LM 335 issues

2.9 volts, 2* 3.3 volts
Those LM335 issues as stated in my last blog entry are still bothering me. I did some additional tests and found out something interesting (at least for me). If the voltage drops below 3 volts, the LM335 begins to post erroneous values. That may be no surprise because the circuit has a typical operating output voltage of 2.98 volts (at 25°C) according to the datasheet (page 2 "Temperature Accuracy").

I wonder if 3.3 volts as used in the preceding experiments are enough power to operate the LM335. I couldn't find any minimum operating voltage specs in the datasheet except an application information on page 6 ("Minimum Temperature Sensing") where V+ is 15 volts for three temperature sensors. My guess is, apart from powering the LM335 with the needed current  through R1 (400µA to 5mA) you need at least 5 volts for stable operation. Tell me if I'm on the wrong track.
I'm also still wondering how to calculate R1, which in my understanding supplies the required current to the LM335. Any input from a reader would be very welcome.
At the moment I'm preparing chapter 6 "Sleeping, then changing the world" where operation with two AA-Batteries is used to power the end nodes. Having the questions above I'm wondering if any operation only from batteries and without voltage regulator is possible at all. The XBees will be no problem - they operate from 2.1 to 3.6 volts according to the datasheet (the PRO needs 3 to 3.4 volts). But what about the LM335?

Update: While surfing the web I found some entries how to calculate R1. As the temperature sensor acts like a voltage divider and everything bases on Ohms law you have to calculate the circuit voltage (in our experiment 3.3 volts) minus the LM335 voltage (depending on the temperature, at 25°C 2.98 volts) and divide it by the needed current (400µA to 5 mA). In short: R1= (Vcc - Vlm335)/ (0.4 to 5mA).
That gives you an envelope which you have to adjust to your expected temperature range.

Sounds a bit confusing? Here are some examples:

Vcc is 3.3 volts, temperature is 25°C (= 2.98volts Vout LM335):

R1 = (3.3V - 2.98V)/ 0.0004A = 800 ohms (400µA = 0.0004A)
R1 = (3.3V - 2.98V)/ 0.005A = 64 ohms (5mA = 0.005A)

So for "room temperature" (25°C) the suggested value of 300 ohms for the circuit works well (everything from 800 to 64 ohms actually should work at least exactly with 25°C ambient temperature according to the datasheet).

What if temperatur changes?

It's 10mV per °K so to reach 3.3V as output from the LM335, everything above 55°C might be a problem because supply voltage almost equals LM335 output voltage (2.73 volts + 0.55 volts = 3.28 volts). At -40°C (= 2,33 volts Vout LM335) and 64 ohms (as calculated above) you would exceed the current with a value of 6.25mA. With 300 ohms you are still safe (3.3mA current for LM335 at -40°C).

So from -40°C to ~55°C and 3.3 volts supply voltage taking 300 ohms for R1 we might be safe.

What if supply voltage changes?

Let's assume supply voltage drops to 2.93 volts generated by two (rechargeable) batteries. At "room temperature" (= 2.98 volts Vout LM335 at 25°C) we might have a problem (at least in a normal climate condition).
Temperature has to drop to about 10°C or lower (2,83 volts Vout LM335) where you could use a R1 from 2 ohms (5mA LM335 current) to 20 ohms (400 µA LM335 current). With the suggested 300 ohms you would fall below the required current for the LM335.

The options:
  1. You could use a second power supply/ step-up converter with at least 3.8 volts (according to the datasheet the LM335 operates up to 100°C). You might have to adjust the resistor value for R1 to your expected temperature range. Same for digital sensors like DS18S20/ DS18B20 because power supply ist 3 to 5.5 volts according to the datasheet.
  2. Take an alternative temperature sensor like the TMP36. This part needs a supply voltage from 2.7 to 5.5 volts which would be almost perfect for two-batterie powered devices and maybe also for a two rechargeable batterie-pack if the voltage stays above 1.35 volts each (I'm not sure how low the voltage might drop before the TMP36 quits working).
For more insight on temperature sensors there is a great tutorial at Ladyada.net.

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