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.

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:

exceeded, the drivers will be disabled for 25 μs.