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SMBUS ALERT FUNCTION

UNDERVOLTAGE LOCKOUT

GENERAL CALL RESET

SHUTDOWN MODE (SD)

IDENTIFICATION REGISTERS

SENSOR FAULT

FILTERING

TMP400

SBOS404 – DECEMBER 2007

The TMP400 supports the SMBus Alert function. The

The TMP400 senses when the power-supply voltage

ALERT pin of the TMP400 may be connected as an

has reached a minimum voltage level for the ADC

SMBus Alert signal. When a master detects an alert

converter to function. The detection circuitry consists

condition on the ALERT line, the master sends an

of a voltage comparator that enables the ADC

SMBus Alert command (00011001) on the bus. If the

converter after the power supply (V+) exceeds 2.45V

ALERT pin of the TMP400 is active, the device

(typical). The comparator output is continuously

acknowledges

the

SMBus

Alert

command

and

checked during a conversion. The TMP400 does not

respond by returning its slave address on the SDA

perform a temperature conversion if the power supply

line. The eighth bit (LSB) of the slave address byte

is not valid. The last valid measured temperature is

indicates whether the temperature exceeding one of

used for the temperature measurement result.

the temperature high limit settings or falling below

one of the temperature low limit settings caused the

alert condition. This bit is high if the temperature is

The TMP400 supports reset via the Two-Wire

greater than or equal to one of the temperature high

General

Call

address

00h

(0000

0000b).

The

limit settings; this bit is low if the temperature is less

TMP400 acknowledges the General Call address and

than one of the temperature low limit settings. See

responds to the second byte. If the second byte is

Figure 17 for details of this sequence.

06h (0000 0110b), the TMP400 executes a software

If multiple devices on the bus respond to the SMBus

reset, while latching the status of the address pins.

Alert command, arbitration during the slave address

This software reset restores the power-on reset state

portion of the SMBus Alert command determines

to all TMP400 registers, aborts any conversion in

which device will clear its alert status. If the TMP400

progress, and clears the ALERT pin. If the second

wins the arbitration, its ALERT pin becomes inactive

byte is 04h (0000 0100b), the TMP400 latches the

at the completion of the SMBus Alert command. If the

status of the address pins, but does not reset. The

TMP400 loses the arbitration, the ALERT pin remains

TMP400 takes no action in response to other values

active.

in the second byte. A 500

µs time delay must be taken

after a general call command.

The TMP400 Shutdown Mode allows the user to save

maximum power by shutting down all device circuitry

The TMP400 allows for the Two-Wire bus controller

other than the serial interface, reducing current

to query the device for manufacturer and device IDs

consumption to typically less than 3

µA; see typical

to allow for software identification of the device at the

characteristic curve Shutdown Quiescent Current vs

particular Two-Wire bus address. The manufacturer

Supply Voltage (Figure 10). Shutdown Mode is

ID is obtained by reading from pointer address FEh.

enabled when the SD bit of the Configuration

The device ID is obtained by reading from pointer

address FFh. The TMP400 returns 55h for the

current conversion is completed. When SD is low, the

manufacturer code and 01h for the device ID. These

device maintains a continuous conversion state.

registers are read-only.

The TMP400 senses a fault at the D+ input resulting

Remote junction temperature sensors are usually

from incorrect diode connection or an open circuit.

implemented in a noisy environment. Noise is most

The

detection

circuitry

consists

voltage

often created by fast digital signals, and it can corrupt

comparator that trips when the voltage at D+ exceeds

measurements. The TMP400 has a built-in 65kHz

(V+) – 0.6V (typical). The comparator output is

filter on the inputs of D+ and D– to minimize the

continuously checked during a conversion. If a fault is

effects of noise. However, a bypass capacitor placed

detected, the result reads 7FFh (0111 1111 1111b)

differentially

across

the

inputs

the

remote

and is used for the temperature measurement result;

temperature sensor is recommended to make the

the OPEN bit (Status Register, bit 2) is set high, and,

application more robust against unwanted coupled

if the alert function is enabled, ALERT asserts low.

signals. The value of the capacitor should be

between 100pF and 1nF. Some applications attain

When not using the remote sensor with the TMP400,

better

overall

accuracy

with

additional

series

the D+ and D– inputs must be connected together to

resistance; however, this increased accuracy is

prevent meaningless fault warnings.

setup-specific. When series resistance is added, the

value should not be greater than 3k

Ω and resistance

correction must be enabled (RC = 1).

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