TCN75AVOA datasheet(27/40 Pages) MICROCHIP

27 / 40 page

 2010 Microchip Technology Inc.

DS21935D-page 27

TCN75A

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.

FIGURE 6-1:

Pull-up Resistors On Serial

Interface.

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.

FIGURE 6-2:

Multiple Devices on I2C™

Bus.

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 VDD 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 VDD 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).

FIGURE 6-3:

Power-supply Filter using a

Single Resistor.

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.

PIC®

SDA

SCL

VDD

MCU

TCN75A

SDA SCL

PIC16F737

Microcontroller

Temperature

Sensor

24LC01

EEPROM

TC654

Fan Speed

Controller

TCN75A

VDD

200



TCN75A

Switching

Regulator

0.1µF

bypass

VDD

200



TCN75A

Switching

Regulator

0.1µF

bypass

Regulator

Linear

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