Datasheet
TCN75A
DS21935A-page 8 © 2005 Microchip Technology Inc.
4.0 FUNCTIONAL DESCRIPTION
The TCN75A temperature sensor consists of a band-
gap type temperature sensor, a ΣΔ Analog-to-Digital
Converter (ADC), user-programmable registers and a
2-wire I
2
C protocol-compatible serial interface.
FIGURE 4-1: Functional Block Diagram.
4.1 Temperature Sensor
The TCN75A uses the difference in the base-emitter
voltage of a transistor while its collector current is
changed from IC
1
to IC
2
. With this method, the ΔV
BE
depends only on the ratio of the two currents and the
ambient temperature, as shown in Equation 4-1.
EQUATION 4-1:
4.2 ΣΔ Analog-to-Digital Converter
A sigma-delta ADC is used to convert ΔV
BE
to a digital
word that corresponds to the transistor temperature.
The converter has an adjustable resolution from 9-bits
(at 30 ms conversion time) to 12-bits (at 240 ms conver-
sion time). Thus, it allows the user to make trade-offs
between resolution and conversion time. Refer to
Section 4.3.4 “Sensor Configuration Register
(CONFIG)” and Section 4.3.4.7 “ΣΔ ADC Resolution”
for details.
Resolution
9-Bit
10-Bit
11-Bit
12-Bit
Temperature
T
HYST
T
SET
Register
Register
Register
Register
Pointer
I
2
C™
Interface
Configuration
Register
ΣΔ ADC
Band-Gap
Temperature
Sensor
One-Shot
Shutdown
Fault Queue
Alert Polarity
Alert Comp/Int
Δ
V
BE
kT
q
----- -
⎝⎠
⎛⎞
ln IC
1
IC
2
⁄()×=
Where:
T = temperature in kelvin
ΔV
BE
= change in diode base-emitter
voltage
k = Boltzmann's constant
q = electron charge
IC
1
and IC
2
= currents with n:1 ratio