Temperature Sensors for Latest DRAM Memory Module Standard

The faster DDR2 and DDR3 (double-data-rate) DRAM memories installed in computers and other systems increase the risk of memory overheating, so temperature sensing, which allows the CPU to regulate the speed of the memory traffic or take further action to prevent overheating, has become more important in system design.

The new STTS424 is a standalone digital temperature sensor, while the STTS424E02 integrates the temperature sensor with a 2 Kbit Serial Presence Detect (SPD) EEPROM that is used on DIMMs to store timing parameters and other information about the module, in accordance with JEDEC requirements. Both devices communicate with the CPU chipset through an I2C/SMBus-compatible serial bus, and are available in 2 x 3mm DFN8 and 4.4 x 3mm TSSOP8 packages.

The STTS424 sensor has a 50% lower operating current than competitive devices and the JEDEC JC42.4 standard. The integrated STTS424E02 can replace the 2 Kbit SPD EEPROM of existing memory module implementations, thereby adding temperature sensing capability with minimum board redesign cost, and with no increase in the number of components. For programming ease, the temperature sensor and the EEPROM in the STTS424E02 each have their own unique I2C/SMBus address.

The new sensor chips – application-specific members of ST’s temperature-sensor family – integrate an analog-to-digital converter (ADC) which monitors and digitizes the temperature with a resolution down to 0.25 degrees C, and an accuracy of +/–1 degree C, over the +75 to +95 degrees C temperature range. The devices are factory calibrated and require no external components for temperature measurement. They can operate on a supply voltage from 2.7V to 3.6V, and over a -40 to +125 degrees C temperature range. The typical supply current at 3.3V is 100 microamps.

An open-drain EVENT output pin is active either if the monitored temperature exceeds a programmed limit or if it strays outside the upper or lower limit of a customer-programmed Alarm window. The pin can be configured to operate either in a comparator mode for thermostat operation, or in the interrupt mode. The 2-Kbit serial EEPROM in the STTS424E02 has the ability to permanently lock the data in the first 128bytes, for use in DDR DIMMs with SPD.