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ELM341 Low Power Thermostat
Description
The ELM341 is a complete temperature measurement and control system in an 8 pin package. It automatically reverts to a very low power sleep mode between measurements, making it suitable for battery operation. This integrated circuit is designed to compare two resistances and drive an output pin depending on the relative value of each. Typically, one of the resistors will be an NTC thermistor, and the other one will be a temperature independent resistor (whether fixed or variable). When the magnitude of the resistance connected to pin 2 exceeds the value of the resistance connected to pin 3, the output pin will be driven to a high state. Hysteresis maintains the output in that state until the relative values differ by approximately 8% (or typically 2C for a 10K thermistor). To reduce the possibility of sporadic outputs, a condition must exist for three successive cycles, or 6 seconds, before the output pin can change state.
Features
* * * * * * * * Low power CMOS design Wide supply range - 3.0 to 5.5 volt operation Built-in proportional hysteresis Measurement in progress output Time delay on operate improves noise immunity Internal pullup resistor on the reset input High current drive outputs - up to 25 mA Sleep mode reduces power requirements
Connection Diagram PDIP and SOIC (top view)
VDD R1
1 2 3 4
8 7 6 5
VSS Out MIP Cap
Applications
* Backup thermostats * Under or over temperature alarm circuits
R2 reset
Block Diagram
VDD reset
4
Control
Sleep Timer
2
Measurement in Progress (busy)
6
MIP
R1 R2
3
Overrange
7
Out
5
Analog to Digital Converter
R1 > R2
3 Consecutive Measurements
ELM341DSB
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ELM341
Pin Descriptions
VDD (pin 1) This pin is the positive supply pin. Internal circuitry connected to this pin is used to provide power on reset of the microprocessor, so an external reset signal is normally not required. Refer to the Electrical Characteristics section for further information. R1 (pin 2) One of the two resistance input pins. A temperature dependent resistance is usually connected to this input for heating or undertemperature alarm type applications. When the value of this resistor is greater than the value of the resistance connected to pin 3 (for three successive measurements) the output will be driven high. R2 (pin 3) The reference resistance is connected to this pin for heating applications (and the temperature dependent resistance is connected here for cooling applications). The other end of this resistor is connected to the integrating capacitor. reset (pin 4) The active low reset input. An internal pullup resistor is provided for convenience. If unused, this pin may be connected to VDD or left open. Cap (pin 5) Temperature measurements are made by determining the time to charge and discharge this integrating capacitor. Pin 5 forces the capacitor to a known voltage for these measurements though, resulting in large current flows. To limit these capacitor currents, and protect the ELM341, a series resistor must be connected to this pin. The value of the resistance, and of the capacitance, is not critical to the measurements. MIP (pin 6) This pin provides a logic high level output while the ELM341 is busy (measurements are in progress). It is suitable for directly driving an LED through a current limiting resistor. As a warning, this output pulses rapidly if either resistor input is found to be open circuited. Out (pin 7) The output pin assumes a logic high state once the resistance of R1 exceeds that of R2 for three successive measurement cycles. The output is maintained until R1 is less than R2 by the hysteresis amount for an additional three counts. Note: Consideration must be given to the fact that this output is in a tri-state (open circuit) mode each time the circuit wakes from sleep to take a measurement. This mode lasts for approximately 300s, which is not generally long enough to affect a relay output, but certainly long enough to be seen by high-speed logic circuits. VSS (pin 8) Circuit common is connected to this pin. This is the most negative point in the circuit.
Ordering Information
These integrated circuits are available in either the 300 mil plastic DIP format, or in the 200 mil SOIC surface mount type of package. To order, add the appropriate suffix to the part number: 300 mil Plastic DIP............................... ELM341P 200 mil SOIC..................................... ELM341SM
All rights reserved. Copyright (c)1999 Elm Electronics. Every effort is made to verify the accuracy of information provided in this document, but no representation or warranty can be given and no liability assumed by Elm Electronics with respect to the accuracy and/or use of any products or information described in this document. Elm Electronics will not be responsible for any patent infringements arising from the use of these products or information, and does not authorize or warrant the use of any Elm Electronics product in life support devices and/or systems. Elm Electronics reserves the right to make changes to the device(s) described in this document in order to improve reliability, function, or design. ELM341DSB
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ELM341
Absolute Maximum Ratings
Storage Temperature....................... -65C to +150C Ambient Temperature with Power Applied....................................-40C to +85C Voltage on VDD with respect to VSS............ 0 to +7.5V Voltage on any other pin with respect to VSS........................... -0.6V to (VDD + 0.6V) Note: Stresses beyond those listed here will likely damage the device. These values are given as a design guideline only. The ability to operate to these levels is neither inferred nor recommended.
