FSCT17A-UH5 datasheet(9/11 Pages) STMICROELECTRONICS

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These interfaces offer very low thermal resistance. For example, the Sil-Pad® 800 family, from

BERGQUIST, which is designed for low cost applications and low mounting pressures, present a typical

thermal impedance of 0.45 °C.in²/W. This impedance can increase up to 0.92 °C.in²/W for a 10 psi pres-

sure, which is below the normal atmospheric one (1 atm = 1013 hPa = 1013 x 100 x 14.5 x 10-3 = 14.7 psi).

Furthermore, a mounting clip will apply a force between 15 to 50 N. This leads to a 25 to 200 psi pressure.

In this case, the thermal impedance varies from 0.6 to 0.29 °C.in²/W for the Sil-Pad® 800 family.

For the example, we take the worst case hypothesis of a 1°C.in²/W interface between the FSCT case and

the heat sink.

We take into account only the tab surface for the heat exchange. This surface equals typically:

S = 4.7 x 5.1 mm2 = 0.037 in2

This yields to a supplementary resistance of: Rthc-h = 1/0.037 = 27°C/W

Then, the maximum power dissipated in the FSCT, for the maximum output power, is given by the following

equation:

OUT

OMS

max =⋅

⋅

The following numerical application gives (for a 150 mA FAN DC current, a 80 gain for the PNP and a 12 V

power supply):

IOUT = 150 / 80 = 1.87mA

PmW

max

=×

−−

187

5 5

Then, the temperature error is:

()

∆TC

=×

0 046

138

The temperature sensing error can then be neglected whan one considers the operation range (from 0 to

100 °C), even with low cost interface material and without any mounting clip. For example, with a higher

pressure, thanks to a clip, this error could be divided by three.

4. TEMPERATURE/VOLTAGE SLOPE CHANGE

The OUT voltage versus temperature characteristic of the FSCT has been designed to fit majority cases of

application, in the field of PC power supplies.

The advantage for the user is to have a minimum count of components while achieving a smart

temperature regulation.

Nevertheless, some applications require dedicated temperature regulation characteristic.

Figure 13

provides an example of a solution which allows to change the ratio between PNP base voltage and the

temperature. This schematic only requires a dual

single-voltage amplifier (in DIL8 package for

example) and less than ten supplementary

resistors.

This schematic keeps the advantage of applying a

constant minimum voltage (VOMS) below TMS tem-

perature. Indeed, the U1A operational amplifier

subtracts 5.1 V (thanks to the D1 Zener diode, re-

fer to VREF) from VOUT. This means that the ratio

change is only taken into account when VOUT is

lower than 5.1 V. For higher voltages (VOMS in mode

ON or VCC in mode OFF), the new base to GND volt-

age (VNEW) remains the same (cf. figure 12).

So, if one wants to increase accurately the

voltage-temperature ratio, i.e. that VOUT will

decrease more quickly when Tj increases, figure

13 schematics should be implemented. Indeed,

the voltage at the operational amplifier U1B (which

acts as a follower) output is:

()

if V

otherw

NEW

OUT

REF

NEW

OUT

REF

OUT

=−

⋅

−

ise









Mode OFF

Mode ON

VOUT

VNEW

T (°C)

VOMS

Step-down

TON

TOFF

Fig. 12: Modified characteristics.

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