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

ERR

1 004

1 008

T

=

273 15

100 C

1 008

T

1 48 C

§

·

u

q

¨

¸

©

¹

q

O

ERR

1.008

T

273.15

T C

1.008

K

§

·

u

¨

¸

©

¹

30

TMP464

SBOS835C – MAY 2017 – REVISED OCTOBER 2019

www.ti.com

Product Folder Links: TMP464

Submit Documentation Feedback

Copyright © 2017–2019, Texas Instruments Incorporated

Typical Application (continued)

where

•

TERR = error in the TMP464 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 TMP464 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 TMP464 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 TMP464 device is

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

TMP464 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 TMP464 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 TMP464

device is measuring. Additionally, the internal power dissipation of the TMP464 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 TMP464 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 TMP464 device

dissipates 0.143 mW (PDIQ = 3.3 V × 43 μA) when both the remote and local channels are enabled.

(6)

(7)