DRV8308_15 Datasheet, PDF (42/60 Pages) Texas Instruments – DRV8308 Brushless DC Motor Controller

DRV8308
SLVSCF7A – FEBRUARY 2014 – REVISED OCTOBER 2014
9 Application and Implementation
www.ti.com
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 Internal Speed Control Loop Constraints
The DRV8308 device is a versatile speed controller and driver for small, 3-phase brushless motors. However,
there are some limitations to its application.
The built-in speed control loop is designed to work optimally with motor electrical speeds from about 50 Hz up to
6.7 kHz. For an 8-pole motor, this translates into about 500 RPM up to more than 100000 RPM. For motors with
higher pole counts, these speeds scale down; for lower pole counts, they scale up.
Operation is possible at slower or faster speeds, but speed control becomes less effective, especially if using the
Hall sensors for speed feedback (as opposed to the FG input).
Typically, the speed loop is optimized (by setting the filter coefficients and gains) at one desired motor speed.
Operation is possible with one set of parameters over a limited speed range (for example, 1000 RPM to 2000
RPM), However, operation over a very wide speed range requires different parameters. The use of the auto gain
and auto advance features can extend the dynamic range up to 4×.
When using the SPI interface to program the registers, the parameters can be updated at any time, even while
the motor is running. In this manner, a wider range of speeds can be accommodated by the speed loop.
When not using the internal speed loop (when controlling the motor using PWM input or register speed control),
the limits imposed by the speed loop do not apply. An external speed control implementation (using a
microcontroller, FPGA, or other logic) can essentially control the motor current directly.
However, if using sine commutation, there are limits to the minimum and maximum speed, which are dictated by
the timers that are used to generate the commutation sequence. The commutation timer is a 25-bit timer clocked
at 50 MHz; therefore, the longest time it can capture is 655 ms. This limits the slowest speed to about 1.5 Hz (or
23 RPM for an 8-pole motor). At the other extreme, there are 960 steps in each sine commutation cycle. To
ensure that there is enough time for the steps, the maximum speed is that which generates 960 counts at 50
MHz, or 52 kHz. This corresponds to a maximum speed of 800000 RPM for an 8-pole motor.
When not using the internal speed loop and using 120° commutation (using all three Hall sensors), there are no
speed limitations. Commutation is performed with combinational logic.
9.1.2 Hall Sensor Configurations and Connections
The Hall sensor inputs on the DRV8308 device are capable of interfacing with a variety of Hall sensors.
Typically, a Hall element is used, which outputs a differential signal on the order of 100 mV. To use this type of
sensor, the VREG5 regulator can be used to power the Hall sensor. Connections are as shown in Figure 26:
42
Submit Documentation Feedback
Product Folder Links: DRV8308
Copyright © 2014, Texas Instruments Incorporated