Datasheet

Detailed Description
The MAX31820 ambient temperature sensor provides
9-bit to 12-bit Celsius temperature measurements with
±0.5°C accuracy over a +10°C to +45°C temperature
range. Over its entire -55°C to +125°C operating range,
the device has ±2.0°C accuracy. The device commu-
nicates over a 1-Wire bus that, by definition, requires
only one data line (and ground) for communication with
a central microprocessor. In addition, the device can
derive power directly from the data line (“parasite power”),
eliminating the need for an external power supply.
Requiring so few pins enables the device to be placed
in a 3-pin TO-92 package. The form factor of this pack-
age allows the device to be placed above the board and
thus measure the ambient temperature of a system, as
opposed to the board temperature that a surface-mount
package would measure.
Each device has a unique 64-bit serial code, allowing
multiple MAX31820 devices to function on the same
1-Wire bus. Therefore, it is simple to use one micro-
processor to control many devices distributed over a
large area. The 64-bit ROM stores the device’s unique
serial code. The scratchpad memory contains the 2-byte
temperature register that stores the digital output from
the temperature sensor. In addition, the scratchpad pro-
vides access to the 1-byte upper and lower alarm trigger
registers (T
H
and T
L
) and the 1-byte configuration regis-
ter. The configuration register allows the user to set the
resolution of the temperature-to-digital conversion to 9, 10,
11, or 12 bits. The T
H
, T
L
, and configuration registers are
nonvolatile (EEPROM), so they retain data when the
device is powered down.
The device uses Maxim Integrated’s exclusive 1-Wire
bus protocol that implements bus communication using
one control signal. The control line requires a weak pullup
resistor since all devices are linked to the bus through a
three-stateoropen-drainport(i.e.,theMAX31820’sDQ
pin). In this bus system, the microprocessor (the master
device) identifies and addresses devices on the bus
using each device’s unique 64-bit code. Because each
device has a unique code, the number of devices that
can be addressed on one bus is virtually unlimited. The
1-Wire bus protocol, including detailed explanations of the
commands and time slots, is covered in the 1-Wire Bus
System section.
The device can also operate without an external power
supply. Power is instead supplied through the 1-Wire
pullupresistorthroughtheDQpinwhenthebusishigh.
The high bus signal also charges an internal capacitor
(C
PP
), which then supplies power to the device when the
bus is low. This method of deriving power from the 1-Wire
bus is referred to as “parasite power.” Alternatively, an
externalsupplyonV
DD
can also power the device.
Operation
Measuring Ambient Temperature
A conventional surface-mount temperature sensor IC has
an excellent thermal connection to the circuit board on
whichitismounted.Heattravelsfromtheboardthrough
the leads to the sensor die. Air temperature can affect
2
3
1
3
2
1
V
DD
DQ
GND
TO-92
FRONT VIEW
MAX31820
SIDE VIEW
PIN NAME FUNCTION
1 GND Ground
2 DQ
DataIn/Out
3 V
DD
OptionalPowerSupply
MAX31820 1-Wire Ambient Temperature Sensor
www.maximintegrated.com
Maxim Integrated │ 5
Pin Description
Pin Conguration