lm27-2plmda National Semiconductor Corporation, lm27-2plmda Datasheet - Page 6

• Manufacturer Part Number: lm27-2plmda
• Description: Accurate, 120 C-150 C Factory Preset Thermostat Lm27 Form
• Manufacturer: National Semiconductor Corporation

www.national.com
Applications Hints
AFTER-ASSEMBLY PCB TESTING
The LM27A's V
by following a simple test procedure. Simply measuring the
V
sembled properly and that its temperature sensing circuitry is
functional. The V
that can be overdriven by 1.5mA. Therefore, one can simply
force the V
state, thereby verifying that the comparator and output cir-
cuitry function after assembly. Here is a sample test proce-
dure that can be used to test a part that would have the part
number LM27-2HJ MDA, which would have a 140°C trip point.
1.
2.
3.
4.
5.
V
The V
40 µA sink). So care should be taken when attaching circuitry
to this pin. Capacitive loading may cause the V
oscillate. Simply adding a resistor in series as shown in Figure
1 will prevent oscillations from occurring. To determine the
value of the resistor follow the guidelines given in Table 1. The
TEMP
TEMP
Turn on V
temperature reading of the LM27A using the equation:
or
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
temperature (reference temperature) should be
measured using an extremely accurate calibrated
temperature sensor, which is in close proximity to and
mounted on the same PCB as the LM27A perhaps even
touching the GND lead of the LM27A if possible. The
LM27A will sence the board temperature not the ambient
temperature.
A.
B.
C. Observe that OS is now low.
D. Release the V
E.
A.
B.
C. When OS goes low, note the V
D. V
E.
A.
B.
TEMP
output voltage will verify that the LM27A has been as-
LOADING
Observe that OS is high.
Drive V
Observe that OS is now high.
Observe that OS is high.
Drive V
Calculate Ttrig using Equation 2.
Gradually raise V
V
Calculate T
TEMP
TEMP
output has very weak drive capability (1 µA source,
TEMP
(−10.69576×10
V
+
TEMP
Trig = V
.
O
voltage to cause the digital output to change
and measure V
TEMP
TEMP
TEMP
= (−3.552×10
output allows after-assembly PCB testing
HYST
to ground.
voltage down gradually.
output has very weak drive capability
TEMP
TEMP
using Equation 2.
−3
TEMP
×(T−30)) + 1.8386V
at OS trigger (HIGH->LOW)
pin.
TEMP
−6
until OS goes HIGH. Note
×(T−30)
. Then calculate the
TEMP
2
) +
voltage.
TEMP
output to
(1)
(2)
6
same value resistor will work for either placement of the re-
sistor. If an additional capacitive load is placed directly on the
LM27A output, rather than across C
a factor of 10 smaller than C
TABLE 1. Resistive compensation for capacitive loading
NOISE CONSIDERATIONS
The LM27A has excellent power supply noise rejection. Listed
below is a variety of signals used to test the LM27A power
supply rejection. False triggering of the output was not ob-
served when these signals where coupled into the V+ pin of
the LM27A.
•
•
•
Testing was done while maintaining the temperature of the
LM27A one degree centigrade way from the trip point with the
output not activated.
FIGURE 1. Resistor placement for capacitive loading
square wave 400kHz, 1Vp-p
square wave 2kHz, 200mVp-p
sine wave 100Hz to 1MHz, 200mVp-p
≤
100nF
b) R in series with signal path
C
≥
10nF
100pF
a) R in series with capacitor
1nF
LOAD
1µF
compensation of V
of V
LOAD
TEMP
.
LOAD
TEMP
, it should be at least
R (Ω)
8200
3000
1000
430
20153017
0
20153018