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TMP451: D+ Pin Bias

Gary Thompson
Prodigy 30 points
Part Number: TMP451
Other Parts Discussed in Thread: TMP411, ,

Does the D+ pin bias change with VCC? I've got a thermal diode in a large package with ESD diodes that kick in above 1V, and I just wanted to make sure the TMP451 wasn't going to turn these on during measurements. I seem to recall earlier device like the TMP411 biased D+ much higher.

Just to pick a nit, I was extremely disappointed to see the TMP451EVM doesn't support running the TMP451 below 3.3V even though it is specified down to 1.8V. It would have been so easy to use an adjustable LDO and allow a switch or even a resistor population to change VCC.

  • 0
    TI__Mastermind 40625 points

    Hi Gary,

    The D+ pin is a current source, not a voltage source. As a current source, the voltage may go as high as the TMP451 VCC level while attempting to reach the target current. Under normal operation, the voltage should only be diode forward voltage (or Vbe) ~0.7V depending on your junction. You are correct that earlier remote sensors from TI had a bias voltage. However, this bias voltage was at the D- pin. It lifted the junction's ~0.7V up off of system ground by 0.3V or 0.7V depending on model. This meant the total voltage at D+ was ~1.0V or ~1.4V under normal operation for the older sensors. TMP451 does not have a bias voltage at its D-, because this would not leave enough voltage headroom for 1.7V system operation.

    The TMP451EVM does support 1.7V operation when connected to an external supply. The switch S1 disables the on-board regulator allowing an external voltage to be applied at the 3P3V/V+ pin. The level translator U3 automatically converts the microcontroller's 3.3V logic to the desired level. This component, U3, does not have a purpose in the circuit otherwise.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 30 points

    Thanks Ren. This aligns with my observations. Just wanted to make sure I wasn't missing something.

    Gary