MAX6620_14 Datasheet, PDF (12/30 Pages) Maxim Integrated Products – Quad Linear Fan-Speed Controller

MAX6620
Quad Linear Fan-Speed Controller
nominally 12V or 5V. The drive to the fans is proportion-
al to V_FAN. See the Fan_ Target Drive Voltage Registers
and the Applications Information sections for more
details.
Fan-Speed Control
DAC (Voltage) Mode. In DAC mode, the MAX6620 sim-
ply sets the voltage that powers the fan. The fan’s
speed is related, but not precisely proportional to, the
drive voltage. The drive voltage is set by the Fan_
Target Drive Voltage registers and may be read from
the Fan_ Drive Voltage registers. Because the output
voltage can ramp to new values at a controlled rate, the
values in the two registers may be different. See the
Register Descriptions and Applications Information sec-
tions for details.
RPM Mode. In RPM mode, the MAX6620 monitors
tachometer output pulses from the fan and adjusts the
fan drive voltage to force the fan’s speed to the desired
value. Fan speed is measured by counting the number
of internal 8192Hz clock cycles that take place during a
selectable number of tachometer periods. The number
of clock cycles counted (11-bit value) is stored in the
Fan_ TACH Count registers, and the desired number of
cycles is stored in the Fan_ Target TACH Count regis-
ters. See the Register Descriptions and Applications
Information sections for details.
Rate-of-Change Control. Sudden changes in fan
speed can be easily heard by users. The MAX6620
helps reduce the audibility of fan-speed changes by
controlling the rate at which the drive to the fan is incre-
mented. Four bits in the Fan_ Dynamics registers set
the rate at which the fan drive voltage is incremented.
This allows the time required for a change in fan speed
to be varied from 0 (in DAC mode only) to several min-
utes. See the Register Descriptions and Applications
Information sections for details.
Monitoring Tachometer Signals. The TACH_ inputs
accept tachometer or “locked-rotor” output signals from
3- or 4-wire fans. When measuring fan speed, the
MAX6620 counts the number of internal 8192Hz clock
cycles that occur during 1, 2, 4, 8, 16, or 32 tachometer
periods. The number of tachometer periods is selec-
table for each fan by using the appropriate Fan_
Dynamics register. Tachometer pulses <25µs in dura-
tion are ignored to minimize the effect of noise on the
tachometer lines.
The TACH count for a given RPM can be obtained from
the following equation:
TACH count =
60
× SR × 8192 = ^{491520 × SR}
NP × RPM
NP × RPM
where:
NP = number of tachometer pulses per revolution. Most
general-purpose brushless DC fans produce two
tachometer pulses per revolution.
SR = 1, 2, 4, 8, 16, or 32. See the Fan_ Speed Range
information in the Fan_ Dynamics Registers (06h, 07h,
08h, 09h)—POR = 0100 1100 section.
The tachometer count consists of 11 bits in the Fan_
TACH Count registers and is available in RPM and DAC
modes. In RPM mode, the desired fan count is written
to the Fan_ Target TACH Count registers.
Fan Failure Detection
When enabled, the MAX6620 monitors the TACH_
inputs to determine when a fan has failed. For fans with
tachometer outputs, failure is detected in various ways
depending on the fan control mode. In every case, four
consecutive fault detections are required to decide
whether the fan has failed. In DAC mode, the Fan_
Target TACH Count registers hold the upper limit for
tachometer count values; a fault condition is identified
when a TACH count exceeds the value written to the
Fan_ Target TACH Count registers for more than 1s. In
RPM mode, a fault condition is identified when any of
the following three conditions occur for more than 1s: 1)
the TACH count exceeds the value of the Fan_ Target
TACH Count registers while the fan drive voltage is at
full-scale, 2) the TACH count exceeds two times the
Fan_ Target TACH Count value, or 3) the TACH count
reaches its full count of 7FFh.
Some fans have locked rotor outputs that produce a
logic-level output to indicate that the fan has stopped
spinning. These signals can be monitored by setting
D2:D1 in the Fan_ Configuration registers. D2 selects
locked rotor or tachometer monitoring and D1 selects the
polarity of the locked rotor signal. A fan fault has occurred
when a locked rotor signal has been present for 1s.
Fan failure is indicated in the Fan Fault register and
also with the open-drain FAN_FAIL output. The
FAN_FAIL output may be masked using the mask bits
in the Fan Fault register. When a fan failure is detected,
drive to the affected fan is removed. Drive may be
restored by writing a new DAC or fan count target to the
fan’s control registers. The global configuration regis-
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