

~Leonard

0 Michael Brantley over 11 years ago in reply to LeonardEllis

TI__Intellectual 1535 points

Like Leonard mentioned, I would run some tests on the RT pin to see the affect on the output. The main thing that I would like to see how is different slew rates on the RT pin will affect the output. Try going from Vin to GND on the RT pin instantaneously, and also try slewing it at a much slower rate (just dial down the voltage from a supply). Sometimes different if you slew the enable pin too slowly or too quickly, the device will not function properly.

Michael

0 Michael Brantley over 11 years ago in reply to Michael Brantley

TI__Intellectual 1535 points

Jamaal,

Also, when you say the output stays on, do you mean that the part is still switching? Or is the output voltage remaining high? If there is significant load capacitance and not enough resistance to discharge, the output could be held high for a significant time even with RT pulled low.

Michael

0 Vijay86929 over 11 years ago in reply to Jamaal_Charles

TI__Genius 11490 points

The circuit seems ok on first look. I will try using the jumper to short RT to GND and see if the part stops switching.

We are not aware of any issues related to RT and shutdown function with this part.

0 Vijay86929 over 11 years ago in reply to Vijay86929

TI__Genius 11490 points

Also what is the VIN and VOUT when you see this? Do these voltage look similar to the normal operation.

0 Jamaal_Charles over 11 years ago in reply to Vijay86929

Genius 3245 points

So, we found the issue. You're suggestion to tie to RT to ground helped us find it. One of the FETs in the design wasn't working properly, and it was allowing power to keep flowing past the regulator. We changed out the FET with a different one and it started working again.

Thanks so much for your help!

Power management

Power management forum

LM25085A RT pin not working