12V

System Fan

Sanyo Denki

109R0612T4H12

LM27

V+

VTEMP

GND

HYST

OS

10k

+5V

0.12F

TJ = TA + "JA(V

+I

Q + (V

+ - V

TEMP)IL_TEMP + VDOIDO)

LM27

SNIS124D – AUGUST 2002 – REVISED JULY 2013

www.ti.com

MOUNTING CONSIDERATIONS

The LM27 can be applied easily in the same way as other integrated-circuit temperature sensors. It can be glued

or cemented to a surface. The temperature that the LM27 is sensing will be within about +0.06°C of the surface

temperature to which the LM27's leads are attached to.

This presumes that the ambient air temperature is almost the same as the surface temperature; if the air

temperature were much higher or lower than the surface temperature, the actual temperature measured would

be at an intermediate temperature between the surface temperature and the air temperature.

To ensure good thermal conductivity, the backside of the LM27 die is directly attached to the GND pin (pin 2).

The temperatures of the lands and traces to the other leads of the LM27 will also affect the temperature that is

being sensed.

Alternatively, the LM27 can be mounted inside a sealed-end metal tube, and can then be dipped into a bath or

screwed into a threaded hole in a tank. As with any IC, the LM27 and accompanying wiring and circuits must be

kept insulated and dry, to avoid leakage and corrosion. This is especially true if the circuit may operate at cold

temperatures where condensation can occur. Printed-circuit coatings and varnishes such as Humiseal and epoxy

paints or dips are often used to ensure that moisture cannot corrode the LM27 or its connections.

The junction to ambient thermal resistance (

θJA) is the parameter used to calculate the rise of a part's junction

temperature due to its power dissipation. For the LM27 the equation used to calculate the rise in the die junction

temperature is as follows:

(4)

where TA is the ambient temperature, V

+ is the power supply voltage, I

Q is the quiescent current, IL_TEMP is the

load current on the VTEMP output, VDO is the voltage on the digital output, and IDO is the load current on the digital

output. Since the LM27's junction temperature is the actual temperature being measured, care should be taken

to minimize the load current that the LM27 is required to drive.

The tables shown in Table 4 summarize the thermal resistance for different conditions and the rise in die

temperature of the LM27 without any loading on VTEMP and a 10k pull-up resistor on an open-drain digital output

with a 5.5V power supply.

Table 4. Thermal resistance (

θJA) and temperature rise due to self heating (TJ−TA)

SOT23-5

SOT23-5

no heat sink

small heat sink

θJA

TJ−TA

θJA

TJ−TA

(°C/W)

(°C)

(°C/W)

(°C)

Still Air

250

0.11

TBD

TBD

Moving Air

TBD

TBD

TBD

TBD

Typical Applications

The fan's control pin has internal pull-up. The 10k pull-down sets a slow fan speed. When the output of the LM27

goes low, the fan will speed up.

Figure 7. Two Speed Fan Speed Control

8

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