1.0 Functional Description

The LM84 temperature sensor incorporates a band-gap type

temperature sensor using a Local or Remote diode and an

8-bit ADC (Delta-Sigma Analog-to-Digital Converter). The

LM84 is compatible with the serial SMBus and I

faces. Digital comparators compare Local and Remote read-

ings

to

user-programmable

setpoints

(LT_CRIT

and

RT_CRIT). Activation of the T_CRIT_A output indicates that

a temperature reading is greater than the limit preset in a

T_CRIT register.

1.1 T_CRIT_A OUTPUT, T_CRIT LIMITS

T_CRIT_A is activated when the Local temperature reading

is greater than the limit preset in the local critical temperature

setpoint register (LT_CRIT) or when the Remote tempera-

ture reading is greater than the limit preset in the remote

critical temperature setpoint register (RT_CRIT), as shown in

Figure 3. The T_CRIT_A mask bit (bit 7 of the Configuration

Register) when set will disable the T_CRIT_A output.

The Status Register can be read to determine which event

caused the alarm. A bit in the Status Register is set high to

indicate T_CRIT temperature alarm, see

Section 1.8.3.

Local and remote temperature diodes are sampled alter-

nately by the A/D converter. The T_CRIT_A output and the

Status Register flags are updated at the completion of a

conversion, which takes approximately 60 ms. T_CRIT_A

and the Status Register flags are reset only after the Status

Register is read and if the temperature is below the setpoint.

1.2 POWER-ON RESET DEFAULT STATES

LM84 always powers up to these known default states:

1.

Local Temperature set to 0˚C

2.

Remote Temperature set to 0˚C until the LM84 senses a

diode present or open circuit on the D+ and D− input

pins.

3.

Status Register set to 00h.

4.

Command Register set to 00h; T_CRIT_A enabled.

5.

Local and Remote T_CRIT set to 127˚C

DS100961-5

FIGURE 2. Temperature-to-Digital Transfer Function (Non-linear scale for clarity)

DS100961-6

FIGURE 3. T_CRIT_A Temperature Response Diagram

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