LM75A
Digital temperature sensor and thermal watchdog
Rev. 04 — 10 July 2007
Product data sheet
1. General description
The LM75A is a temperature-to-digital converter using an on-chip band gap temperature
sensor and Sigma-delta A-to-D conversion technique. The device is also a thermal
detector providing an overtemperature detection output. The LM75A contains a number of
data registers: Configuration register (Conf) to store the device settings such as device
operation mode, OS operation mode, OS polarity and OS fault queue as described in
Section 7 “Functional description”;
temperature register (Temp) to store the digital temp
reading, and set-point registers (Tos and Thyst) to store programmable overtemperature
shutdown and hysteresis limits, that can be communicated by a controller via the 2-wire
serial I
2
C-bus interface. The device also includes an open-drain output (OS) which
becomes active when the temperature exceeds the programmed limits. There are three
selectable logic address pins so that eight devices can be connected on the same bus
without address conflict.
The LM75A can be configured for different operation conditions. It can be set in normal
mode to periodically monitor the ambient temperature, or in shutdown mode to minimize
power consumption. The OS output operates in either of two selectable modes:
OS comparator mode or OS interrupt mode. Its active state can be selected as either
HIGH or LOW. The fault queue that defines the number of consecutive faults in order to
activate the OS output is programmable as well as the set-point limits.
The temperature register always stores an 11-bit 2's complement data giving a
temperature resolution of 0.125
°C.
This high temperature resolution is particularly useful
in applications of measuring precisely the thermal drift or runaway.
The device is powered-up in normal operation mode with the OS in comparator mode,
temperature threshold of 80
°C
and hysteresis of 75
°C,
so that it can be used as a
stand-alone thermostat with those pre-defined temperature set points.
2. Features
I
Pin-for-pin replacement for industry standard LM75 and offers improved temperature
resolution of 0.125
°C
and specification of a single part over power supply range from
2.8 V to 5.5 V
I
Small 8-pin package types: SO8 and TSSOP8
I
I
2
C-bus interface with up to 8 devices on the same bus
I
Power supply range from 2.8 V to 5.5 V
I
Temperatures range from
−55 °C
to +125
°C
I
11-bit ADC that offers a temperature resolution of 0.125
°C
NXP Semiconductors
LM75A
Digital temperature sensor and thermal watchdog
I
Temperature accuracy of:
N
±2 °C
from
−25 °C
to +100
°C
N
±3 °C
from
−55 °C
to +125
°C
I
Programmable temperature threshold and hysteresis set points
I
Supply current of 3.5
µA
in shutdown mode for power conservation
I
Stand-alone operation as thermostat at power-up
I
ESD protection exceeds 2000 V HBM per JESD22-A114, 200 V MM per
JESD22-A115 and 1000 V CDM per JESD22-C101
I
Latch-up testing is done to JEDEC Standard JESD78 which exceeds 100 mA
3. Applications
I
I
I
I
System thermal management
Personal computers
Electronics equipment
Industrial controllers
4. Ordering information
Table 1.
Ordering information
Package
Name
LM75AD
LM75ADP
SO8
TSSOP8
Description
plastic small outline package; 8 leads;
body width 3.9 mm
plastic thin shrink small outline package; 8 leads;
body width 3 mm
Version
SOT96-1
SOT505-1
Type number
4.1 Ordering options
Table 2.
LM75AD
LM75ADP
Ordering options
Topside mark
LM75A
L75A
Temperature range
T
amb
=
−55 °C
to +125
°C
T
amb
=
−55 °C
to +125
°C
Type number
LM75A_4
© NXP B.V. 2007. All rights reserved.
Product data sheet
Rev. 04 — 10 July 2007
2 of 24
NXP Semiconductors
LM75A
Digital temperature sensor and thermal watchdog
5. Block diagram
V
CC
LM75A
BIAS
REFERENCE
POINTER
REGISTER
COUNTER
11-BIT
SIGMA-DELTA
A-to-D
CONVERTER
CONFIGURATION
REGISTER
TEMPERATURE
REGISTER
TOS
REGISTER
THYST
REGISTER
OS
BAND GAP
TEMP SENSOR
TIMER
COMPARATOR/
INTERRUPT
OSCILLATOR
POWER-ON
RESET
LOGIC CONTROL AND INTERFACE
002aad029
A2
A1
A0
SCL SDA
GND
Fig 1. Block diagram
6. Pinning information
6.1 Pinning
SDA
SCL
OS
GND
1
2
8
7
V
CC
A0
A1
A2
SDA
SCL
OS
GND
1
2
3
4
002aad028
8
7
V
CC
A0
A1
A2
LM75AD
3
4
002aad027
6
5
LM75ADP
6
5
Fig 2. Pin configuration for SO8
Fig 3. Pin configuration for TSSOP8
6.2 Pin description
Table 3.
Symbol
SDA
SCL
OS
GND
A2
Pin description
Pin
1
2
3
4
5
Description
Digital I/O. I
2
C-bus serial bidirectional data line; open-drain.
Digital input. I
2
C-bus serial clock input.
Overtemp Shutdown output; open-drain.
Ground. To be connected to the system ground.
