TMP709: TMP709 did not sink low for temperature higher than threshold temp

future chang1

Prodigy 250 points

Part Number: TMP709
Other Parts Discussed in Thread: TMP390, TMP392

Hi TI:

I jump wire to connect TMP709.

Ther Rset is 68K ohm.

So the temperature is set to about 31 degree celsius.

The hyst pin is set to GND.

I use a hair dryer to heat the air temperature.

I think the air temperature will above 31 degree celsius.

The hair dryer is very hot.

But the OTpin did not sink low?

I use pull high 10K ohm for OT pin.

I wonder 10K ohm is not high enough.

So I change 10K ohm to 500K ohm.

But OT pin did not sink low?

I really do not know why OT pin did work?

Can TMP709 detect the air temperature or there is something wrong?

best regards

over 2 years ago

0 Josh Wyatt over 2 years ago

TI__Guru* 90906 points

Dear Future -

Please post your schematic. From the description you have, it seems reasonable and should work, be advised the datasheet does not recommend values over 470k for the pullup on the OT pin.

0 future chang1 over 2 years ago

Prodigy 250 points

Hi Josh:

Please see the following schematic.

I push the button, OT could be low.

Could you help to explain the purpose of the schematic 150 ohm in the datasheet page 1?

Will TMP709 detect the temperature of the air around the chip.

The air temperature heat the chip temperature?

So TMP709 will detect the air temperature.

I think the temperature will rise slowly?

best regards

0 Ren Schackmann over 2 years ago in reply to future chang1

TI__Mastermind 35835 points

The 150 ohm resistor can serve as noise isolation. It's not necessary.

68kohm should trip at 31C. The TMP709 is only rated for temperature accuracy between 60 and 100C. It will idle at high and pull down to low when it trips. Is the sensor inside an enclosure? Are you not waiting long enough for the hot air to reach the sensor? It may take a couple seconds even under direct heat, because thermal transfer of air is not very good.

thanks,

ren

0 future chang1 over 2 years ago in reply to Ren Schackmann

Prodigy 250 points

Hi Ren:

I use hair dryer to heat for one minute.

It did not pull low.

Best regards

+1 Ren Schackmann over 2 years ago in reply to future chang1

TI__Mastermind 35835 points

I'm not sure how much more help we can provide in this case. I would double-check your connections, measure the resistance of your 68k ohm, and test with another TMP709 sample.

As I said, TMP709 is not specified for temperature accuracy in the range you're using it. It is specified for 60 to 100C, which corresponds to set resistors between 15k and 44k ohms.

I would also suggest using TMP390 instead, which supports a 30C trip point.

thanks,

ren

0 future chang1 over 2 years ago in reply to Ren Schackmann

Prodigy 250 points

Hi Ren:

I change to TMP392.

And it works.

Only 2 seconds for hair dryer to trip the point and pull the output low.

Thanks for your help.

best regards

0 Ren Schackmann over 2 years ago in reply to future chang1

TI__Mastermind 35835 points

Glad to hear you like TMP392!

ren

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TMP709: TMP709 did not sink low for temperature higher than threshold temp