DRV8308_15 Datasheet, PDF (34/60 Pages) Texas Instruments – DRV8308 Brushless DC Motor Controller

DRV8308
SLVSCF7A – FEBRUARY 2014 – REVISED OCTOBER 2014
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Device Functional Modes (continued)
In register speed control mode, a 12-bit register SPEED is used to directly provide the speed command.
During sine commutation, the input duty cycle is multiplied by the modulation values for each phase (MOD_U,
MOD_V, and MOD_W) to generate a 12-bit value that determines the output PWM duty cycle of each phase.
Note that in 120° commutation, the MOD values are fixed at a duty cycle that is set by the MOD120 register.
The PWM frequency can be set to either 25, 50, 100, or 200 kHz, with register PWMF. Lower PWM frequencies
are desirable to minimize switching losses; higher PWM frequencies provide better control resolution, especially
at very high motor speeds.
The outputs of the PWM generators are the signals U_PD, V_PD, and W_PD. These contain the duty cycle
information for each phase.
Modulation and PWM generation is shown in Figure 20:
Sine Modulation
PWM generators
MOD120
MOD_U
1
0
100 MHz
X
12-bit
PWM
U_PD
SPD_CMD
MOD_V
1
X
0
PWM_DEG
PWM
Input
MOD_W
1
X
Timer
0
0
MUX
ENL_180
1
SPEED
2
12-bit
PWM
12-bit
PWM
V_PD
W_PD
SPDMODE
CLKIN mode = 0, PWM mode = 1,
Speed Reg mode = 2
MOD_U, MOD_V, and MOD_W values generated
from lookup tables and logic based on the
commutation counter. In 120° commutation mode,
full-scale duty cycle is set by MOD120 register.
Figure 20. Modulation and PWM Generation
8.4.2 Auto Gain and Advance Compensation
The DRV8308 device provides modes to automatically scale the loop gain and the phase advance settings
based on motor speed. This helps improve loop stability and motor performance in cases where the motor must
operate over a wide speed range with a single set of parameters. For applications that run at only one speed,
these functions should be left disabled.
Auto gain compensation is enabled by setting the AUTOGAIN bit. Auto gain will scale the LOOPGAIN of the
system using the following equation:
Computed Gain = (LOOPGAIN / AG_SETPT) × f_CLKIN
(3)
Automatic advance is enabled by setting the AUTOADV bit. The advance setting is scaled such that at zero
speed, there is no phase advance. As speed increases, the phase advance is increased using the equation
below:
Computed Advance = (ADVANCE / AA_SETPT) × f_{Hall_U}
(4)
Both the gain and advance values are latched when LOCK goes active (when the motor is at constant speed).
The auto gain and advance functions are shown in Figure 21:
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