Typical Performance Characteristics

1.0 Functional Description

The LM70 temperature sensor incorporates a band-gap type

temperature

sensor

and

10-bit

plus

sign

∆Σ ADC

(Delta-Sigma Analog-to-Digital Converter). Compatibility of

the LM70’s three wire serial interface with SPI and MI-

CROWIRE allows simple communications with common mi-

crocontrollers and processors. Shutdown mode can be used

to optimize current drain for different applications. A manu-

facture’s ID register identifies the LM70 as National Semi-

conductor product.

1.1 POWER UP AND POWER DOWN

The LM70 always powers up in a known state. The power up

default condition is continuous conversion mode. Immediatly

after power up the LM70 will output an erroneous code until

the first temperature conversion has completed.

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

the LM70 is considered powered down. As the supply volt-

age rises above the nominal 1.6V power up threshold, the in-

ternal registers are reset to the power up default state de-

scribed above.

1.2 SERIAL BUS INTERFACE

The LM70 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-

munication will consist of 32 serial clocks. The first 16 clocks

comprise the transmit phase of communication, while the

second 16 clocks are the receive phase.

should be initiated by taking chip select (CS) low. This

should not be done when SC is changing from a low to high

state. Once CS is low the serial I/O pin (SI/O) will transmit

the first bit of data. The master can then read this bit with the

rising edge of SC. The remainder of the data will be clocked

out by the falling edge of SC. Once the 14 bits of data (one

sign bit, ten temperature bits and 3 high bits) are transmitted

the SI/O line will go into TRI-STATE. CS can be taken high at

any time during the transmit phase. If CS is brought low in

the middle of a conversion the LM70 will complete the con-

version and the output shift register will be updated after CS

is brought back high.

The receive phase of a communication starts after 16 SC pe-

riods. CS can remain low for 32 SC cycles. The LM70 will

read the data available on the SI/O line on the rising edge of

the serial clock. Input data is to an 8-bit shift register. The

part will detect the last eight bits shifted into the register. The

receive phase can last up to 16 SC periods. All ones must be

shifted in order to place the part into shutdown. A zero in any

location will take the LM70 out of shutdown. The following

codes only should be transmitted to the LM70:

•

00 hex (normal operation)

•

01 hex (normal operation)

•

03 hex (normal operation)

•

07 hex (normal operation)

•

0F hex (normal operation)

•

1F hex (normal operation)

•

3F hex(normal operation)

•

7F hex(normal operation)

•

FF hex (Shutdown, transmit manufacturer’s ID)

.

any others may place the part into a Test Mode. Test Modes

are used by National Semiconductor to thoroughly test the

function of the LM70 during production testing. Only eight

bits have been defined above since only the last eight trans-

mitted, before CS is taken HIGH, are detected by the LM70

The following communication can be used to determine the

Manufacturer’s/Device ID and then immediately place the

part into continuous conversion mode. With CS continuously

low:

•

Read 16 bits of temperature data

•

Write 16 bits of data commanding shutdown

•

Read 16 bits of Manufacture’s/Device ID data

•

Write 8 to 16 bits of data commanding Conversion Mode

•

Take CS HIGH.

Note that 250 ms will have to pass for a conversion to com-

plete before the LM70 actually transmits temperature data.

Average Power-On Reset Voltage

vs Temperature

DS101223-23

Static Supply Current vs

Temperature

DS101223-21

Temperature Error

DS101223-22

www.national.com

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