TCN75A_1 Datasheet, PDF (27/40 Pages) Microchip Technology – 2-Wire Serial Temperature Sensor

6.0 APPLICATIONS INFORMATION
6.1 Connecting to the Serial Bus
The SDA and SCL serial interface are open-drain pins
that require pull-up resistors. This configuration is
shown in Figure 6-1.
PIC^®
MCU
V_DD
TCN75A
RR
SDA
SCL
FIGURE 6-1:
Interface.
Pull-up Resistors On Serial
The TCN75A is designed to meet 0.4V (maximum)
voltage drop at 3 mA of current. This allows the
TCN75A to drive lower values of pull-up resistors and
higher bus capacitance. In this application, all devices
on the bus must meet the same pull-down current
requirements.
6.2 Typical Application
Microchip provides several microcontroller product
lines with Master Synchronous Serial Port Modules
(MSSP) that include the I²C interface mode. This
module implements all master and slave functions and
simplifies the firmware development overhead.
Figure 6-2 shows a typical application using the
PIC16F737 as a master to control other Microchip
slave products, such as EEPROM, fan speed
controllers and the TCN75A temperature sensor
connected to the bus.
SDA SCL
PIC16F737
Microcontroller
TC654
Fan Speed
Controller
24LC01
EEPROM
TCN75A
Temperature
Sensor
FIGURE 6-2:
Bus.
Multiple Devices on I²C™
TCN75A
The ALERT output can be wire-ORed with a number of
other open-drain devices. In such applications, the
output needs to be programmed as an active-low
output. Most systems will require pull-up resistors for
this configuration.
6.3 Layout Considerations
The TCN75A does not require any additional
components besides the master controller in order to
measure temperature. However, it is recommended
that a decoupling capacitor of 0.1 µF to 1 µF be used
between the V_DD and GND pins. A high-frequency
ceramic capacitor is recommended. It is necessary for
the capacitor to be located as close as possible to the
power pins in order to provide effective noise
protection.
For applications where a switching regulator is used to
power the sensor, it is recommended to add a 200Ω
resistor in series to V_DD to filter out the switcher noise
from the sensor. It is also recommended to add the
series resistor in applications where a linear regulator
is used to step-down a switching regulator voltage to
power the sensor. For example, if a linearly regulated
3.3V from a 5V switching regulator is used to power the
sensor, add a 200Ω series resistor (refer to Figure 6-3).
Switching
Regulator
TCN75A
200
V_DD
0.1µF
bypass
Switching
Regulator
TCN75A
200
Linear
V_DD
Regulator _0.1µF
bypass
FIGURE 6-3:
Single Resistor.
Power-supply Filter using a
6.4 Thermal Considerations
The TCN75A measures temperature by monitoring the
voltage of a diode located in the die. A low-impedance
thermal path between the die and the Printed Circuit
Board (PCB) is provided by the pins. Therefore, the
TCN75A effectively monitors the temperature of the
PCB. However, the thermal path for the ambient air is
not as efficient because the plastic device package
functions as a thermal insulator.
A potential for self-heating errors can exist if the
TCN75A SDA and SCL communication lines are
heavily loaded with pull-ups. Typically, the self-heating
error is negligible because of the relatively small
current consumption of the TCN75A. However, in order
to maximize the temperature accuracy, the SDA and
SCL pins need to be lightly loaded.
 2010 Microchip Technology Inc.
DS21935D-page 27