A4941GLPTR-T データシートの表示（PDF） - Allegro MicroSystems

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A4941GLPTR-T

Three-Phase Sensorless Fan Driver

Allegro MicroSystems 

A4941

Three-Phase Sensorless Fan Driver

Functional Description

The driver system is a three-phase, BEMF sensing motor control-

ler and driver. Commutation is controlled by a proprietary BEMF

sensing technique.

The motor drive system consists of three half bridge NMOS

outputs, BEMF sensing circuits, adaptive commutation control,

and state sequencer. The sequencer determines which output

devices are active. The BEMF sensing circuits and adaptive com-

mutation circuits determine when the state sequencer advances to

the next state.

A complete self-contained BEMF sensing commutation scheme is

provided. The three half-bridge outputs are controlled by a state

machine with six possible states, shown in figure 1. Motor BEMF

is sensed at the tri-stated output for each state.

BEMF sensing motor commutation relies on the accurate com-

parison of the voltage on the tri-stated output to the voltage at the

center tap of the motor. The BEMF zero crossing, the point where

the tri-stated motor winding voltage crosses the center tap volt-

age, is used as a positional reference. The zero crossing occurs

roughly halfway through one commutation cycle.

Adaptive commutation circuitry and programmable timers

determine the optimal commutation points with minimal

external components. The major blocks within this system are:

the BEMF zero crossing detector, Commutation Delay timer, and

the Blank timer.

BEMF Zero Cross Detection

BEMF zero crossings are detected by comparing the voltage at

the tri-stated motor winding to the voltage at the motor center

tap. Zero crossings are indicated by the FCOM signal, which

goes high at each valid zero crossing and low at the beginning

of the next commutation. In each state, the BEMF detector looks

for the first correct polarity zero crossing and latches it until the

next state. This latching action, along with precise comparator

hysteresis, makes for a robust sensing system. At the beginning

of each commutation event, the BEMF detectors are inhibited for

a period of time set by the Blank timer. This is done so that com-

mutation transients do not disturb the BEMF sensing system.

Commutation Event

See figure 1 for timing relationships. The commutation sequence

is started by a CDCOM pulse or a valid XCOM at startup. After

Output

State

AB CD EF AB CD EF

OUTA

OUTB

OUTC

FCOM

CDCOM

FG

Figure 1. Motor Terminal Output States

Allegro MicroSystems, LLC

4

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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