MIC74_06 Datasheet, PDF (16/18 Pages) Micrel Semiconductor – 2-Wire Serial I/O Expander and Fan Controller

Micrel, Inc.
MIC74
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 interrupting
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
controller. Most adjustable regulators have a feedback
pin and use an external resistor divider to adjust the
output voltage. The MIC74 is designed to take advent-
age of this configuration with its ability to manipulate
multiple feedback resistors connected to the P4–P7
outputs. Individual open-drain output bits are selectively
grounded or allowed to float under the control of the
internal state machine. This action raises or 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 correspond-
ing 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 physical space constraints. Two circuit
examples are shown below.
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
RMIN_SPEED. Once the MIC74’s fan mode is activated
by setting the appropriate bit in the configuration register,
+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
October 2006
Figure 14.Fan Speed Control Using a Buck Converter
16
M9999-101006