0.01

0.1

1.0

10

100

CONVERSION RATE (Hz)

400

600

800

1000

1200

140

0

1600

1800

2000

LM99

SNIS129D – JANUARY 2003 – REVISED MARCH 2013

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FUNCTIONAL DESCRIPTION

diode and a 10-bit plus sign

ΔΣ ADC (Delta-Sigma Analog-to-Digital Converter). The LM99 is compatible with the

serial SMBus version 2.0 two-wire interface. Digital comparators compare the measured Local Temperature (LT)

to the Local High (LHS), Local Low (LLS) and Local T_CRIT (LCS) user-programmable temperature limit

registers. The measured Remote Temperature (RT) is digitally compared to the Remote High (RHS), Remote

Low (RLS) and Remote T_CRIT (RCS) user-programmable temperature limit registers. Activation of the ALERT

output indicates that a comparison is greater than the limit preset in a T_CRIT or HIGH limit register or less than

the limit preset in a LOW limit register. The T_CRIT_A output responds as a true comparator with built in

hysteresis. The hysteresis is set by the value placed in the Hysteresis register (TH). Activation of T_CRIT_A

occurs when the temperature is above the T_CRIT setpoint. T_CRIT_A remains activated until the temperature

goes below the setpoint calculated by T_CRIT

− TH. The hysteresis register impacts both the remote

temperature and local temperature readings.

The LM99 may be placed in a low power consumption (Shutdown) mode by setting the RUN/STOP bit found in

the Configuration register. In the Shutdown mode, the LM99's SMBus interface remains while all circuitry not

required is turned off.

The Local temperature reading and setpoint data registers are 8-bits wide. The format of the 11-bit remote

temperature data is a 16-bit left justified word. Two 8-bit registers, high and low bytes, are provided for each

setpoint as well as the temperature reading. Two offset registers (RTOLB and RTOHB) can be used to

compensate for non–ideality error, discussed further in DIODE NON-IDEALITY. The remote temperature reading

reported is adjusted by subtracting from, or adding to, the actual temperature reading the value placed in the

offset register.

CONVERSION SEQUENCE

The LM99 takes approximately 31.25 ms to convert the Local Temperature (LT), Remote Temperature (RT), and

to update all of its registers. Only during the conversion process the busy bit (D7) in the Status register (02h) is

high. These conversions are addressed in a round–robin sequence. The conversion rate may be modified by the

Conversion Rate Register (04h). When the conversion rate is modified a delay is inserted between conversions;

however, the actual conversion time remains at 31.25 ms. Different conversion rates will cause the LM99 to draw

different amounts of supply current as shown in Figure 4.

Figure 4. Conversion Rate Effect on Power Supply Current

THE ALERT OUTPUT

The LM99's ALERT pin is an active-low open-drain output that is triggered by a temperature conversion that is

outside the limits defined by the temperature setpoint registers. Reset of the ALERT output is dependent upon

the selected method of use. The LM99's ALERT pin is versatile and will accommodate three different methods of

use to best serve the system designer: as a temperature comparator, as a temperature–based interrupt flag, and

as part of an SMBus ALERT system. The three methods of use are further described below. The ALERT and

interrupt methods are different only in how the user interacts with the LM99.

8

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