LM27 datasheet(5/10 Pages) NSC

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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

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.

Turn on V

+ and measure V

TEMP. Then calculate the

temperature reading of the LM27 using the equation:

O = (−3.552x10

−6x(T−30)2)+

(−10.69576x10

−3x(T−30)) + 1.8386V

(1)

(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)

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.

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

TEMPTrig=VTEMP at OS trigger (HIGH->LOW)

E. Calculate Ttrig using Equation (2).

A. Gradually raise V

TEMP until OS goes HIGH. Note

TEMP.

B. Calculate T

HYST using Equation (2).

TEMP LOADING

The V

TEMP 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 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

LOAD, it should be at least a factor of 10 smaller than

LOAD.

20030712

LM27-_ _J

20030713

LM27-_ _K

20030714

LM27-_ _L

20030715

LM27-_ _N

FIGURE 1. Output Pin Options Block Diagrams

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