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FSCTxxA-UH5
September 2003 - Ed: 2A
s
Built in thermal sensor
s
Brushless DC fan speed control
s
Linear control and regulation of the fan speed
according to the temperature
s
Green OFF mode operation with automatic
turn- ON in case of overtemperature
s
Voltage limitation above fan stall voltage
s
No external NTC required
FEATURES
FAN SPEED CONTROLER
s
COMPUTER AND SERVER POWER SUPPLY
s
TELECOM
s
LOW COST VENTILATION (consumer)
APPLICATIONS
s
Low cost integrated fan speed control
s
High integration: only one or two external com-
ponents
s
Reduced acoustic noise because of the linear
control of the external transistor
s
Power supply noise rejection
s
Good thermal coupling with heater
s
Reduced ON/OFF oscillation because of large
hysteresis
s
High current capability (base current higher
than 10 mA)
s
Higher accuracy than discrete circuit with NTC
BENEFITS
IPPAK
1
5
GND
OUT
BIAS
F
ON/OFF
LOW
ON/OFF
HIGH
C
=
100nF
OUT
V
CC
V
T
HYST.-EN
HYSTERESIS
COMPARATOR
DRIVER
ON
REGULATION
VOLTAGE
REF.
TEMP.
SENSOR
ON
FSCTxxA-UH5
Fig. 1: Block Diagram.
A.S.D.
TM
Application Specific Discretes
The FSCTxxA-UH5 is a high-integrated low cost fan speed controller suitable for PC desktops, notebooks
and server power supplies and also all kind of equipments where low cost ventilation system is needed. An
internal thermal sensor connected to the tab is used to regulate the fan speed. A continuous analog voltage
proportional to the tab temperature is produced at the OUT terminal and can control linearly an external
PNP transistor (Fan Transistor) connected in series with a Brushless DC Fan, in order to vary its speed.
The ON control terminal can be used to select two operation modes. If not connected (high impedance
state) ON mode is selected by default.
In Mode ON, the fan speed is regulated according to the tab temperature of the FSCT by adjusting the Vbe
voltage of the PNP fan Transistor. In low temperature conditions, the fan transistor voltage is limited to
about 5.5V (for a 12V fan) in order to guarantee a minimum voltage across the fan above its stalling voltage
and to keep it running (see figure 3).
This regulation is implemented with a high Fan supply voltage rejection.
The mode OFF allows to operate in green Mode. The fan is stopped as long as the temperature remains
below the TON threshold. When the temperature rises above TON the fan is forced to ON and the speed is
regulated according to the temperature as in the same way as mode ON. When temperature falls below
TOFF, the fan is automatically turned OFF. The hysteresis has been dimensioned large enough to reduce
ON/OFF oscillations during the green mode operation.
GENERAL DESCRIPTION
FSCTxxA-UH5
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Symbol
Parameter
Value
Unit
V
CC
Supply voltage referenced to GND
-0.3 to 20
V
T
j
Operating junction temperature
0 to 125
C
Tstg
Storage temperature
-65 to 150
C
T
L
Lead temperature (double wave soldering)
260
C
Maximum voltage at any pin
V
CC
V
Minimum voltage at any pin
-0.3
V
ESD
Electrostatic discharge immunity at each terminal
(human body model)
2
kV
ABSOLUTE MAXIMUM RATINGS
Pint Out
designation
Description
Pin
TEST
Test pin (must not be connected)
1
V
CC
Operating DC supply
2
GND
Ground (internally connected to the tab)
3
ON
Mode ON and OFF selection
4
OUT
Output (connect to PNP fan transistor base)
5
PINT OUT DESIGNATION
Symbol
Parameter
Test conditions
Value
Unit
Min.
Typ.
Max.
SUPPLY
I
CCmax
Operating supply current V
ON
= 5V or 0V
V
CC
= 12V
1.1
3
mA
DRIVER
I
OL
Sinking current capability
at the OUT terminal
V
OUT
= V
OL
and T
j
= 125C
10
mA
I
LEAK
Maximum sink leakage
current at OUT pin (mode
OFF)
V
OUT
= 20V, V
ON
= 0V
T
j
= 50C
10
A
V
OL
Minimum voltage at the
OUT terminal (mode ON)
V
ON
= 5V, T
j
= 125C
0.55
0.7
V
V
OMS
OUT voltage for mini-
mum fan speed (mode
ON)
V
ON
= 5V, T
j
from 0C to T
MS
5.3
5.5
V
Static line regulation
V
CC
= 9V to 15V, V
ON
= 5V
T
j
= 25C
0.5
mV/V
K
SL
V
OUT
temperature slope
V
OUT
= 80%
V
OMS
to
20% V
OMS
FSCT17A-UH5
155
165
175
mV/C
FSCT11A-UH5
100
107
114
FSCT07A-UH5
68
72
77
ELECTRICAL CHARACTERISTICS (Continue)
V
CC
from 9Vdc to 15Vdc, C
out
= 100nF, T
j
= 0C to 125C, otherwise specified
FSCTxxA-UH5
3/11
Symbol
Parameter
Test conditions
Value
Unit
Min.
