24 POWER MODULES https://www.analog.com/en/index.html Issue 2 2024 Power Electronics Europe www.power-mag.com The target control loop bandwidth is 100kHz, 4 to 5 times smaller than the LC resonant frequency. Two 0603 4.7µF capacitors are used for C1. Ferrite bead BLE18PS080SH1 is selected as Lf (its size is 0603, as highlighted in Figure 2). Two 1206 100µF ceramic capacitors are still used as C2. The resonant frequency is 424kHz. The noise measurement comparison is shown in Figure 3. At 2MHz switching frequency, the output switching ripple was 234µV without the LC and is greatly reduced to 15µV after adding a 0603 ferrite bead. Figure 3. Switching noise without the LC (234µV) vs. with LC (15µV). The addition of the second-order LC filter to minimise noise maintains the control loop bandwidth at 100kHz and fast transient response with recovery below 10µs, which is confirmed through bench evaluation with and without the LC filter. Recovering within 10µs allows a negligible blanking time, which is desired in wireless and RF applications. ADI’s LTM4702 resolves the system designer’s challenge of low signal processing efficiency caused by the load transient blanking period. The load transient waveform shown in Figure 4 verifies fast transient response and recovery within 10µs after adding the second-order LC filter, similar to the behaviour of the design example without this filter. Conclusion Supporting high current applications with minimal noise while ensuring high efficiency and stability can be challenging. Adding a secondorder LC filter can significantly reduce noise but may cause instability if not optimised correctly. To minimise noise without compromising stability, use an optimised second-order LC filter. By carefully selecting the required L and C components based on the switching frequency, control loop bandwidth, and resonant frequency, one can minimise the switching noise while maintaining a fast transient response and high bandwidth. Analog Devices: www.analog.com References 1Jim Williams. “AN101: Minimizing Switching Regulator Residue in Linear Regulator Outputs.” Linear Technology, July 2005. Figure 4. Load transient results: without LC vs. with LC (recover within 10µs).
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