Typical Performance Characteristics

Average Power-On Reset Voltage vs Temperature

Static Supply Current vs Temperature (SO-8)

10090923

10090921

Static Supply Current vs Temperature (micro SMD)

Temperature Error (SO-8)

10090925

10090922

1.0 Functional Description

The LM74 temperature sensor incorporates a band-gap type

temperature sensor and 12-bit plus sign

∆Σ ADC (Delta-

Sigma Analog-to-Digital Converter). Compatibility of the

LM74’s three wire serial interface with SPI and MICROWIRE

allows simple communications with common microcontrol-

lers and processors. Shutdown mode can be used to opti-

mize current drain for different applications. A Manufacture’s/

Device

ID

register

identifies

the

LM74

as

National

Semiconductor product.

1.1 POWER UP AND POWER DOWN

When the supply voltage is less than about 1.6V (typical),

the LM74 is considered powered down. The LM74 always

powers up in a known state. When the supply voltage rises

above 1.6V (typical), an internal Power-On Reset (POR)

occurs and the temperature register will then contain a value

of 1111 1111 0000 00XX, where XX indicates undefined

values. See Section 1.5.2 for a diagram of the Temperature

Regisiter contents after POR but before completion of the

first temperature conversion.

The LM74 power-up default condition is continuous conver-

sion mode. After completion of the first full temperature

conversion, the register will contain temperature measure-

ment data in bits D15 (the temperature data MSB) through

D3 (the temperature data LSB). Bit D2 will be fixed high; bits

D1 and D0 are undefined. See Section 1.5.3 for a diagram of

the Temperature Regisiter contents after the first complete

temperature conversion. Note that bit D2 represents a com-

plete conversion flag. During POR it is low and, after the first

temperature conversion is complete, it goes high. This bit

can be polled to indicate when the POR data in the Tem-

perature Register has been replaced with valid temperature

data.

After the first conversion, and any subsequent conversions,

the value in the temperature register does not change until

the completion of the next conversion, at which time the

temperature register is updated with the latest temperature

value.

1.2 SERIAL BUS INTERFACE

The LM74 operates as a slave and is compatible with SPI or

MICROWIRE bus specifications. Data is clocked out on the

falling edge of the serial clock (SC), while data is clocked in

on the rising edge of SC. A complete transmit/receive com-

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