TMP421: DXN connected to ground

Mark Guastaferro

Part Number: TMP421
Other Parts Discussed in Thread: TMP461

Can the TMP421 be used to temp sense a diode-connected PNP where the anode of the diode is grounded?

In this application, a power device provides a temp sense pin to monitor the device temperature. The temp sense pin is configured as a diode-connected pnp, but the anode side of the diode is internally tied to the device ground. This means with a traditional diode connection to the TMP421, the DXN input would be tied to ground. This pin is internally biased a diode drop above ground.

Will this work? Or is there another way to connect the diode to the TMP421?

over 8 years ago

0 Jose Arteaga4 over 8 years ago

TI__Expert 5695 points

Hi Mark,

Please confirm if the anode (positive terminal of the diode) or the cathode (negative terminal of the diode) is connected internally to GND.

As a reference the figure below shows the remote diode cathode and DXN connected to GND.

0 Mark Guastaferro over 8 years ago in reply to Jose Arteaga4

TI__Genius 11645 points

Sorry, you are correct. Cathode is tied to ground. I stated it backwards.

0 Ren Schackmann over 8 years ago in reply to Mark Guastaferro

TI__Mastermind 43885 points

Mark,

The TMP421 provides a small voltage bias on the DXN pin for noise suppression, but is designed to also operate with DXN tied to ground. In fact, all of our remote junction sensors work this way. Please refer to Jose's illustration.

There should be no issue with your application.

You might also consider our TMP461, which is a newer product with a smaller leadless package.

Ren

0 Mark Guastaferro over 8 years ago in reply to Ren Schackmann

TI__Genius 11645 points

Hi Ren,

Thanks for the confirmation, but we'll need more digging now.

In this design, nine TM421s are used to monitor temperature at various points. six of the nine work as expected. The three that do not work correctly all have the DXN pin grounded as described earlier. The other nine do not ground the DXN pin. The data coming out of the three bad TMP421 appears to be way our of whack:

When power device being monitored is operated at low power, TMP421 outputs small,negative temperature (0xEE). Actual temp is 30C. When operated at high power, small positive temperatures are reported (0x08). Actual temp is about 100C. Although reported temp moved in the correct direction, reported values are way off.

The sense transistor is equivalent to a 2N3906 PNP transistor.

0 Ren Schackmann over 8 years ago in reply to Mark Guastaferro

TI__Mastermind 43885 points

Please share your schematic.

Ren

0 Mark Guastaferro over 8 years ago in reply to Ren Schackmann

TI__Genius 11645 points

I will send this to you offline.

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TMP421: DXN connected to ground