MCP960X datasheet(42/57 Pages) MICROCHIP

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DS20005426G-page 42

MCP960X/L0X/RL0X

6.0

APPLICATION INFORMATION

6.1

Layout Considerations

The MCP960X/L0X/RL0X does not require any

additional components to digitize thermocouples.

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 VDD and ground

pins of the device in order to provide effective noise

protection.

In addition, good PCB layout is key for better thermal

conduction from the PCB temperature to the sensor

die. The PCB provides thermal conduction from the die

to the thermocouple cold-junction; therefore, the

component placement positioning and the copper

layout techniques are key for optimum cold-junction

compensation. The recommended implementation for

optimum temperature sensitivity is to extend a copper

ground pad around the device pins, as shown in

Figure 6-1.

FIGURE 6-1:

Recommended PCB Layout.

6.1.1

COLD-JUNCTION COMPENSATION

Copper provides better thermal conductivity than PCB

FR4 to the ambient temperature. It also provides better

thermal conduction than the 5 mm x 5 mm MQFN plastic

package, which houses the temperature sensor die.

Therefore, when connecting the thermocouple wire to

the PCB, it is recommended to place the ground copper

between the thermocouple connector footprint, where

dissimilar conductive material is attached to the PCB

and the MCP960X/L0X/RL0X exposed pad. This allows

temperature to stabilize to the local ambient temperature

(between the thermocouple connector junction and the

PCB copper) and the copper trace conducts the

temperature to the package exposed pad where the

temperature sensor die is placed. The placement of the

sensor exposed pad to the thermocouple connector

junction greatly determines the temperature sensor’s

sensitivity to the local junction temperature changes.

Figure 6-2 demonstrates the recommended techniques.

FIGURE 6-2:

Recommended Component

Placement.

6.2

Thermal Considerations

The potential for self-heating errors exist if the

MCP960X/L0X/RL0X SDA, SCL and alert outputs are

heavily loaded (high current) with pull-up resistors and

circuits, such as high-current LEDs or buzzer loads.

The temperature rise due to self-heat increases the

ambient temperature sensor output, resulting in an

increased temperature offset error compared to the

thermocouple cold-junction ambient temperature.

6.2.1

SELF-HEAT DURING OPERATION

During normal operation, the typical self-heating error

is negligible due to the relatively small current

consumption of the MCP960X/L0X/RL0X. However,

this device integrates a processor to compute the

equations necessary to convert the thermocouple EMF

voltage to degrees Celsius. The processor also

maintains the I2C bus. During I2C communication, the

device operating current increases to IDD = 1.5 mA

(typical), I2C Active specification. If the bus is

continually polled for data at frequent intervals, then the

processor power dissipates heat to the temperature

sensor and the effect of self-heat can be detected.

Therefore, the recommended implementation is to

maintain polling to no more than three times per

temperature conversion period of 320 ms or use the

Burst mode feature to manage self-heat (refer to

Section 6.2.3 “Using Burst Mode to Manage

Self-Heat”). Equation 6-1 can also be used to

determine the effect of self-heat.

Thermal Pad

VIN+/VIN-

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