# Boost Converter Design: Part 5 — The Final Gate: An Output LC Filter **URL:** \ **Category:** Integrated Circuits (ICs)\ **Tags:** analog-devices-inc, tech-tips, power-management-pmic\ **Created:** [September 16, 2026, 7:54am UTC](https://forum.digikey.com/t/boost-converter-design-part-5-the-final-gate-an-output-lc-filter/72019 "2026-09-16T07:54:58Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![Alan\_Yang](https://sea2.discourse-cdn.com/digikey/user_avatar/forum.digikey.com/alan_yang/32/2323_2.png) [@Alan\_Yang](https://forum.digikey.com/u/Alan_Yang)\ **Post date:** [September 16, 2026, 7:54am UTC](https://forum.digikey.com/t/boost-converter-design-part-5-the-final-gate-an-output-lc-filter/72019/1 "2026-09-16T07:54:59Z")
The operating mode has been selected and the IC is in place, but the high-frequency noise from a switching power supply may still exceed the limit. The solution is to add a final “filter gate” at the output. A passive second-order LC filter can attenuate the noise by approximately 47\,\text{dB} before it reaches the load. This article explains where the noise comes from, how to build the filter, how to calculate its component values, and how to verify its performance. ## Where Does Switching Noise Come From? Switching regulators naturally produce high-frequency noise for three reasons: - PCB parasitic inductance and capacitance interact with rapidly changing voltage and current; - the voltage and current change abruptly when the switch transitions; - the switch node rings. These high-frequency components can reach the load through conducted or radiated paths. For sensitive loads such as RF circuits, precision ADCs, and sensors, **even a few millivolts of ripple can cause problems**. Recall Figure 1 from Part 1. It shows the output ripple before filtering—approximately 9\,\text{mV}, with the spikes mainly caused by the switching transitions. It is reproduced below for comparison: ![Figure 1](https://global.discourse-cdn.com/digikey/original/3X/1/a/1a608d530743eb4fb24feaa4d0326e65426a5391.png) **Figure 1:** Ripple and spikes at the LT8330 output before filtering. The simulated ripple is approximately 9\,\text{mV} (image sourced from Analog Devices). ## Option 1: Can an LDO Be Used After the Boost Converter? The most direct approach is to place a low-dropout linear regulator (LDO) after the boost converter and use its high PSRR to suppress ripple. This is not suitable for this example: - With a 34\,\text{V} output, high-voltage LDO options are limited and dropout loss is significant. - The 2\,\text{MHz} switching frequency is higher than the loop bandwidth of most LDOs, so PSRR falls sharply at this frequency. **Conclusion: An LDO is not an efficient or practical solution here.** ## Option 2: A Second-Order LC Output Filter A more suitable approach is to add a **second-order LC filter** at the output. It requires no active devices—only an inductor, several capacitors, and a damping resistor. ![image](https://global.discourse-cdn.com/digikey/original/3X/4/f/4fda4d0086722c5e40803901cde5d6c28be38af7.png) **Figure 2:** Schematic of the additional output filter designed for the LT8330 (image sourced from Analog Devices). Component-selection guidelines: - Choose a series inductor L\_1 in the range of 100\,\text{nH} to 500\,\text{nH}, with as little DCR as possible to avoid additional voltage drop. - C\_1 and L\_1 determine the cutoff frequency. - Choose C\_2 as four to five times C\_1. Together with the damping resistor R\_1, it forms a damping network that suppresses LC resonance. ## How Is the Cutoff Frequency Calculated? For this example, choose L\_1=0.3\,\mu\text{H} and C\_1=4.7\,\mu\text{F}. The cutoff frequency is: f\_C=\frac{1}{2\pi\sqrt{L\times C}}=\frac{1}{2\pi\sqrt{0.3\,\mu\text{H}\times4.7\,\mu\text{F}}}=134\,\text{kHz} \tag{13} Whether this frequency is appropriate can be judged against three reference points: - It is **higher than** the regulator loop bandwidth, approximately 13\,\text{kHz} in this example, so it does not interfere with loop stability. - It is **lower than** the switching frequency, 2\,\text{MHz}, so it provides sufficient attenuation of switching ripple. - As a rule of thumb, place it at least one decade above the loop bandwidth and one decade below the switching frequency. ## How Is the Damping Resistor Calculated? An LC filter has an impedance peak at its resonant frequency. Connecting C\_2 and R\_1 in parallel with C\_1 suppresses this peak: R\_1=\sqrt{\frac{L}{C}}=\sqrt{\frac{0.3\,\mu\text{H}}{4.7\,\mu\text{F}}}=0.25\,\Omega \tag{14} > **Technical tip:** The damping network consumes a small amount of energy and introduces a slight voltage drop. Compared with the noise reduction it provides, this is usually acceptable. ## Simulation Verification 1: Frequency Response The Bode plot provides an intuitive view of how much noise the filter blocks: ![image](https://global.discourse-cdn.com/digikey/original/3X/7/3/73ed112744c8983e765375fea57fe99e18454710.png) **Figure 3:** Simulated frequency response of the additional filter (image sourced from Analog Devices). At the 2\,\text{MHz} switching frequency, the filter provides approximately 47\,\text{dB} of attenuation. **What does 47\,\text{dB} mean? The ripple amplitude is reduced to approximately 1/223 of its original value.