MLX90614 family
Single and Dual Zone
Infra Red Thermometer in TO-39
8.8 Thermal relay
The MLX90614 can be configured to behave as a thermo relay with programmable threshold and hysteresis
on the PWM/SDA pin. The input for the comparator unit of the relay is the object temperature from sensor 1
The output of the MLX90614 is NOT a relay driver but a logical output which should be connected to a
relay driver if necessary.
In order to configure the MLX90614 to work as thermal relay two conditions must be met:
o
o
Set bit TRPWMB high at address 002h in EEPROM
Enable PWM output i.e. EN_PWM is set high
The PWM/SDA pin can be programmed as a push-pull or open drain NMOS (via bit PPODB in EEPROM
PWMCTRL), which can trigger an external device. The temperature threshold data is determined by
EEPROM at address 021h (Tomin) and the hysteresis at address 020h (Tomax).
The logical state of the PWM/SDA pin is as follows:
PWM/SDA pin is high if Tobj 1≥ threshold + hysteresis
PWM/SDA pin is low if Tobj1 ≤ threshold − hysteresis
“1”
hysteresis hysteresis
“0”
threshold
T
Figure 16: Thermal relay : “PWM” pin versus Tobj
The MLX90614 preserves its normal operation when configured as a thermal relay (PWM configuration and
specification applies as a general rule also for the thermal relay) and therefore it can be read using the
SMBus (entering the SMBus mode from both PWM and thermal relay configuration is the same).
For example, the MLX90614 can generate a wake-up alert for a system upon reaching a certain temperature
and then be read as a thermometer. A reset condition (enter-and exit Sleep, for example) will be needed in
order to return to the thermal relay configuration.
Example: threshold 5 °C => (5 + 273.15)*100 = 27815 = 6CA7h
hysteresis is 1°C => 1 * 100 = 100 = 64h
PWM/SDA pin will be low at object temperature below 4 °C
PWM/SDA pin will be high at object temperature higher that 6 °C
3901090614
Rev 003
Page 24 of 40
Data Sheet
03/Oct/2007
MELEXIS [ Melexis Microelectronic Systems ]
