1.0 Functional Description

The LM91 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

LM91 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

LM91 always powers up to these known default states:

1.

Local Temperature set to 0˚C

2.

Remote Temperature set to 0˚C until the LM91 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 T_CRIT set to 127˚C and Remote T_CRIT set to

95˚C.

1.3 SMBus INTERFACE

The LM91 operates as a slave on the SMBus, so the

SMBCLK line is an input (no clock is generated by the LM91)

and the SMBData line is bi-directional. According to SMBus

specifications, the LM91 has a 7-bit slave address. Bit 4 (A3)

of the slave address is hard wired inside the LM91 to a 1.

The remainder of the address bits are controlled by the

address select pins ADD1 and ADD0, and are set by con-

necting these pins to ground for a low, (0) , to V

(1), or left floating (TRI-LEVEL).

Therefore, the complete slave address is:

A6

A5

A4

A3

A2

A1

A0

MSB

LSB

and is selected as follows:

Address Select Pin State

LM91 SMBus Slave

Address

ADD0

ADD1

A6:A0 binary

0

0

001 1000

0

TRI-LEVEL

001 1001

0

1

001 1010

TRI-LEVEL

0

010 1001

TRI-LEVEL

TRI-LEVEL

010 1010

TRI-LEVEL

1

010 1011

1

0

100 1100

1

TRI-LEVEL

100 1101

1

1

100 1110

The LM91 latches the state of the address select pins during

the first read or write on the SMBus. Changing the state of

the address select pins after the first read or write to any

device on the SMBus will not change the slave address of

the LM91.

1.4 TEMPERATURE DATA FORMAT

Temperature data can be read from the Local Temperature,

Remote Temperature, and T_CRIT setpoint registers. Tem-

perature data can only be written to the T_CRIT setpoint

registers. Temperature data is represented by an 8-bit, two’s

complement byte with an LSB (Least Significant Bit) equal to

1˚C:

Temperature

Digital Output

Binary

Hex

+125˚C

0111 1101

7Dh

+25˚C

0001 1001

19h

+1˚C

0000 0001

01h

0˚C

0000 0000

00h

−1˚C

1111 1111

FFh

−25˚C

1110 0111

E7h

−55˚C

1100 1001

C9h

1.5 OPEN-DRAIN OUTPUTS

SMBData and T_CRIT_A outputs are open-drain and do not

have internal pull-ups. A “high” level will not be observed on

these pins until pull-up current is provided from some exter-

nal source, typically a pull-up resistor. Choice of resistor

value depends on many system factors but, in general, the

pull-up resistor should be as large as possible. This will

minimize any local temperature reading errors due to self

heating of the LM91. The maximum resistance of the pull-up,

based on LM91 specification for High Level Output Current,

to provide a 2V high level, is 30 k

Ω.

1.6 DIODE FAULT DETECTION

Before each remote conversion the LM91 goes through an

external diode fault detection sequence. If the D+ input is

20034306

FIGURE 3. T_CRIT_A Temperature Response Diagram

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