1°C Triple SMBus Temperature Sensor

Datasheet

SMSC EMCT03

7

Revision 2.3 (04-19-05)

DATASHEET

Figure 3.2 Block Diagram of Temperature Measurement Circuit

Figure 3.2 shows a detailed block diagram of the temperature measurement circuit. As shown, the

EMCT03 incorporates switched capacitor technology that samples the external remote temperature

diode voltage at two bias currents and holds the difference voltage. The sample frequency is 100kHz

and the current levels Ihigh and Ilow are 170uA and 10uA respectively. The negative terminal for the

remote temperature diode, DN, is internally biased with a forward diode voltage referenced to ground.

The output of the switched capacitor sample and hold circuit interfaces to a single-bit sigma delta

analog-to-digital converter. This ADC runs at 100kHz sample frequency and its output is digitally filtered

and averaged over 2048 samples effectively generating 11 bit accuracy.

The advantages of this architecture over Nyquist rate FLASH or SAR converters are superb linearity

and inherent noise immunity. The linearity can be directly attributed to the sigma delta ADC single-bit

comparator while the noise immunity is achieved by the digital averaging filter. The overall effective

bandwidth of the system is fs/2048 which translates to a 50Hz bandwidth at 100kHz sample rate.

Conversion time equals about 20ms per temperature monitor which equals 60ms total for three

monitors when configured for maximum conversion rate (default).

The 11 bit conversion can be displayed in either legacy format or in extended range format. In Legacy

format, the temperature range covers –64ºC to 127ºC while in extended format, temperature readings

span -64ºC to 191ºC. It should be noted that the latter range is really meant to cover thermal diodes

with a non ideal curvature caused by factor n in equation (1) not being equal to exactly 1.000. In

general, it is not recommended to run silicon based thermal diodes at temperatures above 150ºC.

3.2

System Management Bus Interface Protocol

The EMCT03 communicates with a host controller, such as an SMSC SIO, through the SMBus. The

SMBus is a two wire serial communication protocol between a computer host and its peripheral

devices. Detailed timing diagrams can be found in the electrical characteristics of the SMBus. The

EMCT03 is SMBus 2.0 compatible and supports Write Byte and Read Byte as valid protocols as shown

below:

fs

Delta Vbe

Sample

&

Hold

I

bias

I

low

I

high

fs

1-bit

Sigma

Delta

Modulator

fs

Digital

Averaging

Filter

fs/2048

11-bit

Accurate

Conversion

V

DD

Internal or

Remote Diode

Bias

Diode