MCP9509/10
DS22114A-page 16 © 2008 Microchip Technology Inc.
4.6 Application Information
The MCP9509/10 temperature switch integrates a
temperature sensor and a comparator circuit which out-
puts an alert signal when the user programmed tem-
perature threshold is exceeded. The external resistor
value to set the output threshold can be determined
using Table 4-2. A constant current source I
SET
= 5 µA
(typical) biases the external resistor R
SET
. A thermal
diode is used to measure ambient temperature. When
the voltage across the thermal diode exceeds the volt-
age across R
SET
, V
SET
, the sensor output asserts. The
sensor output de-asserts when the diode voltage drops
below V
SET
and the user selected hysteresis level.
The MCP9509/10 provide Open-drain output where
multiple sensors from multiple PCB hot-spots can be
connected to a single processor I/O input with a
wired-Or Configuration. The MCP9509 requires an
external pull-up resistor which can be used to level-shift
the alert signal. For example, if the sensors are
powered with 5V
DD
and the controller or processor is
powered with 3V
DD
, the external resistor can be
level-shifted by connecting 3V
DD
to the pull-up resistor
as shown in Figure 4-5. The MCP9510 elliminates the
need for an external resistor while providing wired-Or
function (Figure 4-6). The MCP9510 also provides
push-pull output configuration for a direct connection to
the processor with Active-Low or Active-High assert
polarities.
FIGURE 4-5: MCP9509 Wired-Or Output
Configuration with Level-shift.
FIGURE 4-6: MCP9510 Wired-Or Output
Configuration with Internal Pull-up Resistor.
4.6.1 LAYOUT CONSIDERATION AND
THERMAL CONSIDERATION
This family of sensors measure temperature by moni-
toring the voltage level of a thermal diode located in the
die. A low-impedance thermal path between the die
and the PCB is provided by the pins. Therefore, the
sensor effectively monitors PCB temperature. For
efficient performance, it is recommended to layout the
device as close to the heat source as possible. It is also
recommended to use a de-coupling capacitor of 0.1 µF
to 1 µF between V
DD
and GND pins for stability.
When connecting an external resistor to the MCP9509
device, the current through the pull-up resistor must be
considered to prevent self-heat due to power. This can
be determined using Equation 4-3.
EQUATION 4-3: EFFECT OF
SELF-HEATING
SET
OUT
HYST
5V
DD
SET
R
SET
OUT
HYST
5V
DD
I/O
3V
DD
R
PULL_UP
Micro-
controller
R
SET
MCP9509
MCP9509
3V
DD
SET
OUT
HYST
5V
DD
SET
R
SET
OUT
HYST
5V
DD
I/O
Micro-
controller
R
SET
MCP9510
MCP9510
OUTSET
OUTSET
5V
DD
Where:
T
J
= Junction Temperature
T
A
= Ambient Temperature
θ
JA
= Package Thermal Resistance
(220.7 °C/W)
V
OL
= Sensor Output Low Voltage
I
OUT
= Output Current
T
J
T
A
– θ
JA
V
DD
I
DD
× V
OL
I
OUT
×+()=
