LM90

SNIS126A – MAY 2004 – REVISED MARCH 2013

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FUNCTIONAL DESCRIPTION

The LM90 temperature sensor incorporates a delta VBE based temperature sensor using a Local or Remote and

a 10-bit plus sign ADC (Delta-Sigma Analog-to-Digital Converter). The LM90 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 LM90 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 LM90'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 registers.

CONVERSION SEQUENCE

The LM90 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,

the actual conversion time remains at 31.25ms. Different conversion rates will cause the LM90 to draw different

amounts of supply current as shown in Figure 3.

Figure 3. Conversion Rate Effect on Power Supply Current

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