MCP98242-BESLASHST datasheet(39/52 Pages) MICROCHIP

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DS21996C-page 39

MCP98242

6.0

APPLICATIONS INFORMATION

6.1

Connecting to the Serial Bus

The SDA and SCLK serial interface pins 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 number of devices connected to the bus is limited

only by the maximum rise and fall times of the SDA and

SCLK lines. Unlike I2C specifications, SMBus does not

specify a maximum bus capacitance value. Rather, the

SMBus specification requires that the maximum

current through the pull-up resistor be 350 µA and

minimum 100 µA. Because of this, the value of the

pull-up resistors will vary depending on the system’s

bias voltage (VDD). The pull-up resistor values for a

3.3 V system ranges 9 k

Ω to 33 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, with a maximum

voltage drop of 0.4 V, the MCP98242 is designed to

meet a maximum voltage drop of 0.4 V, with 3 mA of

current. This allows lower pull-up resistor values to be

used, allowing the MCP98242 to handle higher bus

capacitance. In such applications, all devices on the

bus

must

meet

the

same

pull-down

current

requirements.

A possible configuration using multiple devices on the

SMBus is shown in Figure 6-2.

FIGURE 6-2:

Multiple Devices on DIMM

SMBus.

6.2

Layout Considerations

The MCP98242 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 and ground pins of the device in order to provide

effective noise protection.

6.3

Thermal Considerations

A potential for self-heating errors can exist if the

MCP98242 SDA, SCLK and Event lines are heavily

loaded with pull-ups (high current). Typically, the

self-heating error is negligible because of the relatively

small current consumption of the MCP98242. A

temperature accuracy error of approximately 0.5°C

could result from self-heating if the communication pins

sink/source the maximum current specified.

For example, if the Event output is loaded to maximum

IOL, Equation 6-1 can be used to determine the effect

of self-heating.

EQUATION 6-1:

EFFECT OF

SELF-HEATING

At room temperature (TA = +25°C) with maximum

IDD = 500 µA and VDD = 3.6V, the self-heating due to

power dissipation TΔ is 0.2°C for the DFN-8 package

and 0.5°C for the TSSOP-8 package.

SDA

SCLK

VDD

MCP98242

Event

Master

Slave

SDA SCLK

MCP98242

Temperature

Sensor

24LCS52

EEPROM

TΔ θJA VDD IDD VOL_Event IOL_Event VOL_SDA IOL_SDA

•

()

Where:

TΔ =TJ - TA

TJ = Junction Temperature

TA = Ambient Temperature

θJA = Package Thermal Resistance

VOL_Event, SDA = Event and SDA Output VOL

(0.4 Vmax)

IOL_Event, SDA = Event and SDA Output IOL

(3 mAmax)

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