LM99

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SNIS129D – JANUARY 2003 – REVISED MARCH 2013

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 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 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%).

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 VDD 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 register will not be set. Since operating the

LM99 at

−128°C 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 previously 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.

COMMUNICATING WITH THE LM99

The data registers in the LM99 are selected by the Command Register. At power-up the Command Register is

set to “00”, the location for the Read Local Temperature Register. The Command Register latches the last

location it was set to. Each data register in the LM99 falls into one of four types of user accessibility:

1. Read only

2. Write only

3. Read/Write same address

4. Read/Write different address

A Write to the LM99 will always include the address byte and the command byte. A write to any register requires

one data byte.

Reading the LM99 can take place either of two ways:

1. If the location latched in the Command Register is correct (most of the time it is expected that the Command

Register will point to one of the Read Temperature Registers because that will be the data most frequently

read from the LM99), then the read can simply consist of an address byte, followed by retrieving the data

byte.

2. If the Command Register needs to be set, then an address byte, command byte, repeat start, and another

address byte will accomplish a read.

The data byte has the most significant bit first. At the end of a read, the LM99 can accept either Acknowledge or

No Acknowledge from the Master (No Acknowledge is typically used as a signal for the slave that the Master has

read its last byte). It takes the LM99 31.25 ms to measure the temperature of the remote diode and internal

diode. When retrieving all 10 bits from a previous remote diode temperature measurement, the master must

insure that all 10 bits are from the same temperature conversion. This may be achieved by using one-shot mode

or by setting the conversion rate and monitoring the busy bit such that no conversion occurs in between reading

the MSB and LSB of the last temperature conversion.

Copyright © 2003–2013, Texas Instruments Incorporated

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