DRV8308_15 Datasheet, PDF (17/60 Pages) Texas Instruments – DRV8308 Brushless DC Motor Controller

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DRV8308
SLVSCF7A – FEBRUARY 2014 – REVISED OCTOBER 2014
Feature Description (continued)
8.3.4 Commutation
For 3-phase brushless DC motors, rotor position feedback is provided from Hall effect transducers mounted on
the motor. These transducers provide three overlapping signals, each 60° apart. The windings are energized in
accordance with the signals from the Hall sensors to cause the motor to move.
In addition to the Hall sensor inputs, commutation is affected by a direction control, which alters the direction of
motion by reversing the commutation sequence. Control of commutation direction is by the DIRPOL register bit
as well as the DIR input terminal. The DIRPOL register bit is combined with the terminal with an exclusive-OR
function as follows:
DIR Terminal
0
0
1
1
Table 1. Direction Behavior
DIRPOL REGISTER BIT
0
1
0
1
RESULTING DIR FOR
COMMUTATION
0
1
1
0
If the commanded direction is changed while the motor is moving, the device either brakes or allows the motor to
coast, depending on the state of the BRKMODE bit, until the motor stops. The stopped condition is determined
by measuring the period of the HALL_U signal; when the period exceeds 160 ms, typical operation resumes and
the motor starts spinning in the commanded direction. This prevents excessive current flow in the output stage if
the motor is reversed while running at speed.
The DRV8308 device supports three commutation modes: standard 120° commutation using three Hall sensors,
120° commutation using a single Hall sensor, and 180° sine-wave-drive commutation.
In standard 120° commutation, mis-positioning of the Hall sensors can cause motor noise, vibration, and torque
ripple. 120° commutation using a single Hall sensor (single-Hall commutation) can improve motor torque ripple
and vibration because it relies on only one Hall edge for timing.
180° sine-wave-drive commutation is even more advanced, and excites the windings with a waveform that
delivers nearly sinusoidal current to each winding.
8.3.4.1 120° 3-Hall Commutation
In standard 120° commutation, the motor phases are energized using simple combination logic based on all
three Hall sensor inputs. Standard 120° commutation is in accordance with Table 2, Figure 7, and Figure 8:
Table 2. Standard 120° Commutation⁽¹⁾
STATE
1
2
3
4
5
6
1X
2X
HALL INPUTS
DIR = 1
DIR = 0
U_H V_H W_H U_H V_H W_H
L
L
H
H
H
L
L
H
H
H
L
L
L
H
L
H
L
H
H
H
L
L
L
H
H
L
L
L
H
H
H
L
H
L
H
L
H
H
H
L
L
L
L
L
L
H
H
H
PHASE U
U_HSGATE
U_LSGATE
L
L
PWM
L / !PWM⁽²⁾
PWM
L / !PWM⁽²⁾
L
L
L
H
L
H
L
L
L
L
PRE-DRIVE OUTPUTS
PHASE V
V_HSGATE
V_LSGATE
PWM
L / !PWM⁽²⁾
L
L
L
H
L
H
L
L
PWM
L / !PWM⁽²⁾
L
L
L
L
PHASE W
W_HSGATE
W_LSGATE
L
H
L
H
L
L
PWM
L / !PWM⁽²⁾
PWM
L / !PWM⁽²⁾
L
L
L
L
L
L
(1) Hall sensor is "H" if the positive input terminal voltage is higher than the negative input terminal voltage. States 1X and 2X are illegal
input combinations.
(2) During states where the phase is driven with a PWM signal, using asynchronous rectification, the LS gate is held off (L); using
synchronous rectification, the LS gate is driven with the inverse of the HS gate.
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