TMP708: Voltage on SET Pin

Bastian Mieck

Part Number: TMP708

Hi everybody,

one of our customers uses the TMP708AIDBVR to detect an over temperature case at 105°C. He had some problems with the stability/precision.

He added a cap at the SET pin and everything was fine.. After that he removed the cap again and measured the voltage on the SET pin.. Attached you find the results (AC Coupled). There are high peaks on the voltage if he doesnt add the cap..

They think the peaks come from the PFC stage which is very close to the Pin/Resistor. But the question is how should the voltage look on the pin? It should be a stable DC voltage right? Or is there any intern circuit which could cause the peaks?

thanks

Bastian

over 7 years ago

0 Kelvin Le THS over 7 years ago

TI__Expert 6475 points

Bastian,

I'll look into this.

-Kelvin

0 Bastian Mieck over 7 years ago in reply to Kelvin Le THS

TI__Prodigy 730 points

Sorry i forgot to upload the picture..

0 Kelvin Le THS over 7 years ago in reply to Bastian Mieck

TI__Expert 6475 points

Bastian,

I checked with our designer and this is what he has to say:

The voltage on the SET pin of the TMP708 is a DC signal (current source on the SET pin creates a voltage when the resistor RSET is connected to the pin). Has the customer checked the ground and supply of the TMP708 in the system to see if there are any anomalies that could explain why the voltage on the SET pin is behaving in this manner?

Definitely check the power supply feeding the tmp708 to see if you get a similar noise pattern.

-Kelvin

0 Bastian Mieck over 7 years ago in reply to Kelvin Le THS

TI__Prodigy 730 points

Ok thats what we thought..

It seems the PFC Stage induces the voltage peaks.. The peaks are about 1V to 1.5V high.. The customer is using the following schematic

Now he put a cap of 4.7uF on the SET Pin. He doesnt see the peaks anymore and has a DC voltage of 500mV. Is this okay or could the cap causing problems?

They are using a 12k SET Resistor to get a threshold of 105°C with 10°C hysteresis