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LTC2990CMS

I2C Temperature, Voltage and Current Monitor

Linear Technology 

LTC2990

Applications Information

The 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

considerations are illustrated in Figure 2.

The diode equation:

VBE

=

η

•

k

•T

q

•



ln 

IC

IS





(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





IC

IS





(2)

The LTC2990 makes differential measurements of diode

voltage to calculate temperature. Proprietary techniques

allow for cancellation of error due to series resistance.

GND SHIELD

TRACE

470pF

NPN SENSOR

0.1µF

LTC2990

V1

VCC

V2

ADR1

V3

ADR0

V4

SCL

GND SDA

2990 F02

Figure 2. Recommended PCB Layout

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 <0.5C) for no additional

cost. Several manufacturers supply suitable transistors,

some recommended sources are listed in Table 10. While

an ideality factor value of 1.004 is typical of target sen-

sors, small deviations can yield significant temperature

errors. Contact LTC Marketing for parts trimmed to ideality

factors other than 1.004. The 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 temperature. Thus, at 25°C, or

298°K, a +1% accurate ideality factor error yields a +2.98

degree error. At 85°C or 358°K, 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 microprocessor or microcontroller.

Factory Ideality Calibration Value:

ηCAL = 1.004

Actual Sensor Ideality Value:

ηACT

Compensated Kelvin Temperature:

TK _COMP

=

ηACT

ηCAL

• TK _MEAS

(3)

Compensated Celsius Temperature

( ) TC_COMP

=







ηACT

ηCAL

•

TC_MEAS + 273





–

273



(4)

A 16-bit unsigned number is capable of representing the

ratio ηACT/ηCAL 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

temperature error of 0.01° at 125°C. Multiplying this 16‑bit

unsigned number and the measured Kelvin (unsigned)

temperature represented as a 16-bit number, yields a

32‑bit unsigned result. To scale this number back to a

13‑bit temperature (9-bit integer part, and a 4-bit fractional

part), divide the number by 215 per Equation (5). Similarly,

Celsius coded temperature values can be scaled using

16-bit fixed-point arithmetic, using Equation (6). In both

cases, the scaled result will have a 9-bit integer (d[12:4])

and the 4LSBs (d[3:0]) representing the 4-bit fractional

part. To convert the corrected result to decimal, divide

the final result by 24 or 16, as you would the register

contents. If ideality factor scaling is implemented in the

2990f



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