LM134 datasheet(10/26 Pages) AD

10 / 26 page

LTC2990

Rev. F

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 2. Recommended PCB Layout

VCC

ADR1

ADR0

SCL

SDA

LTC2990

2990 F02

GND SHIELD

TRACE

NPN SENSOR

470pF

0.1µF

GND

change in sensor voltage per degree temperature change

is 275µV/°C, so environmental noise must be kept to a

minimum. Recommended shielding and PCB trace con-

siderations are illustrated in Figure 2.

The diode equation:

VBE = η•

k • T

• ln

⎛

⎝

⎜

⎞

⎠

⎟

(1)

can be solved for T, where T is Kelvin degrees, IS is a

process dependent factor on the order of 1E-13, η is the

diode ideality factor, k is Boltzmann’s constant and q is

the electron charge.

T =

VBE •q

η • k •In

⎛

⎝

⎜

⎞

⎠

⎟

(2)

The LTC2990 makes differential measurements of diode

voltage to calculate temperature. Proprietary techniques

allow for cancellation of error due to series resistance.

ideality factor of the diode sensor can be considered a

temperature scaling factor. The temperature error for a

1% accurate ideality factor error is 1% of the Kelvin tem-

perature. Thus, at 25°C, or 298K, a +1% accurate ideality

factor error yields a +2.98 degree error. At 85°C or 358K, a

+1% error yields a 3.6 degree error. It is possible to scale

the measured Kelvin or Celsius temperature measured

using the LTC2990 with a sensor ideality factor other than

1.004, to the correct value. The scaling Equations (3) and

(4) are simple, and can be implemented with sufficient

precision using 16-bit fixed-point math in a microproces-

sor or microcontroller.

Factory Ideality Calibration Value:

ηCAL = 1.004

Actual Sensor Ideality Value:

ηACT

Compensated Kelvin Temperature:

TK _COMP =

ηCAL

ηACT

• TK _MEAS

(3)

Compensated Celsius Temperature

TC_COMP =

ηCAL

ηACT

• TC_MEAS + 273

(

)

⎡

⎣

⎢

⎤

⎦

⎥ – 273

(4)

A 16-bit unsigned number is capable of representing the

ratio ηCAL/ηACT in a range of 0.00003 to 1.99997, by

multiplying the fractional ratio by 215. The range of scal-

ing encompasses every conceivable target sensor value.

The ideality factor scaling granularity yields a worst-case

Ideality Factor Scaling

The LTC2990 is factory calibrated for an ideality factor of

1.004, which is typical of the popular MMBT3904 NPN

transistor. The semiconductor purity and wafer-level pro-

cessing limits device-to-device variation, making these

devices interchangeable (typically <0.5°C) for no addi-

tional cost. Several manufacturers supply suitable transis-

tors, some recommended sources are listed in Table 1.

Discrete 2-terminal diodes are not recommended as tem-

perature sensors. While an ideality factor value of 1.004

is typical of target sensors, small deviations can yield

significant temperature errors. Contact LTC Marketing for

parts trimmed to ideality factors other than 1.004. The

Table 1. Recommended Transistors to Be Used as Temperature

Sensors

MANUFACTURER

PART NUMBER

PACKAGE

Fairchild Semiconductor

MMBT3904

FMMT3904

SOT-23

Central Semiconductor

CMPT3904

CET3904E

SOT-23

SOT-883L

Diodes, Inc.

MMBT3904

SOT-23

On Semiconductor

MMBT3904LT1

SOT-23

NXP

MMBT3904

SOT-23

Infineon

MMBT3904

SOT-23

Rohm

UMT3904

SC-70

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