MIC74 datasheet(15/20 Pages) MICREL

15 / 20 page

August 1, 2000

MIC74

Micrel

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

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

MIC74

GND

OUT

RF2

/FS2

RF1

1.8k

/FS1

RF0

3.5k

RMIN_SPEED

/FS0

/SHDN

FAN

A-Speed

HP2A-B3

or similar

RFB

RPU

100k

220

µF

MIC29152

µF

0.1

µF

+12V

+3.3V

VDD

SMBCLK

SMBDATA

SMBALERT

GND

SMBus

Host

Figure 13. Fan Speed Control Using an Adjustable Low-Dropout Regulator

MIC74

SHDN

PGND

SGND

RF2

/FS2

RF1

1.8k

/FS1

RF0

3.5k

RMIN_SPEED

/FS0

/SHDN

FAN

A-Speed

HP2A-B3

or similar

RFB

3300pF

100k

RBASE

150k

RPU

200k

220

µF

MIC4574

µF

0.1

µF

+12V

+3.3V

VDD

SMBCLK

SMBDATA

SMBALERT

GND

SMBus

Host

+3.3V

L1 100

µH

2N3906

Figure 14. Fan Speed Control Using a Buck Converter

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