TC642EV Microchip Technology, TC642EV Datasheet - Page 12

• Manufacturer Part Number: TC642EV
• Description: KIT EVALUATION FOR TC642
• Manufacturer: Microchip Technology

Specifications of TC642EV

• Processor To Be Evaluated: TC642 and TC646
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: Q698359

TC642
Table 5-1 lists recommended values for R
on the nominal operating current of the fan. Note that
the current draw specified by the fan manufacturer may
be a worst-case rating for near-stall conditions and may
not be the fan’s nominal operating current. The values
in Table 5-1 refer to actual average operating current. If
the fan current falls between two of the values listed,
use the higher resistor value. The end result of employ-
ing Table 5-1 is that the signal developed across the
sense resistor is approximately 450 mV in amplitude.
TABLE 5-1:
5.5
The TC642 is designed to drive an external transistor
or MOSFET for modulating power to the fan. This is
shown as Q
and 5-9. The V
of 5 mA and a minimum sink current of 1 mA. Bipolar
transistors or MOSFETs may be used as the power
switching element, as shown in Figure 5-7. When high
current gain is needed to drive larger fans, two transis-
tors may be used in a Darlington configuration. Three
possible circuit topologies are shown in Figure 5-7: (a)
shows a single NPN transistor used as the switching
element; (b) illustrates the Darlington pair; and (c)
shows an N-channel MOSFET.
One major advantage of the TC642’s PWM control
scheme versus linear speed control is that the power
dissipation in the pass element is kept very low. Gener-
ally, low cost devices in very small packages, such as
TO-92 or SOT, can be used effectively. For fans with
nominal operating currents of no more than 200 mA, a
single transistor usually suffices. Above 200 mA, the
Darlington or MOSFET solution is recommended. For
the fan sensing function to work correctly, it is impera-
tive that the pass transistor be fully saturated when
“on”.
Table 5-2 gives examples of some commonly available
transistors and MOSFETs. This table should be used
as a guide only since there are many transistors and
MOSFETs which will work just as well as those listed.
The critical issues when choosing a device to use as
Q
DS21444C-page 12
Nominal Fan Current (mA)
1
are: (1) the breakdown voltage (V
Output Drive Transistor Selection
100
150
200
250
300
350
400
450
500
1
50
OUT
in Figures 3-1, 5-1, 5-4, 5-6, 5-7, 5-8
R
pin has a minimum source current
SENSE
VS. FAN CURRENT
R
SENSE
(BR)CEO
9.1
4.7
3.0
2.4
2.0
1.8
1.5
1.3
1.2
1.0
SENSE
( )
or V
based
DS
(MOSFET)) must be large enough to withstand the
highest voltage applied to the fan (Note: This will occur
when the fan is off); (2) 5 mA of base drive current must
be enough to saturate the transistor when conducting
the full fan current (transistor must have sufficient
gain); (3) the V
ficiently drive the gate of the MOSFET to minimize the
R
be within the transistor's/MOSFET's current handling
capability; and (5) power dissipation must be kept
within the limits of the chosen device.
A base-current limiting resistor is required with bipolar
transistors (Figure 5-6).
FIGURE 5-6:
R
The correct value for this resistor can be determined as
follows:
V
cal Characteristics”; V
transistor’s data sheet. It is now possible to solve for
R
EQUATION
OH
DS(on)
BASE
BASE
V
OH
is specified as 80% of V
.
= 80% V
V
V
V
I
.
BASE
of the device; (4) rated fan current draw must
OH
R SENSE
R BASE
R
BASE
DD
OUT
=
V
=
= I
= R
= I
voltage must be high enough to suf-
OH
V
FAN
FAN
R SENSE
BASE
- V
+ V
Circuit For Determining
BE (SAT)
R
/ h
x R
BE (SAT)
R BASE
BASE
2002 Microchip Technology Inc.
I
x I
FE
BASE
SENSE
DD
+ V
+ V
BASE
is given in the chosen
V
in Section 1.0, “Electri-
- V
–
BE (SAT)
R SENSE
BE (SAT)
R SENSE
+
GND
–
–
V
Fan
+ V
DD
R BASE
Q
R
1
SENSE