** ## Simulation Verification 2: Time-Domain Output Ripple Finally, examine the time-domain waveform after adding the filter and compare it with Figure 1: ![image](https://global.discourse-cdn.com/digikey/original/3X/0/6/06153ac9b26e1f8603d84442d22fc74c70b2d4e6.png) **Figure 4:** Simulated LT8330 output ripple after adding the filter (image sourced from Analog Devices). Comparing Figures 1 and 4, the high-frequency spikes are significantly suppressed and the output becomes clean and flat. This is the value of a second-order LC filter in a noise-sensitive application. ## Important Reminder: Keep the Filter Outside the Control Loop The filter is connected after the feedback-sense point, on the load side. In other words, the feedback network senses the voltage before the filter rather than the “clean” filtered output. Why? - If the filter is placed inside the control loop, the LC resonance introduces additional phase shift into the loop gain and may compromise stability. - When placed outside the loop, the filter only attenuates output noise and does not affect loop compensation. - The tradeoff is a small load-regulation loss caused by the filter’s DC resistance and the inductor DCR, which is usually minor. ## Summary - A second-order LC filter is an economical and effective solution for suppressing high-frequency switching noise. - The 134\,\text{kHz} cutoff frequency lies between the loop bandwidth (13\,\text{kHz}) and the switching frequency (2\,\text{MHz}). - The damping network, R\_1=0.25\,\Omega and C\_2=22\,\mu\text{F}, suppresses the LC resonant peak. - The simulation shows approximately 47\,\text{dB} of attenuation at 2\,\text{MHz}. ### Key Takeaways - A second-order LC filter is economical and effective; this example achieves approximately 47\,\text{dB} of attenuation at 2\,\text{MHz}. - Set the cutoff frequency to 134\,\text{kHz}, between the loop bandwidth (13\,\text{kHz}) and switching frequency (2\,\text{MHz}). - The damping network, R\_1=0.25\,\Omega and C\_2=22\,\mu\text{F}, suppresses the LC resonant peak. - The filter must be placed after the feedback-sense point, outside the control loop, to avoid compromising stability. > **Source:** This article references the Analog Devices technical article _Boost Converter Design for Sensitive Loads at Light Load_. ### Related Products - [LT8336 | 40 V, 2.5 A low-IQ synchronous boost converter](https://www.digikey.com/en/products?keywords=LT8336) - [LT8330 | 60 V, 1 A low-IQ boost/SEPIC converter](https://www.digikey.com/en/products?keywords=LT8330) ### Additional References 1. [LT8336 data sheet (Analog Devices)](https://www.analog.com/media/en/technical-documentation/data-sheets/lt8336.pdf) 2. [LT8330 data sheet (Analog Devices)](https://www.analog.com/media/en/technical-documentation/data-sheets/lt8330.pdf) ### More Content - [Boost Converter Design: Part 1 — The Light-Load Challenge: Choosing Between Efficiency and Noise](https://forum.digikey.com/t/boost-converter-design-part-1-the-light-load-challenge-choosing-between-efficiency-and-noise/72014) - [Boost Converter Design: Part 2 — Design Goals and Input Current / Duty Cycle Calculations](https://forum.digikey.com/t/boost-converter-design-part-2-design-goals-and-input-current-duty-cycle-calculations/72015) - [Boost Converter Design: Part 3 — Selecting the Inductor: Larger or Smaller?](https://forum.digikey.com/t/boost-converter-design-part-3-selecting-the-inductor-larger-or-smaller/72016) - [Boost Converter Design: Part 4 — IC Selection and Light-Load Operating Modes](https://forum.digikey.com/t/boost-converter-design-part-4-ic-selection-and-light-load-operating-modes/72018) - [Boost Converter Design: Part 5 — The Final Gate: An Output LC Filter](https://forum.digikey.com/t/boost-converter-design-part-5-the-final-gate-an-output-lc-filter/72019) - [Boost Converter Design: Part 6 — Reviewing the Seven-Step Design Process](https://forum.digikey.com/t/boost-converter-design-part-6-reviewing-the-seven-step-design-process/72020)