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參數(shù)資料
| 型號(hào)： | MIC74BQS TR |
| 廠商： | Micrel Inc |
| 文件頁(yè)數(shù)： | 8/18頁(yè) |
| 文件大?。?/td> | 0K |
| 描述： | IC I/O EXPANDER I2C 8B 16QSOP |
| 標(biāo)準(zhǔn)包裝： | 2,500 |
| 接口： | I²C，SM 總線 |
| 輸入/輸出數(shù)： | 8 |
| 中斷輸出： | 是 |
| 電源電壓： | 2.7 V ~ 3.6 V |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 16-SSOP（0.154"，3.90mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 16-QSOP |
| 包裝： | 帶卷 (TR) |
| 其它名稱： | MIC74BQSTR MIC74BQSTR-ND |
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Micrel, Inc.
MIC74
October 2006
16
M9999-101006
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,
MIC74
IN
EN
GND
OUT
FB
RF2
1k
/FS2
RF1
1.8k
/FS1
RF0
3.5k
RMIN_SPEED
1k
P3
P2
P1
P0
/FS0
/SHDN
FAN
A-Speed
HP2A-B3
or similar
RFB
3k
RPU
100k
C3
220F
MIC29152
C1
10F
C4
0.1F
+12V
+3.3V
VDD
SMBCLK
SMBDATA
SMBALERT
A2
A1
A0
GND
SMBus
Host
Figure 13.Fan Speed Control Using an Adjustable Low-Dropout Regulator
MIC74
IN
SHDN
PGND
SGND
SW
FB
RF2
1k
/FS2
RF1
1.8k
/FS1
RF0
3.5k
RMIN_SPEED
1k
P3
P2
P1
P0
/FS0
/SHDN
FAN
A-Speed
HP2A-B3
or similar
RFB
3k
C2
3300pF
100k
RBASE
150k
RPU
200k
C3
220F
MIC4574
C1
10F
C4
0.1F
+12V
+3.3V
VDD
SMBCLK
SMBDATA
SMBALERT
A2
A1
A0
GND
SMBus
Host
+3.3V
D1
L1 100H
Q1
2N3906
Figure 14.Fan Speed Control Using a Buck Converter
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