MIC74 Datasheet, PDF (15/20 Pages) Micrel Semiconductor – 2-Wire Serial I/O Expander and Fan Controller Advance Information

MIC74
the new image to DATA. Interrupts would be disabled briefly
while DATA is being modified.
Regardless of which procedure is used, it is important that
only one software routine at a time attempts to make changes
to the output data. In a system where polling is the exclusive
method for servicing inputs, this is usually not a problem. If
interrupts are employed to any degree in dealing with MIC74
inputs, care must be taken to insure that a software routine in
the midst of making changes to outputs is not interrupted by
another routine that proceeds to make its own changes. The
risk is that the value in DATA will be changed by an interrupt-
ing routine after it is read by a different routine in the process
of making its own changes. If this occurs, the value written to
DATA by the first routine may be incorrect. The most straight-
forward solution to this potential problem is to disable system
interrupts while the data register is actually being modified.
Application Circuits
The MIC74, in conjunction with a linear low-dropout or
switching regulator, can be configured as a fan speed control-
ler. Most adjustable regulators have a feedback pin and use
an external resistor divider to adjust the output voltage. The
MIC74 is designed to take advantage of this configuration
with its ability to manipulate multiple feedback resistors
connected to the P4–P7 outputs. Individual open-drain out-
put bits are selectively grounded or allowed to float under the
control of the internal state machine. This action raises or
Micrel
lowers the equivalent resistance seen in the regulator’s
feedback path, thus changing the output voltage.
Any conventional adjustable regulator is usually suitable for
use with the MIC74. The output voltage corresponding to
each value to be programmed into the fan speed register can
be determined by selecting the resistors in the circuit. The
regulator itself can be chosen to meet the needs of the
application, such as input voltage, output voltage, current
handling capability, maximum power dissipation, and physi-
cal space constraints. Two circuit examples are shown be-
low.
The circuit of Figure 13 illustrates use of a typical LDO linear
regulator such as the MIC29152. A switching regulator-
based fan control circuit using the MIC4574 200kHz Simple
0.5A Buck Regulator is shown in Figure 14. Both circuits
assume a 12V fan power supply but will accommodate much
higher input voltages if required (MIC4574: 24V, MIC29152:
26V). Care must be taken, however, to insure that the
maximum power dissipation of the regulator is not exceeded.
If the regulator overheats, its internal thermal shutdown
circuitry will deactivate it. (See MIC29152 or MIC4574
datasheet.)
Since the MIC74 powers up with all its I/O’s inputs (floating),
both circuits will power-up with the fan running at a minimum
speed determined by the value of R_{MIN_SPEED}. Once the
MIC74’s fan mode is activated by setting the appropriate bit
+3.3V
SMBus
Host
C4
0.1µF
+12V
C1
10µF
MIC74
VDD
/SHDN
SMBCLK /FS2
SMBDATA /FS1
SMBALERT /FS0
A2
P3
A1
P2
A0
P1
GND
P0
R_PU
100k
MIC29152
IN
OUT
EN
FB
GND
R_F2
R_{F1 1k}
R_{F0 1.8k}
3.5k
R_FB
3k
C3
220µF
FAN
A-Speed
HP2A-B3
or similar
R_{MIN_SPEED}
1k
Figure 13. Fan Speed Control Using an Adjustable Low-Dropout Regulator
+3.3V +12V
+3.3V
SMBus
Host
C4
0.1µF
R_PU
200k
R_BASE
150k
100k
2N3906
Q1
C1
10µF
MIC74
VDD
/SHDN
SMBCLK /FS2
SMBDATA /FS1
SMBALERT /FS0
A2
P3
A1
P2
A0
P1
GND
P0
MIC4574
IN
SW
SHDN
FB
SGND PGND
L1 100µH
C2
3300pF
D1
R_F2
R_{F1 1k}
1.8k ^RF0
3.5k
R_FB
3k
C3
220µF
FAN
A-Speed
HP2A-B3
or similar
R_{MIN_SPEED}
1k
August 1, 2000
Figure 14. Fan Speed Control Using a Buck Converter
15
MIC74