Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).

Note 8: Local temperature accuracy does not include the effects of self-heating. The rise in temperature due to self-heating is the product of the internal power

dissipation of the LM90 and the thermal resistance. See (Note 5) for the thermal resistance to be used in the self-heating calculation.

Note 9: Quiescent current will not increase substantially with an SMBus.

Note 10: This specification is provided only to indicate how often temperature data is updated. The LM90 can be read at any time without regard to conversion state

(and will yield last conversion result).

Note 11: Default values set at power up.

SMBData and SMBCLK pins to a high impedance state.

1.0 Functional Description

The LM90 temperature sensor incorporates a delta V

BE

based temperature sensor using a Local or Remote and a

10-bit plus sign ADC (Delta-Sigma Analog-to-Digital Con-

verter). The LM90 is compatible with the serial SMBus ver-

sion 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 mea-

sured Remote Temperature (RT) is digitally compared to the

Remote High (RHS), Remote Low (RLS) and Remote

T_CRIT (RCS) user-programmable temperature limit regis-

ters. Activation of the ALERT output indicates that a com-

parison 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 compara-

tor 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 tem-

perature goes below the setpoint calculated by T_CRIT −

TH. The hysteresis register impacts both the remote tem-

perature 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

Section 4.1 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.

1.1 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 pro-

cess the busy bit (D7) in the Status register (02h) is high.

These conversions are addressed in a round robin se-

quence. The conversion rate may be modified by the Con-

version 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 2.

1.2 THE ALERT OUTPUT

The LM90’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. Re-

set of the ALERT output is dependent upon the selected

method of use. The LM90’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 de-

scribed below. The ALERT and interrupt methods are differ-

ent only in how the user interacts with the LM90.

Each temperature reading (LT and RT) is associated with a

T_CRIT setpoint register (LCS, RCS), a HIGH setpoint reg-

ister (LHS and RHS) and a LOW setpoint register (LLS and

RLS). At the end of every temperature reading, a digital

comparison determines whether that reading is above its

HIGH or T_CRIT setpoint or below its LOW setpoint. If so,

the corresponding bit in the STATUS REGISTER is set. If the

ALERT mask bit is not high, any bit set in the STATUS

REGISTER, with the exception of Busy (D7) and OPEN

(D2), will cause the ALERT output to be pulled low. Any

temperature conversion that is out of the limits defined by the

temperature setpoint registers will trigger an ALERT. Addi-

tionally, the ALERT mask bit in the Configuration register

must be cleared to trigger an ALERT in all modes.

1.2.1 ALERT Output as a Temperature Comparator

When the LM90 is implemented in a system in which it is not

serviced by an interrupt routine, the ALERT output could be

used as a temperature comparator. Under this method of

20033739

FIGURE 2. Conversion Rate Effect on Power Supply

Current

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