MCP9803 datasheet(22/30 Pages) MICROCHIP

22 / 30 page

MCP9800/1/2/3

DS21909B-page 22

 2004 Microchip Technology Inc.

6.0

APPLICATIONS INFORMATION

6.1

Connecting to the Serial Bus

The SDA and SCLK 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.

For the SMBus protocol, the number of devices con-

nected to the bus are limited only by the maximum rise

and fall times of the SDA and SCLK lines. Unlike the

I2C specifications, SMBus does not specify a maximum

bus capacitance value. Rather, it specifies 350 µA

(max.) current through the pull-up resistor. Therefore,

the value of the pull-up resistors will vary depending on

the system’s supply voltage (VDD). The pull-up resistor

values for a 5V system ranges 14.3 k

Ω to 50 kΩ.

Minimizing bus capacitance is still very important, as it

directly affects the rise and fall times of the SDA and

SCLK lines.

Although SMBus specifications only require the SDA

and SCLK lines to pull down 350 µA (max.) with 0.4V

(max.) voltage drop, the MCP9800/1/2/3 is designed to

meet 0.4V (max.) voltage drop at 3 mA of current. This

allows the MCP9800/1/2/3 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 I2C interface mode. This module

implements all master and slave functions and simpli-

fies 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 MCP980X

temperature sensor connected to the bus.

FIGURE 6-2:

Multiple Devices on SMBus.

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 MCP9800/1/2/3 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.

6.4

Thermal Considerations

The

MCP9800/1/2/3

measures

temperature

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 MCP9800/1/2/3 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

MCP9800/1/2/3 SDA and SCLK communication lines

are heavily loaded with pull-ups. Typically, the self-

heating error is negligible because of the relatively

small current consumption of the MCP9800/1/2/3.

However, in order to maximize the temperature

accuracy, the SDA and SCLK pins need to be lightly

loaded.

SDA

SCLK

VDD

MCP980X

SDA SCLK

PIC16F737

Microcontroller

MCP980X

Temperature

Sensor

24LC01

EEPROM

TC654

Fan Speed

Controller

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