National Semiconductor LM26 Series, Manual

Page: 5 / 9

Functional Description
LM26 OPTIONS
The LM26 can be factory programmed to have a trip point
anywhere in the range of −55˚C to +110˚C.
Applications Hints
AFTER-ASSEMBLY PCB TESTING
The LM26’s V
TEMP
output allows after-assembly PCB testing
by following a simple test procedure. Simply measuring the
V
TEMP
output voltage will verify that the LM26 has been
assembled properly and that its temperature sensing cir-
cuitry is functional. The V
TEMP
output has very weak drive
capability that can be overdriven by 1.5mA. Therefore, one
can simply force the V
TEMP
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
LM26CIM5-TPA which has an 85˚C trip point.
1. Turn on V
+
and measure V
TEMP
. Then calculate the
temperature reading of the LM26 using the equation:
V
O
= (−3.479x10
−6
x(T−30)
2
) + (−1.082x10
−2
x(T−30)) +
1.8015V (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.
3.
A. Observe that OS is high.
B. Drive V
TEMP
to ground.
C. Observe that OS is now low.
D. Release the V
TEMP
pin.
E. Observe that OS is now high.
4.
A. Observe that OS is high.
B. Drive V
TEMP
voltage down gradually.
C. When OS goes low, note the V
TEMP
voltage.
D. V
TEMP
Trig = V
TEMP
at OS trigger (HIGH-
>
LOW)
E. Calculate Ttrig using Equation (2) .
5.
A. Gradually raise V
TEMP
until OS goes HIGH. Note
V
TEMP
.
B. Calculate T
HYST
using Equation (2) .
V
TEMP
LOADING
The V
TEMP
output has very weak drive capability (40µA
source, 1µA sink). So care should be taken when attaching
circuitry to this pin. Capacitive loading may cause the V
TEMP
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 LM26 output, rather than across
C
LOAD
, it should be at least a factor of 10 smaller than
C
LOAD
.
10132312
LM26-_ _A
10132313
LM26-_ _B
10132314
LM26-_ _C
10132315
LM26-_ _D
FIGURE 1. Output Pin Options Block Diagrams
LM26
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