TC648_13 Datasheet, PDF (10/28 Pages) Microchip Technology – Fan Speed Controller with Auto-Shutdown and Over-Temperature Alert

TC648
5.1 Temperature Sensor Design
The temperature signal connected to V_IN must output a
voltage in the range of 1.25V to 2.65V (typical) for 0%
to 100% of the temperature range of interest. The
circuit in Figure 5-2 illustrates a convenient way to pro-
vide this signal using a temperature dependent voltage
divider circuit.
VDD
IDIV
RT₁
NTC Thermistor
100 kΩ @25˚C
R₁ = 100 kΩ
VIN
R₂ = 23.2 kΩ
FIGURE 5-2:
Circuit.
Temperature Sensing
RT₁ is a conventional NTC thermistor and R₁ and R₂
are standard resistors. The supply voltage (V_DD) is
divided between R₂ and the parallel combination of
RT₁ and R₁. For convenience, the parallel combination
of RT₁ and R₁ will be referred to as R_TEMP. The resis-
tance of the thermistor at various temperatures is
obtained from the manufacturer’s specifications.
Thermistors are often referred to in terms of their resis-
tance at 25°C.
Generally, the thermistor shown in Figure 5-2 is a non-
linear device with a negative temperature coefficient
(also called an NTC thermistor). In Figure 5-2, R₁ is
used to linearize the thermistor temperature response
and R₂ is used to produce a positive temperature
coefficient at the V_IN node. As an added benefit, this
configuration produces an output voltage delta of 1.4V,
which is well within the range of the V_C(SPAN)
specification of the TC648. A 100 kNTC thermistor is
selected for this application in order to keep I_DIV to a
minimum.
For the voltage range at V_IN to be equal to 1.25V to
2.65V, the temperature range of this configuration is
0°C to 50°C. If a different temperature range is required
from this circuit, R₁ should be chosen to equal the
resistance value of the thermistor at the center of this
new temperature range. It is suggested that a maxi-
mum temperature range of 50°C be used with this cir-
cuit due to thermistor linearity limitations. With this
change, R₂ is adjusted according to the following
equations:
DS21448D-page 10
EQUATION
V_DD x R₂
= V(T₁)
R_TEMP (T₁) + R₂
V_DD x R₂
= V(T₂)
R_TEMP (T₂) + R₂
Where T₁ and T₂ are the chosen temperatures and
R_TEMP is the parallel combination of the thermistor
and R₁.
These two equations facilitate solving for the two
unknown variables, R₁ and R₂. More information about
thermistors may be obtained from AN679, “Tempera-
ture Sensing Technologies”, and AN685, “Thermistors
in Single Supply Temperature Sensing Circuits”, which
can be downloaded from Microchip's web site at
www.microchip.com.
5.2 Minimum Speed Mode
The TC648 is configured for minimum speed mode by
grounding V_AS and designing the temperature sensor
network such that V_IN operates the fan at relatively con-
stant, minimum speed when the thermistor is at
minimum temperature. Figure 5-3 shows operation in
minimum speed mode. The 0% and 100% fan speeds
correspond to V_IN values of 1.25V and 2.65V, typical.
Minimum system temperature (T_MIN) is defined as the
lowest measured temperature at which proportional fan
speed control is required by the system. The fan
operates at minimum speed for all temperatures below
T_MIN and at speeds proportional to the measured
temperature between T_MIN and T_MAX.
Fan Speed
100%
Minimum
Speed
0%
TMIN
TMAX
FIGURE 5-3:
Operation.
Minimum Fan Speed Mode
Temperature sensor design consists of a two-point
calculation: one at T_MIN and one at T_MAX. At T_MIN, the
ohmic value of the thermistor must be much higher
than that of R₁ so that minimum speed is determined
primarily by the values of R₁ and R₂. At T_MAX, the
ohmic value of the thermistor must result in a V_IN of
2.65V nominal. The design procedure consists of ini-
tially choosing R₁ to be 10 times smaller than the
 2001-2012 Microchip Technology Inc.