LM34: LM34DM

Chandra Shekar L M

Part Number: LM34
Other Parts Discussed in Thread: LMV331

Hi,

In one of our Ethernet board design, we are used temperature sensor LM34DM. LM34 is interfaced to comparator LMV331.

We have set the LM34DM to 153 F Turn On Temperature to shut down the board.

We have seen some error output on LM34DM even if the temperature is not reaching 153 F, the system is getting shutting down and coming-up.

Please let us know what was the problem.

Thanks

Chandra

over 4 years ago

0 David Vaseliou over 4 years ago

TI__Expert 6820 points

Hi Chandra,

Can you please provide your schematic and scope captures of the error of the output of the LM34DM?

Thanks,
David

0 Chandra Shekar L M over 4 years ago

Prodigy 50 points

Hi David,

Please find the schematics below

We are measured the voltages across.

1. Voltage threshold setting coming to U21 Pin 3 is 1.163V(This is measured on multi-meter)

2. The voltage across TP2(which is output from temperature sensor U14 pin 1) captured through oscilloscope.

In oscilloscope we set the trigger level to 1.16V and started trigger. Before board goes to shut-down, we can see the trigger.

The following oscilloscope capture figure will depicts the same.

We are captured the voltage across R133 when board gets shut-off.

Before board get shut-off, the voltage across R133 = 0.920V

Thanks & Regards
L M Chandra shekara
GL Communications India Private Ltd
#1, Ground Floor, Uma Admiralty
Bannerghatta Road
Bangalore - 560029

Voice : +91-080-40488415

E-Mail :chandrashekara@gl.com
http://www.gl.com
IP Phone : 301 670 4784 x360

0 David Vaseliou over 4 years ago in reply to Chandra Shekar L M

TI__Expert 6820 points

Hi Chandra,

From the scopeshot on your other post it looks like there is a voltage drop of about 240mV when High and 40mV when low across R133.

Can you help clarify a few things?

First, when you say "The scope capture, yellow shows before R133 and blue shows after R133.", is yellow at voltage from TP2 to GND and blue voltage of Pin1 of U21 to GND?

Secondly, when shutting down board, what exactly are you shutting down? Is the LM34 being shutdown?

Lastly, what temperature do you expect the device to be at? A reading of 1.163V at TP2 to GND should be around 116.3 degrees F.

Thanks,
David

0 Chandra Shekar L M over 4 years ago

Prodigy 50 points

Hi David,

As per our schematic, we are expecting trip at 147.3 degree F, but it was going down at 116.3 degree F.

Please suggest us what was problem with schematic?

0 David Vaseliou over 4 years ago in reply to Chandra Shekar L M

TI__Expert 6820 points

Hi Chandra,

Can you please clarify the above information. Also. can you also explain to be your test procedure? Are you ramping temperature until you see the comparator go high?

Can you get a scopeshot of LM34 pin 1 with the U21 pins 1,3,4?

Thanks,
David

0 Chandra Shekar L M over 4 years ago

Prodigy 50 points

Hi David,

Please find the following details for testing procedure.

For over temperature shut-down in 10G Ethernet analyzer design , we used LM34DM with LMV331.

When we start Ethernet analyzer application , after within 2-3 minutes, the board shuts-off even if temperature is within a limits.

The temperature is monitored by multi-meter by placing multi-meter lead on top of Ethernet Analyzer ASIC, and it was shown 107 degree F (41 degree Celsius).

The board shut-down has to happen for 116 degree F, but it was happening below 116 degree F.

Please let us know why LM34 is behaving incorrectly?

The temperature starts ramping from 95 degree F to 107 degree F.

1. LM34 pin1

2. U21 Pin 3 is measured on multi-meter is 1.163V

3. U21 Pin 1

4. U21 pin 4 after transistor

0 David Vaseliou over 4 years ago in reply to Chandra Shekar L M

TI__Expert 6820 points

Hi Chandra,

The LM34 temperature output will be the die temperature of our device which may not be the same as measuring the case temperature of the Ethernet Analyzer ASIC. If the devices are connected through a copper trace/plane the die temperature could be higher than what you are reading from your multimeter and is shutting down the board at the expected temperature level.

Thanks,
David

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LM34: LM34DM