Digital input. User-defined address bit 2.
LM75A_4
© NXP B.V. 2007. All rights reserved.
Product data sheet
Rev. 04 — 10 July 2007
3 of 24
NXP Semiconductors
LM75A
Digital temperature sensor and thermal watchdog
Pin description
…continued
Pin
6
7
8
Description
Digital input. User-defined address bit 1.
Digital input. User-defined address bit 0.
Power supply.
Table 3.
Symbol
A1
A0
V
CC
7. Functional description
7.1 General operation
The LM75A uses the on-chip band gap sensor to measure the device temperature with
the resolution of 0.125
°C
and stores the 11-bit 2's complement digital data, resulted from
11-bit A-to-D conversion, into the device Temp register. This Temp register can be read at
any time by a controller on the I
2
C-bus. Reading temperature data does not affect the
conversion in progress during the read operation.
The device can be set to operate in either mode: normal or shutdown. In normal operation
mode, the temp-to-digital conversion is executed every 100 ms and the Temp register is
updated at the end of each conversion. In shutdown mode, the device becomes idle, data
conversion is disabled and the Temp register holds the latest result; however, the device
I
2
C-bus interface is still active and register write/read operation can be performed. The
device operation mode is controllable by programming bit B0 of the configuration register.
The temperature conversion is initiated when the device is powered-up or put back into
normal mode from shutdown.
In addition, at the end of each conversion in normal mode, the temperature data (or Temp)
in the Temp register is automatically compared with the overtemperature shutdown
threshold data (or Tos) stored in the Tos register, and the hysteresis data (or Thyst) stored
in the Thyst register, in order to set the state of the device OS output accordingly. The
device Tos and Thyst registers are write/read capable, and both operate with 9-bit
2's complement digital data. To match with this 9-bit operation, the Temp register uses
only the 9 MSB bits of its 11-bit data for the comparison.
The way that the OS output responds to the comparison operation depends upon the OS
operation mode selected by configuration bit B1, and the user-defined fault queue defined
by configuration bits B3 and B4.
In OS comparator mode, the OS output behaves like a thermostat. It becomes active
when the Temp exceeds the T
os
, and is reset when the Temp drops below the T
hyst
.
Reading the device registers or putting the device into shutdown does not change the
state of the OS output. The OS output in this case can be used to control cooling fans or
thermal switches.
In OS interrupt mode, the OS output is used for thermal interruption. When the device is
powered-up, the OS output is first activated only when the Temp exceeds the T
os
; then it
remains active indefinitely until being reset by a read of any register. Once the OS output
has been activated by crossing T
os
and then reset, it can be activated again only when the
Temp drops below the T
hyst
; then again, it remains active indefinitely until being reset by a
read of any register. The OS interrupt operation would be continued in this sequence:
T
os
trip, Reset, T
hyst
trip, Reset, T
os
trip, Reset, T
hyst
trip, Reset, etc.
LM75A_4
© NXP B.V. 2007. All rights reserved.
Product data sheet
Rev. 04 — 10 July 2007
4 of 24
NXP Semiconductors
LM75A
Digital temperature sensor and thermal watchdog
In both cases, comparator mode and interrupt mode, the OS output is activated only if a
number of consecutive faults, defined by the device fault queue, has been met. The fault
queue is programmable and stored in the two bits, B3 and B4, of the Configuration
register. Also, the OS output active state is selectable as HIGH or LOW by setting
accordingly the configuration register bit B2.
At power-up, the device is put into normal operation mode, the T
os
is set to 80
°C,
the T
hyst
is set to 75
°C,
the OS active state is selected LOW and the fault queue is equal to 1. The
temp reading data is not available until the first conversion is completed in about 100 ms.
The OS response to the temperature is illustrated in
Figure 4.
T
os
T
hyst
reading temperature limits
OS reset
OS active
OS output in comparator mode
OS reset
OS active
(1)
(1)
(1)
OS output in interrupt mode
002aad032
(1) OS is reset by either reading register. It is assumed that the fault queue is met at each T
os
and
T
hyst
crossing point.
Fig 4. OS response to temperature
7.2 I
2
C-bus serial interface
The LM75A can be connected to a compatible 2-wire serial interface I
2
C-bus as a slave
device under the control of a controller or master device, using two device terminals, SCL
and SDA. The controller must provide the SCL clock signal and write/read data to/from the
device through the SDA terminal. Notice that if the I
2
C-bus common pull-up resistors have
not been installed as required for I
2
C-bus, then an external pull-up resistor, about 10 kΩ,
is needed for each of these two terminals. The bus communication protocols are
described in
Section 7.10.
7.3 Slave address
The LM75A slave address on the I
2
C-bus is partially defined by the logic applied to the
device address pins A2, A1 and A0. Each of them is typically connected either to GND for
logic 0, or to V
CC
for logic 1. These pins represent the three LSB bits of the device 7-bit
address. The other four MSB bits of the address data are preset to ‘1001’ by hard wiring
inside the LM75A.
Table 4
shows the device’s complete address and indicates that up to
LM75A_4
© NXP B.V. 2007. All rights reserved.
Product data sheet
Rev. 04 — 10 July 2007
5 of 24
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