TMP300 temperature switch.

Kumar,

I am currently looking into this and will get back to you shortly.

Kumar,

Can you please attach the schematic you have referenced. 

Hello Chris,

We have used this Schemaitc and found that , it is working for positive temp @30*c but what we should do to get working this device for -20*C

From Data sheet , we know it will be operational temp -40 to 150*c

Pl send ur comments at earliest

regards

prem

Prem,

I don't see anything wrong with the schematic. You asked before whether the TMP300 is capable of driving a PMOS load. The answer is no. The OUT pin is an open drain and is not buffered. You would have to buffer the output in order to drive the large gate capacitance. When you see the signal falling slowly, is the device connected to the PMOS device? 

Can you provide an oscilloscope shot showing this behavior with the time per division shown? This will help us better understand how fast the output is driven low. 

Prema,

I have modeled your setup with the output pin driving a capacitive load. This capacitive load is a typical C_gs (Gate source capacitance) for a PMOS that can handle the kind of current you have referenced. You can see below that the output will not immediately transition from a high to a low with this load. As this capacitance is increased the fall time increases as well. My recommendation is to drive the PMOS through a buffer.

dear Chris,

We had tested the TMP300 output without MOSFET drive, only with external pull up.

The output is checked on Digital multimeter. The rate of falling was quite slow(in sec).

If TMP300 can't work at negative temperature can you suggest any other temp switch from TI which will work at threshold -15/-20 deg cel.

regards,

prem

Prem,

I will check the device with the setup you have provided at negative temperatures and see if I run into the same issue. If you could provide the board layout then that would be useful in determining if we are dealing with any parasitic effects that could cause this behavior.

Dear Chris,

Thank you very much.

My project is on hold due to this issue. your early reply is realy appreciated.

prem

Prema:

On the schematic you sent, pin 5 VTEMP is shown connected to ground. I do not believe this will allow the part to function properly.

Please refer to Figure 14 of the data sheet TMP300B-Q1, SBOS586, and try that configuration.

Thanks

Mark

Dear Mr. Chris Featherstone,

The schematic which I have send to you, is without bypass capacitors. Earlier we checked with bypass capacitor that time I was not able to see any downfall at output.

Temp set was -25 deg Cel. and We were trying to cool the temperature switch using "ICE bag". But we don't know exactly whether temperature is really reaching to -25 deg cel or not.

Well, I would like to know, the above captured waveforms are for what temperature set. 

And if it is on negative side, how you have subjected IC in negative temp environment.

I will try again with pin 5 floating and/or with by pass capacitor and update you the result.

Thanking you,

Regards

Prem

Prem,

This is what I used during my tests.

TMP300 Configuration:

I used M.G. Chemicals Super Cold 134 Plus as well as Thermal Cool hot and therapeutic pad from walgreens and obtained the same results above. The Super Cold spray reaches temperatures of -51^oC as seen below. This was sprayed on the device package.  The thermal cool pad was placed in the freezer for 5 hours. The pad reached 32^oF or 0^oC. I placed the pad on the device package and ground lead in two separate tests.

Super Cold Spray:

Thermal Cool Pad Cold retention properties:

Dear Mr. cheris,

We have tested the circuit with pin 5 floating and with bypass cap too. The output is tripping.....

But I have a doubt.....As shown in datasheet....pin 3 (OUT#) is a active low pin...which means...it should be high @ normal condition and should go low when set temperature is reached.

We have observed something reverse. (Which is actually same as the waveforms that you have attached)

1. At room temperature (T> Tset) Vout=0.028V (Low)

2. After cooling (when T<Tset) ...Vout=8.5V  (High)

3. After Heating IC again  (T>Tset).....Vout=0.028V    (Low)......

This is I found opposite to pin3 active low behavior... ..

Kindly clarify,

Prem

Chris, 

Thanks for your replay.

So finally the conclusion is I have to use an inverted gate driver to drive a PMOS.

Because I need PMOS to ON when output is high and OFF for low output.

thanks for quick reply.

Regards,

Prem

Prem,

No problem. I'm glad we could help out.