LTC2997H View Datasheet(PDF) - Linear Technology

Part Name

Description

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LTC2997H

Remote/Internal Temperature Sensor

Linear Technology 

LTC2997

OPERATION

The Block Diagram shows the main components of the

LTC2997.

The LTC2997 measures temperature using either a remote

or internal diode and provides a buffered voltage propor-

tional to absolute temperature (VPTAT) and a buffered 1.8V

reference voltage. Remote temperature measurements

usually use a diode connected transistor as a tempera-

ture sensor, allowing the remote sensor to be a discrete

NPN (ex. MMBT3904) or an embedded PNP device in a

microprocessor or FPGA.

Temperature measurements are conducted by measur-

ing the diode voltage at multiple test currents. The diode

equation can be solved for T, where T is degrees Kelvin,

IS is a process dependent factor on the order of 10–13A,

η is the diode ideality factor, k is the Boltzmann constant

and q is the electron charge:

T

=

q

η•

k

•

VDIODE

ln

⎛⎜ID

⎞

⎟

⎝IS⎠

This equation has a relationship between temperature and

voltage, dependent on the process-dependent variable IS.

Measuring the same diode (with the same value IS) at two

different currents yields an expression which is independent

of IS. The value in the natural logarithm term becomes the

ratio of the two currents, which is process independent.

T

=

q

η•

k

•

VDIODE2 – VDIODE1

ln

⎛

⎜

ID2

⎞

⎟

⎝ ID1⎠

Series Resistance Cancellation

Resistance in series with the remote diode causes a positive

temperature error by increasing the measured voltage at

each test current. The composite voltage equals:

VDIODE +

VERROR

= η kT

q

•

ln⎛⎜ID

⎝IS

⎞

⎟

⎠

+

R

S

•ID

where RS is the series resistance.

The LTC2997 removes this error term from the sensor

signal by subtracting a cancellation voltage (see Figure 1).

A resistance extraction circuit uses one additional current

(I3) to determine the series resistance in the measurement

www.DataSheet.net/

path. Once the correct value of the resistor is determined

VCANCEL equals VERROR. Now the temperature to voltage

converter's input signal is free from errors due to series

resistance and the sensor temperature can be determined

using currents I1 and I2.

I1, I2

I3

RSERIES

D+

VERROR

VBE

D–

RESISTANCE

–+

EXTRACTION

CIRCUIT

VCANCEL = VERROR

TEMPERATURE

VBE

TO VOLT

CONVERTER

2997 F01

Figure 1. Series Resistance Cancellation

VPTAT

8

2997fa

Datasheet pdf - http://www.DataSheet4U.co.kr/

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