LM27 National Semiconductor, LM27 Datasheet - Page 5

• Manufacturer Part Number: LM27
• Description: Factory Preset Thermostat
• Manufacturer: National Semiconductor

Functional Description
LM27 OPTIONS
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 V
by following a simple test procedure. Simply measuring the
V
assembled properly and that its temperature sensing cir-
cuitry is functional. The V
capability that can be overdriven by 1.5mA. Therefore, one
can simply force the V
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
2. Verify that the temperature measured in step one is
TEMP
temperature reading of the LM27 using the equation:
or
within (
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
output voltage will verify that the LM27 has been
±
(−10.69576x10
3˚C + error of reference temperature sensor) of
TEMP
V
+
O
and measure V
= (−3.552x10
output allows after-assembly PCB testing
TEMP
LM27-_ _J
LM27-_ _L
TEMP
−3
x(T−30)) + 1.8386V
voltage to cause the digital
−6
output has very weak drive
x(T−30)
TEMP
FIGURE 1. Output Pin Options Block Diagrams
. Then calculate the
2
) +
20030714
20030712
(1)
(2)
5
3.
4.
5.
V
The V
source, 40 µA sink). So care should be taken when attaching
circuitry to this pin. Capacitive loading may cause the V
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
C
C
TEMP
LOAD
LOAD
board temperature not the ambient temperature (see
Section Titled Mounting Considerations)
A. Observe that OS is high.
B. Drive V
C. Observe that OS is now low.
D. Release the V
E. Observe that OS is now high.
A. Observe that OS is high.
B. Drive V
C. When OS goes low, note the V
D. V
E. Calculate Ttrig using Equation (2).
A. Gradually raise V
B. Calculate T
, it should be at least a factor of 10 smaller than
.
TEMP
LOADING
V
TEMP
TEMP
output has very weak drive capability (1 µA
Trig = V
.
TEMP
TEMP
HYST
to ground.
voltage down gradually.
TEMP
TEMP
LM27-_ _K
LM27-_ _N
using Equation (2).
TEMP
pin.
at OS trigger (HIGH-
until OS goes HIGH. Note
TEMP
voltage.
www.national.com
20030715
20030713
>
LOW)
TEMP