Selecting MOSFET Drivers for the LX4580 Analog Front-End IC for Motor-Actuator Control Systems

The LX4580 is an analog front-end for high-reliability motor-actuator control systems. The device operates under stringent standards such as DO-160 for avionics in airborne systems. The LX4580 interfaces with dual microcontrollers (MCUs) or Field Programmable Gate Arrays (FPGAs) in redundant COM/MON system architectures and features Elliptic Curve Cryptography (ECC) encoding to provide 1-bit error correction and 2-bit error detection.  Sequential logic is implemented with Triple Mode Redundancy (TMR) to protect against Single-Event Upsets (SEUs).

Figure 1 shows a typical motor-actuator system using a resolver to provide accurate Permanent Magnet Synchronous Motors (PMSM) rotor position, and a Linear Variable Differential Transformer (LVDT) for actuator linear position feedback. The LX4580 combines analog sensor acquisition and motor control Pulse-Width Modulation (PWM) synthesis. For the motor control, there are eight independent PWM outputs to drive upper and lower Field-Effect Transistor (FET) gate drivers in the motor half-bridge stages. FET drivers and current sense amplifiers are usually the only external active components required. This post discusses the needs and tradeoffs when selecting the FET drivers. A separate post will cover selecting the ground-side current sense amplifiers shown in the block diagram.

Figure 1 Typical Motor Actuator System

A typical list of requirements for the FET drivers for an LX4580 half-bridge stage is:

• High-side drive for the upper FET allowing the half-bridge to use two NFETs, not a PFET/NFET pair
• Suitable gate drive voltage swing to suit either standard FETs (9V to 15V) for high voltage motors, logic-level FETs for low voltage motors or enhancement GaN FETs (typically 5V)
• Sufficient gate drive current capability and slew rate to turn on the FETs in a short time with respect to the motor drive PWM frequency to allow PWM duty ratios close to 0% and 100%
• Independent controls for the high-side and low-side FETs to allow FET commutation dead-time management by the control system
• A high-reliability build standard, such as AEC-Q100

For high-voltage motors up to 85V, the MIC4605-1YM is a suitable half-bridge driver for dual NFETs and is AEC-Q100 qualified. The gate drive supply range is 5.25V to 16V, making the driver a good choice for standard high gate voltage FETs where a gate drive voltage in the range 9V to 15V is commonly selected. The MIC4605 is also intended for switching power supplies, so it operates happily at PWM frequencies as high as 1MHz.

Figure 2 shows the driver block diagram. Power to the high-side driver is supplied by a bootstrap circuit comprising an external capacitor and internal diode. When the lower FET is on, the external capacitor charges up to the gate drive supply, V_DD, through the internal diode. When the lower FET is off and the upper FET is turned on, the internal diode becomes reverse biased, and the external capacitor provides a high-side supply to the upper FET driver.

Figure 2 Driver Block Diagram

The MCP14700 is a similar half-bridge driver for dual NFETs, again AEC-Q100 qualified, but targets low voltage FETs intended for a 5V gate drive voltage, and low voltage motors up to 30V. Like the MIC4605, the MCP14700 targets switching power supplies as well as motor drivers, and works at PWM frequencies up to 2MHz.

For more information on the LX4580, contact Dorian.Johnson@microchip.com.