In the first five parts, we moved from understanding why a light-load boost converter is difficult to designing one that meets the noise requirement. This final article condenses the complete design into a single workflow. We will first look at the final circuit, review each step, and then summarize the recommendations for noise-sensitive loads.
The Complete Final Design
Figure 1: Complete circuit with the LT8330 and a second-order LC output filter. This is the final form of the design developed in this series (image sourced from Analog Devices).
The Seven-Step Design Review
1. Calculate Input Current Before Selecting the IC
The switch current in a boost converter is not the load current; it is the inductor current viewed from the input. Calculate the average and peak input current from P_{OUT}, efficiency, and input voltage before selecting the IC switch-current rating. (See Part 2.)
2. For a Light-Load, Low-Noise Design, Maintain CCM Whenever Possible
CCM provides a fixed switching frequency and fixed ripple frequency, making filtering easier and EMI more predictable. If the converter enters DCM or Burst Mode at light load, the effective frequency decreases and both noise and EMI become worse. (See Parts 1 and 3.)
3. Prefer Pulse-Skipping Mode or Forced CCM
If the IC supports pulse-skipping mode, select it first. Forced CCM is also worth considering. Although Burst Mode provides higher efficiency, its noise penalty is too high for noise-sensitive loads. (See Part 4.)
4. Provide Enough Switch-Current Rating, but Do Not Oversize It
An excessive switch-current rating raises the IC overcurrent threshold I_{OCL}. This in turn requires the inductor saturation current to satisfy I_{SAT}>1.2\times I_{OCL}, increasing inductor size, core loss, and EMI. (See Part 4.)
5. Prefer the Lower Input Rail for Light-Load Boost Operation
The lower the input voltage, the higher the input current, and the easier it is for the larger inductor current to maintain CCM. In this example, the 12\,\text{V} input is more favorable for maintaining continuous conduction than the 28\,\text{V} input. (See Parts 2 and 3.)
6. Use a Second-Order LC Filter to Suppress High-Frequency Noise
Even with the right IC and operating mode, switching ripple and spikes may still exceed the noise budget of a sensitive load. A second-order LC filter at the output can attenuate the 2\,\text{MHz} switching noise by approximately 47\,\text{dB}. (See Part 5.)
7. Place the Filter Outside the Control Loop
The feedback-sense point should be before the filter to prevent LC resonance from introducing additional phase shift and compromising loop stability. The filter is responsible only for post-processing the output noise. (See Part 5.)
Conclusion
Designing a boost converter for a sensitive load, such as an RF circuit or analog sensor, requires attention to noise at every stage:
- IC selection: Start with the input-current calculation and choose a device with an appropriate switch-current rating. Avoid an oversized inductor.
- Operating mode: Pulse-skipping mode can maintain CCM at light load, keeping the effective switching frequency stable and EMI predictable.
- Input-rail selection: A lower input-voltage rail is generally more favorable for maintaining CCM at light load.
- Output filtering: When switching noise is still too high, a second-order LC filter outside the control loop can provide approximately 47\,\text{dB} of attenuation without compromising loop stability.
Combining these techniques makes it possible to meet a strict noise budget in a space-constrained design. In one sentence: calculate first, choose the right mode, and then add one more filter stage.
Key Takeaways
- The seven-step process is: calculate input current → select the IC → maintain CCM → use pulse-skipping mode or forced CCM → provide only the necessary switch-current rating → boost from the lower input rail → add a second-order LC filter outside the loop.
- Prefer the lower input voltage rail because it helps maintain CCM at light load.
- Combining these techniques makes it possible to meet a strict noise budget in a space-constrained design.
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
- LT8330 | 60 V, 1 A low-IQ boost/SEPIC converter
Additional References
More Content
- Boost Converter Design: Part 1 — The Light-Load Challenge: Choosing Between Efficiency and Noise
- Boost Converter Design: Part 2 — Design Goals and Input Current / Duty Cycle Calculations
- Boost Converter Design: Part 3 — Selecting the Inductor: Larger or Smaller?
- Boost Converter Design: Part 4 — IC Selection and Light-Load Operating Modes
- Boost Converter Design: Part 5 — The Final Gate: An Output LC Filter
- Boost Converter Design: Part 6 — Reviewing the Seven-Step Design Process