DRV8308_15 Datasheet, PDF (47/60 Pages) Texas Instruments – DRV8308 Brushless DC Motor Controller

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DRV8308
SLVSCF7A – FEBRUARY 2014 – REVISED OCTOBER 2014
9.2.2.4 Required Flutter (Speed Jitter)
Flutter is a measure of motor speed consistency. The best possible flutter largely depends on motor
characteristics, loading, and tuning of the DRV8308 registers. BLDC motors with high detent torque and discrete
positions will have higher flutter. The DRV8308EVM User's Guide SLVUA41 describes the important registers
and a tuning process.
9.2.2.5 Configuration Method
The DRV8308 must have its registers set in order to function. There are 3 methods:
1. Pre-program an external EEPROM, and set pin SMODE High.
2. Set pin SMODE Low, and write register data over SPI while the DRV8308 is powered.
3. Set pin SMODE Low, write register data over SPI while the DRV8308 is powered, and burn it to the internal
EPROM (OTP). Then on future power ups, the DRV8308 will load the custom configuration data.
If the DRV8308 will be used in an open-loop PWM mode, the following register settings provide good baseline
settings:
ADDRESS
0x00
0x01
0x02
0x03
0x04
0x05
0x06
0x07
0x08
0x09
0x0A
0x0B
VALUE
0x0911
0x0000
0x04FF
0x6800
0x40D2
0x0000
0x0000
0x0000
0x0000
0x0000
0xF000
0x0000
9.2.2.6 Hall Element Current
Hall elements output a differential voltage that is proportional to the amount of bias current. An absolute max
current is specified, as well as the element resistance over temperature.
The DRV8308 regulated outputs VREG or VSW can be used to supply Hall element current, along with a series
resistor to limit element current. Its sizing depends on the element equivalent resistance (they can be arranged in
parallel or serial), and the VM voltage if VSW is used.
9.2.2.7 Power FET Switching Time
The switching time on the external FETs is the V_GS rise time, and it can be easily controlled with DRV8308
register IDRIVE. The 10mA setting causes a switching time that is 5 times the 50mA setting. Larger FETs that
have higher current capabilities have a larger gate charge (Q_g), and require higher IDRIVE settings for
reasonable switching times.
However, fast switching times can cause extra voltage noise on VM and GND. This can be especially due to a
relatively slow reverse-recovery time of the low-side body diode, where it conducts reverse-bias momentarily,
being similar to shoot-through. To minimize noise, lower IDRIVE settings are often beneficial, and the 10mA
setting has worked well with many types of FETs operating below 5A.
Copyright © 2014, Texas Instruments Incorporated
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