Datasheet

4-
Temperature
sensor
AVSS
UCD90124A
12 V
4-wire Fan
Temperature
sensor
MONx
FPWM
GPIO
3
DC Fan
12V
TACH
GND
PWM
15kHz – 30kHz
3.3V PWM signal
changes fan speed
with duty cycle
3.3V TACH output
to GPI/GPIO for fan
speed monitoring
UCD90124A
www.ti.com SLVSAN8 –JANUARY 2012
Figure 25. Four-Wire Fan Connection
The UCD90124A autocalibrate feature automatically finds and records the turn-on, turn-off and maximum speeds
and duty cycles for any fan. Fans have a minimum speed at which they turn on, a turn-off speed that is usually
slightly lower than the turn-on speed, and a maximum speed that occurs at slightly less than 100% duty cycle.
Each speed has a PWM duty cycle that goes with it. Every fan is slightly different, even if the model numbers are
the same. The built-in temperature control algorithms use the actual measured operating speed range instead of
0 RPM to rated speed of the fan to improve the fan control algorithms. The user can choose whether to use
autocalibrate or to manually enter the fan data.
The UCD90124A can control up to four independent fans as defined in the PMBus standard. When enabled, the
FAN-PWM control output provides a digital signal with a configurable frequency and duty cycle, with a duty cycle
that is set based on the FAN_COMMAND_1 PMBus command. The PWM can be set to frequencies between 1
Hz and 125 MHz based on the UCD90124A PWM type selected for the fan control. The duty cycle can be set
from 0% to 100% with 1% resolution. The FAN-TACH fan-control input counts the number of transitions in the
tachometer output from the fan in each 1-second interval. The tachometer can be read by issuing the
READ_FAN_SPEED_1 command. The speed is returned in RPMs.
Fault limits can also be set for the tachometer speed by issuing the FAN_SPEED_FAULT_LIMIT command and
the status checked by issuing the STATUS_FAN_1_2 command. See the UCD90xxx Sequencer and System
Health Controller PMBus Command Reference for a complete description of each command.
The UCD90124A also supports two fan control algorithms.
Hysteretic Fan Control
Temp
ON
and Temp
OFF
levels are input by the user. Temp
ON
is higher than Temp
OFF
. A GPIO pin is used to turn
the fan or fans on at full speed when the monitored temperature reaches Temp
ON
and to turn the fans off when
the temperature drops below Temp
OFF
.
Copyright © 2012, Texas Instruments Incorporated Submit Documentation Feedback 31
Product Folder Link(s) :UCD90124A