TMP400 datasheet(19/25 Pages) TI

19 / 25 page

www.ti.com

REMOTE SENSING

MEASUREMENT ACCURACY AND THERMAL

ERR

1.008

[273.15+T( C)]

(4)

ERR

1.004

1.008

(273.15+100 C)

= 1.48

ERR

°C

(5)

TMP400

SBOS404 – DECEMBER 2007

If filtering is needed, the suggested component

2. Base-emitter voltage < 0.95V at 120

µA, at the

values are 100pF and 50

Ω on each input. Exact

lowest sensed temperature.

values are application specific. Resistance correction

3. Base resistance < 100

Ω.

must be enabled to avoid offset correction.

4. Tight control of VBE characteristics indicated by

small variations in hFE (that is, 50 to 150).

Based

these

criteria,

two

recommended

The TMP400 is designed to be used with either

small-signal transistors are the 2N3904 (NPN) or

discrete transistors or substrate transistors built into

2N3906 (PNP).

processor chips and ASICs. Either NPN or PNP

transistors can be used, as long as the base-emitter

junction is used as the remote temperature sense.

CONSIDERATIONS

Either a transistor or diode connection can also be

used; see Figure 11.

The temperature measurement accuracy of the

TMP400

depends

the

remote

and/or

local

Errors in remote temperature sensor readings are

temperature sensor being at the same temperature

generally the consequence of the ideality factor and

as the system point being monitored. Clearly, if the

current excitation used by the TMP400 versus the

temperature sensor is not in good thermal contact

manufacturer-specified operating current for a given

with the part of the system being monitored, then

transistor. Some manufacturers specify a high-level

there will be a delay in the response of the sensor to

and low-level current for the temperature-sensing

a temperature change in the system. For remote

substrate transistors. The TMP400 uses 6

µA for I

LOW

temperature sensing applications using a substrate

and 120

µA for I

HIGH. The TMP400 allows for different

transistor (or a small, SOT23 transistor) placed close

n-factor values; see the N-Factor Correction Register

to the device being monitored, this delay is usually

section.

not a concern.

The ideality factor (n) is a measured characteristic of

The local temperature sensor inside the TMP400

a remote temperature sensor diode as compared to

monitors the ambient air around the device. The

an ideal diode. The ideality factor for the TMP400 is

thermal

time

constant

for

the

TMP400

trimmed to be 1.008. For transistors whose ideality

approximately two seconds. This constant implies

factor does not match the TMP400, Equation 4 can

that if the ambient air changes quickly by 100

°C, it

be used to calculate the temperature error. Note that

would take the TMP400 about 10 seconds (that is,

for

the

equation

used

correctly,

actual

five thermal time constants) to settle to within 1

°C of

temperature (

°C) must be converted to Kelvin (°K).

the final value. In most applications, the TMP400

package is in electrical (and therefore, thermal)

contact with the printed circuit board (PCB), as well

as subjected to forced airflow. The accuracy of the

Where:

measured temperature directly depends on how

accurately the PCB and forced airflow temperatures

n = Ideality factor of remote temperature sensor

represent the temperature that the TMP400 is

°C) = actual temperature

measuring. Additionally, the internal power dissipation

TERR = Error in TMP400 reading due to n ≠ 1.008

of the TMP400 can cause the temperature to rise

Degree delta is the same for

°C and °K

above the ambient or PCB temperature. The internal

power dissipated as a result of exciting the remote

For n = 1.004 and T(

°C) = 100°C:

temperature sensor is negligible because of the small

currents used. For a 5.5V supply and maximum

conversion rate of eight conversions per second, the

TMP400 dissipates 1.82mW (PDIQ = 5.5V × 420µA).

If the ALERT pin is sinking 1mA, an additional power

of 0.4mW is dissipated (PDOUT = 1mA × 0.4V =

If a discrete transistor is used as the remote

0.4mW). Total power dissipation is then 2.22mW

temperature sensor with the TMP400, the best

(PDIQ + PDOUT) and, with an θJA of 150°C/W, causes

accuracy can be achieved by selecting the transistor

the

junction

temperature

rise

approximately

according to the following criteria:

0.333

°C above the ambient.

1. Base-emitter voltage > 0.25V at 6

µA, at the

highest sensed temperature.

Submit Documentation Feedback

Product Folder Link(s): TMP400

TMP400 Datasheet(PDF) 19 Page - Texas Instruments

TMP400 Datasheet(HTML) 19 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Datasheet Download

Link URL