Philips Semiconductors

Product data sheet

SA56004X

±1 °C accurate, SMBus-compatible, 8-pin, remote/local

digital temperature sensor with over temperature alarms

2004 Oct 06

17

Remote diode selection

To measure the remote temperature or the temperature of an

externally attached diode, the device automatically forces two

successive currents of about 160

µA and 10 µA at D+ pin. It

measures the voltage (VBE) between D+ and D–, detects the

difference between the two VBEs or the

∆VBE and then converts the

∆VBE into a temperature data using the basic PTAT voltage formula

as shown below. The device typically takes about 38 ms to perform

a measurement during each conversion period or cycle, which is

selectable by programming the conversion rate register.

DVBE + n

kT

q

ln I2

I1

where:

n: Diode ideality factor

k: Boltzmann’s constant

T: Absolute temperature (

° K) = 273 °C + T (°C)

q: Electron charge

ln: Natural logarithm

I2, I1: Two source currents

Because the device does not directly convert the sensed VBE as in

the old method of temperature measurement systems, the VBE

calibration is not required. Furthermore, the device remote

temperature error is adjusted at the manufacturer to meet the

specifications with the use of the reference diode-connected

transistors such as the 2N3904/2N3906. The diode type to be used

in customer applications must have the characteristics as close to

the 2N3904/2N3906 as possible in order to obtain optimal results.

Finally, to prevent the effects of system noise on the measured VBE

signals, an external capacitor of about 2200 pF connected between

the D+ and D– pins as well as the grounded-shield cable for the

diode connection wires are recommended.

Diode fault detection

The SA56004X is designed with circuitry to detect the fault

or floating, the Remote Temperature High Byte (RTHB) register is

loaded with +127

°C, the Remote Temperature Low Byte (RTLB)

register is loaded with 0

°C, and the OPEN bit (bit 2 of the Status

floating, if the Remote T_CRIT setpoint is set less than +127

°C, and

T_CRIT Mask are disabled, then, the T_CRIT output pins will be

pulled LOW. Furthermore, if the Remote HIGH Setpoint High Byte

(RHSHB) register is set to a value less than +127

°C and the Alert

Mask is disabled, then the ALERT output will be pulled LOW.

Note: the OPEN bit itself will not trigger an ALERT.

When the D+ pin is shorted to ground or to D–, the Remote

Temperature High Byte (RTHB) register is loaded with –128

°C

(1000 0000) and the OPEN (bit 2 in the Status register) will not be

set. Since operating the SA56004X is beyond its normal limits, this

temperature reading represents this shorted fault condition. If the

value in the Remote Low Setpoint High Byte (RLSHB) register is

more than –128

°C and the Alert Mask is disabled, the ALERT

output will be pulled LOW.

SMBus INTERFACE

The device can communicate over a standard two-wire serial

using SCLK and SDATA. The device employs four standard SMBus

protocols: Write Byte, Read Byte, Receive Byte, and Send Byte.

Data formats of four protocols are shown in Figure 17. The following

key points of protocol are important:

1. The SMBus master initiates data transfer by establishing a

START condition (S) and terminates data transfer by generating

a STOP condition (P).

2. Data is sent over the serial bus in sequences of 9 clock pulses

according to each 8-bit data byte followed by 1-bit status of

device acknowledgement (A).

3. The 7-bit slave address is equivalent to factory-programmed

address of the device.

4. The command byte is equivalent to the address of the selected

device register.

5. The receive byte format is used for quicker transfer data from a

device reading register which was previously selected.

Serial interface reset

If the SMBus master attempts to reset the SA56004X while the

SA56004X is controlling the data line and transmitting on the data

line, the SA56004X must be returned to a known state in the

communication protocol. This may be accomplished in two ways:

1. When the SDATA is LOW, the SA56004X SMBus state machine

resets to the SMBus idle state if SCLK is held LOW for more

than 35 ms (maximum TIMEOUT period). According to

SMBus

specification 2.0, all devices are required to time-out when the

SCLK line is held LOW for 25 to 35 ms. Therefore, to insure a

time-out of all devices on the bus, the SCLK line must be held

LOW for at least 35 ms.

2. When the SDATA is HIGH, the master initiates an SMBus start.

The SA56004X will respond properly to a SMBus start condition

only during the data retrieving cycle. After the start, the

SA56004X will expect a SMBus Address byte.