TMP421 datasheet(23/30 Pages) TI

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ERR

1.004 * 1.008

1.008

273.15 ) 100°C

ERR

+ 1.48°C

(5)

LAYOUT CONSIDERATIONS

MEASUREMENT ACCURACY AND THERMAL

TMP421

TMP422

TMP423

SBOS398B – JULY 2007 – REVISED MARCH 2008

For n = 1.004 and T(

°C) = 100°C:

power dissipated as a result of exciting the remote

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

TMP421/22/23 dissipate 2.3mW (PDIQ = 5.5V ×

415

µA). A θ

of 100

°C/W causes the junction

temperature to rise approximately +0.23

°C above the

If a discrete transistor is used as the remote

ambient.

temperature sensor with the TMP421/22/23, the best

accuracy can be achieved by selecting the transistor

according to the following criteria:

Remote temperature sensing on the TMP421/22/23

1. Base-emitter voltage > 0.25V at 6

µA, at the

measures

very

small

voltages

using

very

low

highest sensed temperature.

currents; therefore, noise at the IC inputs must be

2. Base-emitter voltage < 0.95V at 120

µA, at the

minimized.

Most

applications

using

the

lowest sensed temperature.

TMP421/22/23 will have high digital content, with

3. Base resistance < 100

Ω.

several clocks and logic level transitions creating a

4. Tight control of VBE characteristics indicated by

noisy environment. Layout should adhere to the

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

following guidelines:

1. Place the TMP421/22/23 as close to the remote

Based

these

criteria,

two

recommended

junction sensor as possible.

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

2N3906 (PNP).

2. Route the DXP and DXN traces next to each

other and shield them from adjacent signals

through the use of ground guard traces; see

CONSIDERATIONS

Figure 20. If a multilayer PCB is used, bury these

traces between ground or VDD planes to shield

The temperature measurement accuracy of the

them

from

extrinsic

noise

sources.

mil

TMP421/22/23 depends on the remote and/or local

(0.127mm) PCB traces are recommended.

temperature sensor being at the same temperature

3. Minimize

additional

thermocouple

junctions

as the system point being monitored. Clearly, if the

caused by copper-to-solder connections. If these

temperature sensor is not in good thermal contact

junctions are used, make the same number and

with the part of the system being monitored, then

approximate

locations

copper-to-solder

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

connections

both

the

DXP

and

DXN

a temperature change in the system. For remote

connections to cancel any thermocouple effects.

temperature-sensing applications using a substrate

transistor (or a small, SOT23 transistor) placed close

4. Use a 0.1

µF local bypass capacitor directly

to the device being monitored, this delay is usually

between the V+ and GND of the TMP421/22/23;

not a concern.

see

Figure

21.

Minimize

filter

capacitance

between DXP and DXN to 1000pF or less for

The

local

temperature

sensor

inside

the

optimum

measurement

performance.

This

TMP421/22/23 monitors the ambient air around the

capacitance

includes

any

cable

capacitance

device.

The

thermal

time

constant

for

the

between the remote temperature sensor and the

TMP421/22/23 is approximately two seconds. This

TMP421/22/23.

constant implies that if the ambient air changes

5. If

the

connection

between

the

remote

quickly by 100

°C, it would take the TMP421/22/23

temperature sensor and the TMP421/22/23 is

about

seconds

(that

is,

five

thermal

time

less than 8 in (20.32 cm) long, use a twisted-wire

constants) to settle to within 1

°C of the final value. In

pair connection. Beyond 8 in, use a twisted,

most applications, the TMP421/22/23 package is in

shielded pair with the shield grounded as close to

electrical, and therefore thermal, contact with the

the TMP421/22/23 as possible. Leave the remote

printed circuit board (PCB), as well as subjected to

sensor connection end of the shield wire open to

forced

airflow.

The

accuracy

the

measured

avoid ground loops and 60Hz pickup.

temperature directly depends on how accurately the

PCB and forced airflow temperatures represent the

6. Thoroughly clean and remove all flux residue in

temperature that the TMP421/22/23 is measuring.

and around the pins of the TMP421/22/23 to

Additionally, the internal power dissipation of the

avoid temperature offset readings as a result of

TMP421/22/23 can cause the temperature to rise

leakage paths between DXP or DXN and GND,

above the ambient or PCB temperature. The internal

or between DXP or DXN and V+.

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