PWM and PFM Operation of DC/DC Converters for Portable Applications

This topic discusses the trade-offs between fixed-frequency pulse-width modulation (PWM) and pulsefrequency modulation (PFM) for portable DC/DC converter applications. PWM converters offer the benefit of predictable operating frequency, which may simplify the design of circuitry that suppresses electromagnetic interference (EMI). They also offer low output ripple characteristics and high efficiency during moderateto high-load conditions. However, PWM converters suffer from very poor conversion efficiency at light-load or standby conditions. PFM or “power-save” operating modes used with PWM converters offer a solution to this problem. Hysteretic or PFM architectures can also offer other benefits for boost converters. Because no loop compensation is required, the converter circuits are relatively simple and easy to design. Under the operating constraints of a typical battery-operated system, the resulting frequency range of a PFM converter may be narrow enough to allow its use even in some relatively noise-sensitive environments. PFM converters may not be appropriate for all applications. By understanding some basic details regarding the differences between PWM and PFM approaches, the system designer can make a more intelligent choice regarding the best architecture for a particular application.