TC648 datasheet(12/28 Pages) MICROCHIP

12 / 28 page

TC648

DS21448C-page 12

 2002 Microchip Technology Inc.

Per Section 1.0, “Electrical Characteristics”, the leak-

age current at the VAS pin is no more than 1 µA. It is

conservative to design for a divider current, IDIV, of

100 µA. If VDD = 5.0V then…

EQUATION

We can further specify R1 and R2 by the condition that

the divider voltage is equal to our desired VAS. This

yields the following:

EQUATION

Solving for the relationship between R1 and R2 results

in the following equation:

EQUATION

For this example, R1 = (2.27) R2. Substituting this rela-

tionship back into the original equation yields the

resistor values:

R2 = 15.3 kΩ, and R1 = 34.7 kΩ

In this case, the standard values of 34.8 k

Ω and

15.4 k

Ω are very close to the calculated values and

would be more than adequate.

5.4

Output Drive Transistor Selection

The TC648 is designed to drive an external transistor

or MOSFET for modulating power to the fan. This is

shown as Q1 in Figures 5-1, 5-6, 5-7,and 5-8. The

VOUT pin has a minimum source current of 5 mA and a

minimum sink current of 1 mA. Bipolar transistors or

MOSFETs may be used as the power switching ele-

ment, as is shown in Figure 5-6. When high current

gain is needed to drive larger fans, two transistors may

be used in a Darlington configuration. These circuit

topologies are shown in Figure 5-6: (a) shows a single

NPN transistor used as the switching element; (b) illus-

trates the Darlington pair; and (c) shows an N-channel

MOSFET.

One major advantage of the TC648’s PWM control

scheme versus linear speed control is that the power

dissipation in the pass element is kept very low.

Generally, 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 power dissipation to be kept

low, it is imperative that the pass transistor be fully sat-

urated when "on".

Table 5-1 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

Q1 are: (1) the breakdown voltage (V(BR)CEO or VDS

(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 VOUT voltage must be high enough to suf-

ficiently drive the gate of the MOSFET to minimize the

RDS(on) of the device; (4) rated fan current draw must

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. The correct value for this resistor can be

determined as follows:

VOH

=VBE(SAT) + VRBASE

VRBASE =RBASE x IBASE

IBASE

=IFAN / hFE

VOH is specified as 80% of VDD in Section 1.0,

“Electrical Characteristics”; VBE(SAT) is given in the

chosen transistor data sheet. It is now possible to solve

for RBASE.

EQUATION

Some applications benefit from the fan being powered

from a negative supply to keep motor noise out of the

positive supply rails. This can be accomplished by the

method shown in Figure 5-7. Zener diode D1 offsets

the -12V power supply voltage, holding transistor Q1 off

when VOUT is low. When VOUT is high, the voltage at

the anode of D1 increases by VOH, causing Q1 to turn

on. Operation is otherwise the same as in the case of

fan operation from +12V.

R1 + R2

IDIV = 1e

–4A =

, therefore

5.0V

R1 + R2 =

= 50,000

Ω = 50 kΩ

1e–4A

5.0V

VDD x R2

R1 + R2

VAS =

VDD - VAS

VAS

R1 = R2 x

R2 x (5 - 1.53)

1.53

VOH - VBE(SAT)

RBASE =

IBASE

TC648 Datasheet(PDF) 12 Page - Microchip Technology

TC648 Datasheet(HTML) 12 Page - Microchip Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL