TMP468 datasheet(32/49 Pages) TI2

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Average Conversion Current = (Local Conversion Time) × (Conversions Per Second) × (Local Active IQ ) +

(Remote Conversion Time) × (Conversions Per Second) × (Remote Active IQ) × (Number of Active Channels +

(Standby Mode) × [1 ± ((Local Conversion Time) + (Remote Conversion Time) × (Number of Active

Channels)) × (Conversions Per Second)]

ERR

1 004

1 008

273 15

100 C

1 008

1 48 C

ERR

1.008

273.15

T C

1.008

TMP468

SBOS762B – NOVEMBER 2016 – REVISED JUNE 2017

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Typical Application (continued)

NOTE

For Equation 4 to be used correctly, the actual temperature (°C) must be converted to

Kelvin (K).

where

•

TERR = error in the TMP468 device because η ≠ 1.008

•

η = ideality factor of the remote temperature sensor

•

T(°C) = actual temperature, and

(4)

In Equation 4, the degree of delta is the same for °C and K.

For η = 1.004 and T(°C) = 100°C:

(5)

If a discrete transistor is used as the remote temperature sensor with the TMP468 device, then select the

transistor according to the following criteria for best accuracy:

•

Base-emitter voltage is > 0.25 V at 7.5 μA, at the highest-sensed temperature.

•

Base-emitter voltage is < 0.95 V at 120 μA, at the lowest-sensed temperature.

•

Base resistance is < 100 Ω.

•

Tight control of VBE characteristics indicated by small variations in hFE (50 to 150).

Based on these criteria, TI recommends using a MMBT3904 (NPN) or a MMBT3906 (PNP) transistor.

8.2.2 Detailed Design Procedure

The local temperature sensor inside the TMP468 is influenced by the ambient air around the device but mainly

monitors the PCB temperature that it is mounted to. The thermal time constant for the TMP468 device is

approximately two seconds. This constant implies that if the ambient air changes quickly by 100°C, then the

TMP468 device takes approximately 10 seconds (that is, five thermal time constants) to settle to within 1°C of

the final value. In most applications, the TMP468 package is in electrical (and therefore thermal) contact with the

printed-circuit board (PCB), and subjected to forced airflow. The accuracy of the measured temperature directly

depends on how accurately the PCB and forced airflow temperatures represent the temperature that the TMP468

device is measuring. Additionally, the internal power dissipation of the TMP468 device can cause the

temperature to rise above the ambient or PCB temperature. The internal power is negligible because of the small

current drawn by the TMP468 device. Equation 6 can be used to calculate the average conversion current for

power dissipation and self-heating based on the number of conversions per second and temperature sensor

channel enabled. Equation 7 shows an example with local and all remote sensor channels enabled and

conversion rate of 1 conversion per second; see the Electrical Characteristics table for typical values required for

these calculations. For a 3.3-V supply and a conversion rate of 1 conversion per second, the TMP468 device

dissipates 0.224 mW (PDIQ = 3.3 V × 68 μA) when both the remote and local channels are enabled.

(6)

TMP468 Datasheet(PDF) 32 Page - Texas Instruments

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