LM99 datasheet(10/20 Pages) NSC

10 / 20 page

1.0 Functional Description (Continued)

Remote temperature data is represented by an 11-bit, two’s

complement word with an LSB (Least Significant Bit) equal

to 0.125˚C. The data format is a left justified 16-bit word

available in two 8-bit registers:

Actual vs. LM99 Remote Temperature Conversion

Actual Remote Diode

Temperature,˚C

LM99 Remote Diode

Temperature Register, ˚C

Binary Results in LM99

Remote Temperature Register

Hex Remote

Temperature

120

+104

0110 1000 0000 0000

6800h

125

+109

0110 1101 0000 0000

6D00h

126

+110

0110 1110 0000 0000

7100h

130

+114

0111 0010 0010 0000

7200h

135

+119

0111 0111 0000 0000

7700h

140

+124

0111 1100 0000 0000

7200h

Output is 11-bit two’s complement word. LSB = 0.125 ˚C.

Actual vs. Remote T_Crit Setpoint

Actual Remote Diode

T_Crit Setpoint,˚C

Factory-Programmed

Remote T_CRIT High

Setpoint, ˚C

Binary Remote T_CRIT High

Setpoint Value

Hex Remote T_CRIT

High Setpoint Value

126

+110

0110 1110

71h

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

complement byte with an LSB (Least Significant Bit)

equal to 1˚C:

Local

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

1.7 OPEN-DRAIN OUTPUTS

The SMBData, ALERT and T_CRIT_A outputs are open-

drain outputs and do not have internal pull-ups. A “high” level

will not be observed on these pins until pull-up current is

provided by some external source, typically a pull-up resis-

tor. Choice of resistor value depends on many system fac-

tors but, in general, the pull-up resistor should be as large as

possible. This will minimize any internal temperature reading

errors due to internal heating of the LM99. The maximum

resistance of the pull-up to provide a 2.1V high level, based

on LM99 specification for High Level Output Current with the

supply voltage at 3.0V, is 82 k

Ω (5%) or 88.7 kΩ (1%).

1.8 DIODE FAULT DETECTION

The LM99 is equipped with operational circuitry designed to

detect fault conditions concerning the remote diode. In the

event that the D+ pin is detected as shorted to V

DD 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, and the OPEN bit (D2) in

the status register is set. As a result, if the Remote T_CRIT

setpoint register (RCS) is set to a value less than +127˚C the

ALERT and T_Crit output pins will be pulled low, if the Alert

Mask and T_Crit Mask are disabled. If the Remote HIGH

Setpoint High Byte Register (RHSHB) is set to a value less

than +127˚C then ALERT will be pulled low, if the Alert Mask

is disabled. The OPEN bit itself will not trigger and ALERT.

In the event that the D+ pin is shorted to ground or D−, the

Remote Temperature High Byte (RTHB) register is loaded

with −128˚C (1000 0000) and the OPEN bit (D2) in the status

is beyond it’s operational limits, this temperature reading

represents this shorted fault condition. If the value in the

Remote Low Setpoint High Byte Register (RLSHB) is more

than −128˚C and the Alert Mask is disabled, ALERT will be

pulled low.

Remote diode temperature sensors that have been previ-

ously released and are competitive with the LM99 output a

code of 0˚C if the external diode is short-circuited. This

change is an improvement that allows a reading of 0˚C to be

truly interpreted as a genuine 0˚C reading and not a fault

condition.

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