Designing a 60V Buck-Boost LED driver with up to 98% efficiency: Page 3 of 3

November 03, 2016 //By Keith Szolusha
Designing a 60V Buck-Boost LED driver with up to 98% efficiency
Synchronous buck-boost converters with four power switches can deliver very high efficiency while providing both step-up and step-down DC/DC conversion.

Spread Spectrum Frequency Modulation (SSFM) Reduces EMI

SSFM reduces EMI in switching regulators. Although the switching frequency is most often chosen to be outside the AM band (530kHz to 1.8MHz), unmitigated switching harmonics can still violate stringent automotive EMI requirements within the AM band. