Three-Phase Sensorless Fan Driver
A4941
5
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
the commutation delay period, a CDCOM is asserted, starting
the Blank timer. The Blank signal disables the BEMF detector so
the comparator is not active during the commutation transients.
The next zero crossing, detected on the tri-stated output, causes
FCOM to go high. This triggers the Commutation Delay timer
and the sequence repeats.
CTAP
Connection pin for motor center-tap if available. If not available
(such as with delta type motors), the null point will be generated
internally.
Startup
At startup, commutations are provided by an onboard oscillator.
These commutations are part of the startup scheme, to step the
motor to generate BEMF until legitimate BEMF zero crossings
are detected and normal BEMF sensing commutation is achieved.
Until an appropriate number of FCOM pulses are achieved (96),
100% PWM will be applied to the motor windings.
Standby Mode
Driving PWM low for 500 μs causes the IC to enter a low power
standby mode.
Lock Detect
Valid FCOM signals must be detected to ensure the motor is not
stalled. If a valid FG is not detected for 2 s, the outputs will be
disabled for 5 s before an auto-restart is attempted.
FG Output
The FG output provides fan speed information to the system.
FG is an open drain output.
PWM Input
The duty cycle applied to the PWM pin is translated directly
to an average duty cycle applied across the motor windings
to control speed.
• For voltage controlled applications, where V
BB
controls the
speed, PWM can be left open circuit. PWM is internally pulled-
up to logic high level.
• PWM also can be used as a control input to start and stop the
motor.
• For PWM applications, input frequencies in the range
15 to 30 kHz are applied directly to the motor windings. If the
PWM duty cycle is very small, then the IC will apply a mini-
mum pulse width of typically 6 μs. This minimum pulse width
effects the minimum speed. As a result of having a minimum
pulse width, the IC can startup and operate down to very short
duty cycles.
SLEW Input
Enables or disables soft switching by connection as follows:
SLEW Pin Connection Soft Start Status
GND Enabled
Open Disabled
FC Input
This is the logic input to set force commutation time at startup, by
connection as follows:
FC Pin Connection
Startup Commutation Time
(ms)
GND 100
VBB 50
Open 200
Overcurrent Protection
If needed, a sense resistor can be installed to limit current. (See
Applications Information section for more details.) The current
limit trip point would be set by:
I
OCL
= 200 mV / R
S
.
When the trip point is reached, if the threshold voltage, V
OCL
, is
exceeded, the drivers will be disabled for 25 μs.
