LM35 / SOT23?

[Fred Bartoli]

I've a very small PCB, diam 15(mm of course ) as full as it can be.

I need a temp sensor 10mV/K - 0°C->0V - SOT23 or smaller.

There's the LM45-SOT23, but I do need better supply voltage compliance, and
I can't find the LM35 or equiv. in a small package.

Any hint?

 

[Spehro Pefhany]

I've a very small PCB, diam 15(mm of course ) as full as it can be.

I need a temp sensor 10mV/K - 0°C->0V - SOT23 or smaller.

There's the LM45-SOT23, but I do need better supply voltage compliance, and
I can't find the LM35 or equiv. in a small package.

Any hint?

Microchip MCP9700 except 0°C is 500mV. It works down to 2.3V w/output
+10mV/K. The LM50 is functionally similar, but without the low voltage
capability.


Best regards,
Spehro Pefhany

 

[John Larkin]

I've a very small PCB, diam 15(mm of course ) as full as it can be.

I need a temp sensor 10mV/K - 0°C->0V - SOT23 or smaller.

There's the LM45-SOT23, but I do need better supply voltage compliance, and
I can't find the LM35 or equiv. in a small package.

Any hint?

What do you mean by "supply voltage compliance"?

John

 

[Fred Bartoli]

Microchip MCP9700 except 0°C is 500mV. It works down to 2.3V w/output
+10mV/K. The LM50 is functionally similar, but without the low voltage
capability.

Thanks Spehro.

I've already spotted those and also the TC4017, but neither will do.

Unfortunately I realise I wasn't clear enough about the voltage compliance:
they have to be mounted inside a constant current supplied 2 wires sensor
that settles its average bias point from 6V to maybe 16-17V on the whole
temperature range.
And the 0degreeC->0V is mandatory too, since we have to align on
competition, which is a defacto standard.

The LM35 is the ideal device, except the packaging. I was hoping of maybe a
SOT23 packaged second source.

 

[Fred Bartoli]

What do you mean by "supply voltage compliance"?

It has to accept a supply voltage range from 5-6V up to at least 20V.

 

[John Larkin]

Thanks Spehro.

I've already spotted those and also the TC4017, but neither will do.

Unfortunately I realise I wasn't clear enough about the voltage compliance:
they have to be mounted inside a constant current supplied 2 wires sensor
that settles its average bias point from 6V to maybe 16-17V on the whole
temperature range.
And the 0degreeC->0V is mandatory too, since we have to align on
competition, which is a defacto standard.

The LM35 is the ideal device, except the packaging. I was hoping of maybe a
SOT23 packaged second source.

The '35 and '45 are (I think) the same chip, with the LM35 having a
higher max voltage rating based on self-heating effects.

But be warned: both have a lot of personality. Small spikes can make
either latch up, especially if the supply voltage is above 5. And the
datasheet suggests you can pull the output below ground to measure
negative temps, but that can make it latch, too. And they oscillate
with capacitive loads.

Bob Pease promised me he'd fix it, 10 years ago or so.

One remaining advantage of the Fahrenheit temp scale is that you can
get LM34's, which output zero volts at zero F!


John

 

[Spehro Pefhany]

The '35 and '45 are (I think) the same chip, with the LM35 having a
higher max voltage rating based on self-heating effects.

They look more to me like they were designed to be similar. Check out
the turn-on response curve and noise voltage graph, for example. More
relevant to Fred's application would be the abs max supply voltage
limit, unfortunately.

But be warned: both have a lot of personality. Small spikes can make
either latch up, especially if the supply voltage is above 5. And the
datasheet suggests you can pull the output below ground to measure
negative temps, but that can make it latch, too.

Hmmm.. the 1994, 1999 and 2000 data sheets all have the -1V abs max
output voltage limit (which is potentially exceeded by a circuit in
their 1986 application note).

Do you have an earlier datasheet? I think it came out in '83 (and it's
not in my 1982 NS databook).

And they oscillate with capacitive loads.

Being voltage-output devices..

Bob Pease promised me he'd fix it, 10 years ago or so.

Maybe they just 'fixed' the datasheet. ;-)

One remaining advantage of the Fahrenheit temp scale is that you can
get LM34's, which output zero volts at zero F!


John

What's the point of a circuit that will only work down to a couple
degrees C above freezing anyway?


Best regards,
Spehro Pefhany

 

[John Larkin]

They look more to me like they were designed to be similar. Check out
the turn-on response curve and noise voltage graph, for example. More
relevant to Fred's application would be the abs max supply voltage
limit, unfortunately.


Hmmm.. the 1994, 1999 and 2000 data sheets all have the -1V abs max
output voltage limit (which is potentially exceeded by a circuit in
their 1986 application note).

There is apparently a substrate diode to Vout. Or more precisely, a
substrate SCR.

What's especially nasty is if you do the pulldown thing before you
apply Vcc. A tiny amount of substrate current can then make it latch
up when the main power is applied.

Do you have an earlier datasheet? I think it came out in '83 (and it's
not in my 1982 NS databook).


Being voltage-output devices..


Maybe they just 'fixed' the datasheet. ;-)


What's the point of a circuit that will only work down to a couple
degrees C above freezing anyway?

If you use the F part, the LM34, you can measure down to ... computes
furiously ... -17.8 C without doing the dangerous negative pulldown
thing.

After all, it never gets colder than 0F anywhere in the civilized
world.

John

 

[Robert Baer]

Thanks Spehro.

I've already spotted those and also the TC4017, but neither will do.

Unfortunately I realise I wasn't clear enough about the voltage compliance:
they have to be mounted inside a constant current supplied 2 wires sensor
that settles its average bias point from 6V to maybe 16-17V on the whole
temperature range.
And the 0degreeC->0V is mandatory too, since we have to align on
competition, which is a defacto standard.

The LM35 is the ideal device, except the packaging. I was hoping of maybe a
SOT23 packaged second source.

Buy the chips, mount them on the board, and wire bond to the traces.
Slightly smaller than the package...