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| MIC74 Datasheet, PDF (16/20 Pages) Micrel Semiconductor – 2-Wire Serial I/O Expander and Fan Controller Advance Information | |||
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MIC74
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 tSTART
(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 regulatorâs internal reference voltage is 1.24V. If the
regulator uses a different reference voltage, that value should
be used instead.
FAN_SPEED Fan Speed
Value
Selected
RFB
0000 0000b power-up
3k
0000 0000b
fan off
3k
0000 0001b
lowest
3k
0000 0010b 2nd lowest 3k
0000 0011b 3rd lowest 3k
0000 0100b medium
3k
0000 0101b 3rd highest 3k
0000 0110b 2nd highest 3k
0000 0111b
highest
3k
RMIN
1k
1k
1k
1k
1k
1k
1k
1k
1k
Micrel
The following equations show how to calculate the resistor
values for the fan controllers. For example, when the fan
speed register contains 011b, which is the 3rd lowest speed,
RF1 and RF0 are parallel to RMIN to give the equivalence
resistor (REQ) value of 545â¦.
REQ = RF1 || RF0 || RMIN
REQ = 1.8k || 3.6k ||1k
REQ = 545â¦
The output voltage is calculated by using:
VOUT
=
1.24V

ï£ï£¬1+
RFB
REQ


VOUT
= 1.24V ï£ï£¬1+
3k
545â¦

VOUT = 8.06V
RF2
open
open
open
open
open
1k
1k
1k
1k
RF1
open
open
open
1.8k
1.8k
open
open
1.8k
1.8k
RF0
open
open
3.6k
open
3.6k
open
3.6k
open
3.6k
REQ
1k
1k
783
643
545
500
439
391
353
VOUT
4.96V
0V
5.99V
7.03V
8.06V
8.68V
9.71V
10.75V
11.78V
Table 3. Fan Speed Selection
MIC74
16
August 1, 2000
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