Rainbow Electronics LM27 - Manual (Page 5)

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
TEMP
output allows after-assembly PCB testing
by following a simple test procedure. Simply measuring the
V
TEMP
output voltage will verify that the LM27 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
LM27CIM5X-2HJ which has a 140˚C trip point.
1. Turn on V + and measure V
TEMP
. Then calculate the
temperature reading of the LM27 using the equation:
V
O
= (−3.552x10 −6 x(T−30) 2 ) +
(−10.69576x10 −3 x(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 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 LM27 output, rather than across
C
LOAD
, it should be at least a factor of 10 smaller than
C
LOAD
.
20030712
LM27-_ _J
20030713
LM27-_ _K
20030714
LM27-_ _L
20030715
LM27-_ _N
FIGURE 1. Output Pin Options Block Diagrams
LM27
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