Driver handles three-phase brushless motors
By Richard A Quinnell, Contributing Editor -- EDN, December 22, 2005
The high cost of electronic control for BLDC (brushless-dc) motors has long been one of the barriers to their use in system designs. Now, Apex Microtechnology is offering a low-cost motor-driver IC to remove that barrier for users of three-phase BLDC motors requiring currents as great as 5A per phase. The company’s SA305 combines drive transistors with logic control, current sensing, and fault-protective circuits to simplify design-in. The device’s high switching speed and driver power efficiency help reduce manufacturing costs.
The term “motor driver” does not quite do the SA305 justice; the term “intelligent switching amplifier” seems more accurate. The device offers three independent drive channels that can each source or sink current on an output line based on digital signals coming from a DSP or a microcontroller that controls the SA305. The device is not simply a passive driver, however. It has built-in circuitry to prevent simultaneous “source” and “sink” drive signals from creating an internal short circuit. Instead, the SA305 automatically inserts dead time to one drive command if the other drive command is still active. This prevents shoot-through-current situations from damaging the drive transistors.
Other built-in safety features include junction-temperature sensing and output-short-circuit protection. If the junctions of the drive transistors reach 160°C the device shuts off the drive transistors and asserts the fault line. A short-circuit across output pins has the same effect, although designers should mount the device to handle a brief 500W power surge. An external reset signal restores the device to normal operation. The SA305 can handle continuous currents of 5A and peak currents of 10A, making it suitable for BLDC motors as powerful as 1/3 hp. The drive transistors’ channel resistance of less than 0.4Ω and PWM-controlled drive current (switching at 100 kHz) means that the device generates little heat. This feature allows the device to operate with limited or no heat-sinking, reducing implementation costs.
You can independently control the output transistors for the three channels, which means that the SA305 is not limited to motor applications, according to Apex’s vice president of engineering, Tom Price. The unit can just as easily handle relays, lighting, and other high-current loads. The SA305 comes in an SIP package with grounded heat tab. The cost is $13.10 (10,000).
Apex Microtechnology Corp, www.apexmicrotech.com.
