Boost Converter Design: Part 4 — IC Selection and Light-Load Operating Modes

Semiconductor Integrated Circuits (ICs)
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Based on the preceding calculations, the peak switch current is approximately 0.377\,\text{A}. This article compares two candidate ICs, LT8336 and LT8330, with an emphasis on two light-load operating modes—Burst Mode and pulse-skipping mode—and their effects on noise and EMI.

Candidate ICs: LT8336 and LT8330

LT8336

  • Integrated 40\,\text{V}/2.5\,\text{A} switch.
  • Synchronous operation.
  • Integrated Silent Switcher® Gen 2 architecture, which reduces magnetic-field radiation from the hot loop through an internal reverse-current path.
  • Supports Burst Mode and pulse-skipping mode, with optional spread-spectrum frequency modulation (SSFM).

LT8330

  • Asynchronous topology with an external catch diode.
  • Integrated 60\,\text{V}/1\,\text{A} switch.
  • Does not include Silent Switcher technology.
  • Its lower switch-current rating makes it possible to use a smaller inductor.

The LT8336 has a higher switch-current rating and therefore appears to provide more margin. However, a higher switch-current rating is not always better.

Why Is a Higher Switch-Current Rating Not Always Better?

In a monolithic regulator, the overcurrent limit I_{OCL} is set internally. To prevent inductor saturation, the inductor saturation current should satisfy:

I_{SAT}>1.2\times I_{OCL}

The typical I_{OCL} of the LT8336 is approximately 3\,\text{A}, which requires an inductor saturation current greater than 3.6\,\text{A}. Even when the actual load current is only 0.1\,\text{A}, the inductor must still be selected for 3.6\,\text{A}. This increases its size and core loss, and may also increase EMI.

By contrast, the LT8330 has a switch-current rating of only 1\,\text{A}, so the requirement for inductor saturation current is much lower, which helps reduce the solution size. This is why the LT8330 should still be evaluated even though the LT8336 appears to be the more capable device.

Light-Load Operating Modes: Burst Mode vs. Pulse-Skipping Mode

The LT8336 SYNC / MODE pin can select two light-load modes:

Burst Mode

  • At light load, the inductor current falls to 0\,\text{A} and the converter operates in DCM.
  • The converter sends a burst of pulses only when the output voltage falls below the threshold, then becomes quiet.
  • Advantage: Highest light-load efficiency.
  • Disadvantage: The effective switching frequency changes with load, and the current peak is set internally by the IC and may be large. EMI and output noise are therefore more difficult to predict.

Pulse-Skipping Mode

  • The converter continues switching at the programmed 2\,\text{MHz} frequency.
  • When the load does not need as much energy, the converter skips complete cycles rather than changing the frequency.
  • Advantage: The switching frequency remains fixed, the inductor current stays in CCM, and EMI and ripple are more predictable.
  • Disadvantage: Light-load efficiency is slightly lower than in Burst Mode.

For a noise-sensitive load, low noise is usually more important than maximum efficiency. Therefore, pulse-skipping mode is the preferred choice.

Simulation Results: LT8336 with a 12\,\text{V} Input

The following LTspice schematic shows the LT8336. The SYNC / MODE connection determines whether the converter operates in Burst Mode or pulse-skipping mode.

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Figure 1: The SYNC / MODE connection determines the light-load operating mode (image sourced from Analog Devices).

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Figure 2: Inductor-current comparison between Burst Mode (blue) and pulse-skipping mode (yellow) with a 12\,\text{V} input.

In Burst Mode, the inductor-current peak reaches approximately 0.7\,\text{A}, more than eight times the load current. The current is discontinuous, and the effective frequency is below 2\,\text{MHz}. In pulse-skipping mode, the current is continuous and the frequency remains fixed at 2\,\text{MHz}, consistent with the preceding calculations.

Simulation Results: LT8336 with a 28\,\text{V} Input

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Figure 3: Inductor-current comparison between Burst Mode (cyan) and pulse-skipping mode (yellow) with a 28\,\text{V} input.

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Figure 4: Zoomed view of Burst Mode and pulse-skipping mode with a 28\,\text{V} input.

With a 28\,\text{V} input, the duty cycle is small and the inductor charging interval is short. Even with the 10\,\mu\text{H} inductor calculated previously, pulse-skipping mode only barely maintains continuous conduction, and the current briefly approaches 0\,\text{A}. This shows that a higher input voltage makes the converter more likely to enter DCM.

Simulation Results: LT8330

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Figure 5: 12\,\text{V}-input schematic based on the LT8330.

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Figure 6: LT8330 inductor current with a 12\,\text{V} input.

With a 12\,\text{V} input, the LT8330 inductor current is continuous. Its peak and valley values agree with the initial calculations and with the LT8336 pulse-skipping-mode result.

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Figure 7: LT8330 inductor current with a 28\,\text{V} input and a 10\,\mu\text{H} inductor.

The current is now discontinuous, producing alternating large and small spikes. This behavior is caused by cycle-by-cycle current variation and switch-node ringing.

Selection Conclusions

  • If light-load efficiency is the highest priority and the noise margin is generous, Burst Mode can be selected.
  • If the load is noise-sensitive, select pulse-skipping mode or forced CCM, even if this sacrifices some efficiency.
  • When possible, select an IC with a lower switch-current rating to reduce the inductor size and saturation-current requirement.
  • An IC with Silent Switcher technology can further reduce EMI, but it is usually more expensive.

For this example, the LT8330 solution remains the final choice, with a post-regulator filter used to address the remaining noise.

Key Takeaways

  • A higher switch-current rating is not always better. A higher rating leads to a higher I_{OCL}, which requires a larger inductor and can worsen EMI.
  • For noise-sensitive loads, pulse-skipping mode or forced CCM provides more predictable noise.
  • This example selects the LT8330 because its lower switch-current rating helps reduce the solution size; the remaining noise is handled by the post-regulator filter.

Source: This article references the Analog Devices technical article Boost Converter Design for Sensitive Loads at Light Load.

Related Products

Additional References

  1. LT8336 data sheet (Analog Devices)
  2. LT8330 data sheet (Analog Devices)

More Content

Boost Converter Design: Part 3 — Selecting the Inductor: Larger or Smaller?
Boost Converter Design: Part 2 — Design Goals and Input Current / Duty Cycle Calculations
Boost Converter Design: Part 1 — The Light-Load Challenge: Choosing Between Efficiency and Noise