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PDF LM27 Data sheet ( Hoja de datos )
| Número de pieza | LM27 | |
| Descripción | Factory Preset Thermostat | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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September 2006
www.DataSheet4U.com
LM27
SOT-23, ±3˚C Accurate, 120˚C-150˚C Factory Preset
Thermostat
General Description
The LM27 is a precision, single digital-output, low-power
thermostat comprised of an internal reference, DAC, tem-
perature sensor and comparator. Utilizing factory program-
ming, it can be manufactured with different trip points as well
as different digital output functionality. The trip point (TOS)
can be preset at the factory to any temperature in the range
of +120˚C to +150˚C in 1˚C increments. The LM27 has one
digital output (OS/OS/US/US), one digital input (HYST) and
one analog output (VTEMP). The digital output stage can be
preset as either open-drain or push-pull. In addition, it can be
factory programmed to be active HIGH or LOW. The digital
output can be factory programmed to indicate an over tem-
perature shutdown event (OS or OS) or an under tempera-
ture shutdown event (US or US). When preset as an over-
temperature shutdown (OS) it will go LOW to indicate that
the die temperature is over the internally preset TOS and go
HIGH when the temperature goes below (TOS–THYST). Simi-
larly, when preprogrammed as an undertemperature shut-
down (US) it will go HIGH to indicate that the temperature is
below TUS and go LOW when the temperature is above
(TUS+THYST). The typical hysteresis, THYST, can be set to
2˚C or 10˚C and is controlled by the state of the HYST pin. A
VTEMP analog output provides a voltage that is proportional
to temperature and has a −10.7mV/˚C output slope.
Currently, there are several standard parts available, see
ordering information for details. For other part options, con-
tact a National Semiconductor Distributor or Sales Repre-
sentative for information on minimum order qualification. The
LM27 is currently available in a 5-lead SOT-23 package.
Applications
n Microprocessor Thermal Management
n Appliances
n Portable Battery Powered Systems
n Fan Control
n Industrial Process Control
n HVAC Systems
n Electronic System Protection
Features
n Internal comparator with pin selectable 2˚C or 10˚C
hysteresis
n No external components required
n Open-drain or push-pull digital output; supports CMOS
logic levels
n Internal temperature sensor with VTEMP output pin
n VTEMP output allows after-assembly system testing
n Internal voltage reference and DAC for trip-point setting
n Currently available in 5-pin SOT-23 plastic package
n Excellent power supply noise rejection
Key Specifications
j Power Supply Voltage
j Power Supply Current
j Hysteresis Temperature
j Temperature Trip Point Accuracy
2.7V to 5.5V
40µA(max)
15µA(typ)
2˚C or 10˚C(typ)
±3˚C (max)
LM27CIM5-2HJ Simplified Block Diagram and Connection Diagram
The LM27CIM5-2HJ has a fixed trip point of 140˚C.
For other trip point and output function availability,
please see ordering information or contact National Semiconductor.
20030701
© 2006 National Semiconductor Corporation DS200307
www.national.com
1 page  
Functional Description
LM27 OPTIONS
LM27-_ _J
20030712
LM27-_ _K
20030713
LM27-_ _L
20030714
LM27-_ _N
20030715
FIGURE 1. Output Pin Options Block Diagrams
The LM27 can be factory programmed to have a trip point
anywhere in-between 120˚C to 150˚C.
Applications Hints
AFTER-ASSEMBLY PCB TESTING
The LM27’s VTEMP output allows after-assembly PCB testing
by following a simple test procedure. Simply measuring the
VTEMP output voltage will verify that the LM27 has been
assembled properly and that its temperature sensing cir-
cuitry is functional. The VTEMP output has very weak drive
capability that can be overdriven by 1.5mA. Therefore, one
can simply force the VTEMP voltage to cause the digital
output to change state, thereby verifying that the comparator
and output circuitry function after assembly. Here is a
sample test procedure that can be used to test the
LM27CIM5X-2HJ which has a 140˚C trip point.
1. Turn on V+ and measure VTEMP. Then calculate the
temperature reading of the LM27 using the equation:
VO = (−3.552x10−6x(T−30)2) +
(−10.69576x10−3x(T−30)) + 1.8386V
(1)
or
(2)
2. Verify that the temperature measured in step one is
within (±3˚C + error of reference temperature sensor) of
the ambient/board temperature. The ambient/board tem-
perature (reference temperature) should be measured
using an extremely accurate calibrated temperature sen-
sor, which is in close proximity to and mounted on the
same PCB as the LM27 perhaps even touching the GND
lead of the LM27 if possible. The LM27 will sence the
board temperature not the ambient temperature (see
Section Titled Mounting Considerations)
3.
A. Observe that OS is high.
B. Drive VTEMP to ground.
C. Observe that OS is now low.
D. Release the VTEMP pin.
E. Observe that OS is now high.
4.
A. Observe that OS is high.
B. Drive VTEMP voltage down gradually.
C. When OS goes low, note the VTEMP voltage.
D. VTEMPTrig = VTEMP at OS trigger (HIGH->LOW)
E. Calculate Ttrig using Equation (2).
5.
A. Gradually raise VTEMP until OS goes HIGH. Note
VTEMP.
B. Calculate THYST using Equation (2).
VTEMP LOADING
The VTEMP output has very weak drive capability (1 µA
source, 40 µA sink). So care should be taken when attaching
circuitry to this pin. Capacitive loading may cause the VTEMP
output to oscillate. Simply adding a resistor in series as
shown in Figure 2 will prevent oscillations from occurring. To
determine the value of the resistor follow the guidelines
given in Table 1. The same value resistor will work for either
placement of the resistor. If an additional capacitive load is
placed directly on the LM27 output, rather than across
CLOAD, it should be at least a factor of 10 smaller than
CLOAD.
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| PDF Descargar | [ Datasheet LM27.PDF ] | |
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