FSCTxxA-UH5

5/11

DETAILED DESCRIPTION & APPLICATION INFORMATION

1. OUTPUT CHARACTERISTIC VERSUS TEMPERATURE

1.1 FAN speed control

As it's well known, BRUSHLESS DC motors present the advantage of a quasi-linear speed-voltage char-

acteristic. Hence, the fan speed is varying linearly versus the voltage which is applied across its terminals.

The FSCT Integrated Circuit provides at its OUT terminal, a voltage which changes versus the temperature

sensed through its tab. This OUT voltage decreases as temperature increases, following two modes of op-

eration. By connecting this OUT terminal to the Base of a PNP transistor, as shown in

figure 1, the FAN

voltage will increase with the sensed temperature.

The FAN voltage is given by the following equation:

VV

V

V

FAN

FAN

EB

OUT

=−

−

+

(1)

FAN V

OUT V

()

()

=−

11

(2)

It can be noticed that the PNP operates as a linear amplifier. This avoids EMI and acoustic noise compared

to Pulse Width Modulation control circuits.

1.2 ON/OFF Mode Selection

The

figure 6 gives the algorithm flowchart of the FSCT behavior. First, two modes of operation are distin-

guished by the ON pin signal.

s

Mode ON: activated when ON pin is at High Level or Not Connected (thanks to an internal Pull-up

s

Mode OFF: activated when ON pin is at Low Level.

During Mode ON, the OUT voltage follows a three-parts characteristic, according to its junction tempera-

s

s

s

During Mode OFF, an Hysteresis control allows

the system to switch to mode ON only if the tem-

back off, if the system is cooled enough to lower

The

figure 7 sums up the FSCT Output character-

istic versus the temperature, for the two operation

modes.

1.3 Minimum speed (mode ON)

Mode ON allows users to ensure that the FAN will

be always ON, whatever the ambient temperature

is.

minimum voltage applied across the FAN is 5.5 V

(according to

Equation 2). This voltage is above

stalling value of most fans and will then ensure that

the controlled FAN will always run, in mode ON,

avoiding spurious turn-off due to too low voltage at

low temperature. Annoying noises due to FAN

repetitive starts up are also suppressed.

yes

ON is

LOW

?

BEGIN

no

T

?

yes

FAN OFF

V

FAN ON

V

OR

V

no

FAN ON

V

OR

V

OR

V

T

?

no

yes

FAN OFF (V

yes

ON is

LOW

?

BEGIN

no

T

?

yes

FAN OFF

V

FAN ON

V

OR

V

no

FAN ON

V

OR

V

OR

V

T

?

no

yes

FAN OFF (V

Fig. 6: FSCT behavior flow-chart.