Interfacing PWM logic sources to battery-powered motor drivers can be a challenge. While an Linear Resonant Actuator (LRA) is driven with an AC bipolar waveform near the LRA's resonant frequency, an Eccentric Rotating Mass (ERM) motor is driven primarily with a DC voltage. Because of this DC requirement, there can be no AC coupling capacitor in series with the driver IC's input. Designing a PWM interface to the driver IC therefore requires a bit of algebra to ensure proper operation.
This article shows a design methodology for interfacing a motor driver to a PWM source, including equations and component guidance, and details the design procedure for interfacing the LM4570 Eccentric Rotating Mass and Linear Resonant Actuator driver IC to a PWM source. (The LM4570 driver IC is intended for mobile phones and other portable media devices.) The circuit includes an example with accuracy and sensitivity analysis.
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About the author, and other articles by this author:
John Guy has been involved in the semiconductor industry since 1984. He graduated from San Jose State University with a BSEE in 1992. Prior to joining National Semiconductor, John also worked at Maxim, Analog Devices, and Precision Monolithics. John has authored over 40 articles and design ideas. In his spare time he enjoys cooking, carpentry, and building esoteric contraptions. Among his other articles are:
- “Headphone Amplifiers: Choose the right topology for your application”
- “Class D Amplifier FAQ – Part 2: Zobel Circuits, Testing, PCB Layout & Grounding”
- “Class D amplifier FAQ – Part 1: Overview, amplifier selection and filters”
- “Supervisor IC indicates fan failure”
- “Hand-soldering of surface-mount ICs”
- “Mixed-signal Audio Subsystem Enables High-performance Audio in Portable Electronics”
- “Headphone attenuator includes microphone monitor”