2005-2013 Microchip Technology Inc. DS21977C-page 25
MCP9805
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 I
2
C 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 (V
DD
). The pull-up resistor values for a
3.3 V system ranges 9 k to 33 k. 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 MCP9805 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 MCP9805 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 MCP9805 does not require any additional compo-
nents 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 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
MCP9805 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 MCP9805. 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
I
OL
, Equation 6-1 can be used to determine the effect
of self-heating.
EQUATION 6-1: EFFECT OF
SELF-HEATING
At room temperature (T
A
= +25°C) with maximum
I
DD
= 500 µA and V
DD
= 3.6V, the self-heating due to
power dissipation T
is 0.2°C for the DFN-8 package
and 0.5°C for the TSSOP-8 package.
SDA
SCLK
V
DD
R
R
Microcontroller
MCP9805
Event
R
Master Slave
SDA
SCLK
MCP9805
Temperature
Sensor
24LCS52
EEPROM
T
JA
V
DD
I
DD
V
OL_Event
I
OL_Event
V
OL_SDA
I
OL_SDA
++=
Where:
T
=T
J -
T
A
T
J
= Junction Temperature
T
A
= Ambient Temperature
JA
= Package Thermal Resistance
V
OL_Event, SDA
= Event and SDA Output V
OL
(0.4 V
max
)
I
OL_Event, SDA
= Event and SDA Output I
OL
(3 mA
max
)