Electrical Characteristics
All values are for operation at 25C and a 5V supply, unless otherwise noted. For further information, refer to note 1 below.
Characteristic
Supply Voltage, VDD VDD rate of rise Average Supply Current, IDD
Minimum
3.0 0.05 0.008 0.002
Typical
5.0
Maximum Units
5.5 V V/ms
Conditions
see note 2 VDD = 5V, see note 3 VDD = 3V, see note 3 see note 4 see note 5 see note 6
1.0 0.6 2.0
2.4 2.4
mA mA sec
Frequency of measurements Reset pin internal pullup resistance R1C or R2C time constant Input low voltage - reset pin Input high voltage - reset pin Output low voltage Output high voltage VDD - 0.7 300 500 VSS 0.85 VDD
470
600 500,000 0.15 VDD VDD 0.6
K s V V V V
Current (sink) = 8.7mA Current (source) = 5.4mA
Notes: 1. This integrated circuit is produced with a Microchip Technology Inc.'s PIC12C5XX as the core embedded
microcontroller. For further device specifications, and possibly clarification of those given, please refer to the appropriate Microchip documentation. 2. This spec must be met in order to ensure that a correct power on reset occurs. It is quite easily achieved using most common types of supplies, but may be violated if one uses a slowly varying supply voltage, as may be obtained through direct connection to solar cells, or some charge pump circuits. 3. Integrated circuit only. Does not include any LED or drive currents. Minimum currents represent those which are typically found between measurements when in the low power sleep mode. 4. If a measured resistance is determined to be out of limits, the frequency of measurements is increased to provide visual feedback as well as a faster recovery. 5. The value of the pullup resistance is supply and temperature dependent. 6. One should also maintain R1 and R2 to not less than about 5K. When C is chosen, select the pin 5 current limiting resistance so that RLIMC is less than 1msec, and RLIM is greater than 1K.
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ELM341
Example Application
Figure 1 shows the ELM341 in an example heating control circuit. A closed contact output occurs whenever the temperature measured by RTEMP falls to a value less than that determined by RSET. It is anticipated that this type of circuit could possibly be used to control temperatures over the range of -40C to +40C. Power for the control circuit is from a 3V battery, while the output relay is powered from a 12V supply. The output relay type is not important, as long as consideration is given to the coil drive requirements, and the capabilities of the ELM341. In this example, a relay with a 400 coil resistance was chosen so that a 2N3904 could drive it directly. For further reductions in current requirements, consideration could be given to using a power MOSFET transistor instead of this bipolar one. Temperature measuring is performed by RTEMP, which is a negative temperature coefficient type thermistor. It has a resistance of 10K at 25C, and this value decreases with increasing temperature. This value was chosen both because it is commonly available, and because it limits the 0.1F integrating capacitor currents to less than 1mA over the typical range of operation (keeping the thermistor self-heating to a minimum). If the thermistor is mounted any appreciable distance from the ELM341, consideration must be given to cabling effects such as capacitive and induced currents. Generally the integrated circuit can be adequately protected by mounting a small value (220) resistor physically close to the ELM341 as shown 3V 12V Relay 1N4001 To the heating control below. Take into account it's value when determining the setpoint, though. For this design, RSET was selected to be equal to the resistance of RTEMP at 10C, so that the relay contact closes for any measured temperatures less than 10C. The resistance value was determined from specs given by the manufacturer, but could have been determined experimentally as well. An LED has been provided for visual feedback of the circuit operation. It is connected to the `measurement in progress' output, so that it is energized each time a measurement is being made. Typically, this would be for about 25mS every 2 seconds. Current requirements for this entire circuit have been measured to be about 2A minimum and 23A average, at room temperature and the relay deenergized, even with the LED blinking. Measurement times, and thus current consumption, vary with temperature, so this should only be used as a guideline. With a 5V supply, these values escalate to about 8A and 70A, respectively. Just a reminder that consideration must be given to the fact that the pin 7 output is in a tri-state (open circuit) mode just prior to making a measurement. This mode lasts for approximately 300s, sufficient to possibly affect some output circuits. It is generally not sufficient for an electromechanical relay to drop out, however, especially with a `kick-back' diode across the coil.
+12V
see text
1 2 3 4
8 7 6 5
1.5K 2N3904
RTEMP 10K @25C
560 RSET 18K 10K LED 0.1F Figure 1. Backup Heating Control Thermostat
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