MIC74

Micrel

MIC74

16

August 1, 2000

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1

Table 3. Fan Speed Selection

in the configuration register, the fan will be shutdown by the

assertion of the /SHDN output if FAN_SPEED is zero. If

FAN_SPEED is programmed with any nonzero value, the fan

will be driven to its maximum speed for the duration of t

START

(about 1 second) and then assume the programmed speed.

Note that the circuit in Figure 14 contains an additional

transistor, Q1, as an inverter because the regulator in this

example has an active-high shutdown input rather than an

enable input. Otherwise the circuits function identically.

Table 3 lists the output voltages corresponding to all the fan

speeds and system states possible with these circuits. The

following equations are used to calculate the resistor values

used in MIC74 fan speed control circuits. It is assumed here

that the regulator’s internal reference voltage is 1.24V. If the

regulator uses a different reference voltage, that value should

be used instead.

The following equations show how to calculate the resistor

values for the fan controllers. For example, when the fan

speed register contains 011

R

resistor (R

RR

R

R

EQ

MIN

=

F1

F0

||

||

R

3.6k 1k

||

||

R

545

The output voltage is calculated by using:

V

R

R

OUT

FB

EQ

=+









1.24V 1

V

Ω









1.24V 1

3k

545

V