Typ.
Max.
TEMPERATURE MANAGEMENT (all T threshold are guaranteed by design)
T
MS
Temperature regulation
threshold
V
OUT
= V
OMS
V
ON
= 5V
32
37
42
C
T
ON
OFF mode switch ON
temperature
V
CC
= 12V
V
ON
= 0V
66
71
76
C
T
OFF
OFF mode switch OFF
temperature
V
CC
= 12V
V
ON
= 0V
34
39
44
C
T
OL
Minimum temperature
for maximum speed
V
CC
= 12V
V
ON
= 5V
V
OUT
= V
OL
FSCT17A-UH5
61
66
71
C
FSCT11A-UH5
76
81
86
FSCT07A-UH5
97
102
107
T
V3
Tab temperature for
V
OUT
= 3V
(*)
(see figure 3)
V
OUT
= 3V
V
ON
= 5V
FSCT17A-UH5
49
52
55
C
FSCT11A-UH5
56
59
62
FSCT07A-UH5
66
69
72
ON SIGNAL
V
IH
Voltage range for ON mode
1
V
V
IL
Voltage range for OFF mode
0.3
V
I
S
Source current at the
ON pin
(see figure 2)
V
CC
= 12V
ON terminal shorted to ground
10
25
A
(*) Absolute temperature dissipation applicable on all V
OUT
range between V
OMS
and V
OL
ELECTRICAL CHARACTERISTICS (Continue)
V
CC
within the supply voltage range, C
out
= 100nF, T
j
= 0C to 125C, otherwise specified
Symbol
Parameter
Value
Unit
Rth
(j-c)
Thermal resistance junction to case
3
C/W
THERMAL PARAMETERS
50k
To other blocks
I
S
V
CC
ON
Fig. 2: "ON" input schematic diagram.
FSCTxxA-UH5
4/11
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
9
11
13
15
17
19
V
(V)
CC
I
(mA)
CC
Tj = 25C
Tj = 125C
Fig. 4: I
CC
versus V
CC
for mode ON.
V
CC
V
CC
GND
ON
OUT
I
CC
3.8k
100nF
V
FAN
12V
V
ON
1V
A
Fig. 5: I
CC
versus V
CC
test circuit (mode ON).
V
(V
)
OUT
DC
T (C)
j
V
= 5.3V
OMS
V
= 3V
OUT
V
= 0.6V
OL
T
=37
MS
T
V3
T
V3
T
V3
T
=81
OL
T
=102
OL
FSCT07A-UH5
72mV/C
T
=66
OL
FSCT17A-UH5
165mV/C
FSCT11A-UH5
107mV/C
Fig. 3: Temperature slopes.
FSCTxxA-UH5
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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:
V
V
V
V
FAN
FAN
EB
OUT
=
-
-
+
(1)
Typically, for a 12 V V
FAN+
and a 1 V V
EB
voltage, this gives:
V
V
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
curren source "I
S
")
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-
ture (T
j
):
s
T
j
< T
MS
: in this case, V
OUT
is limited to V
OMS
to avoid stalling the fan rotor
s
T
MS
< T
j
< T
OL
: V
OUT
is linearly regulated versus the junction temperature
s
T
j
> T
OL
: in this case, V
OUT
is clamped to its minimum value (V
OL
).
During Mode OFF, an Hysteresis control allows
the system to switch to mode ON only if the tem-
perature exceeds the T
ON
value. The Fan is turned
back off, if the system is cooled enough to lower
the temperature below T
OFF
.
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.
The OUT voltage is clamped to V
OMS
for low tem-
perature (below T
MS
= 37 C typically).
The V
OMS
has been set to 5.5 V max , thus the
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
j
> T
ON
?
yes
FAN OFF
V
OUT
= V
CC
FAN ON
V
OUT
= V
OMS
k
SL
.(T
j
T
MS
)
OR
V
OUT
= V
OL
IF T
j
> T
OL
no
FAN ON
V
OUT
= V
OMS
k
SL
.(T
j
T
MS
)
OR
V
OUT
= V
OMS
IF T
j
< T
MS
OR
V
OUT
= V
OL
IF T
j
> T
OL
T
j
< T
OFF
?
no
yes
FAN OFF (V
OUT
= V
CC
)
yes
ON is
LOW
?
BEGIN
no
T
j
> T
ON
?
yes
FAN OFF
V
OUT
= V
CC
FAN ON
V
OUT
= V
OMS
k
SL
.(T
j
T
MS
)
OR
V
OUT
= V
OL
IF T
j
> T
OL
no
FAN ON
V
OUT
= V
OMS
k
SL
.(T
j
T
MS
)
OR
V
OUT
= V
OMS
IF T
j
< T
MS
OR
V
OUT
= V
OL
IF T
j
> T
OL
T
j
< T
OFF
?
no
yes
FAN OFF (V
OUT
= V
CC
)
Fig. 6: FSCT behavior flow-chart.
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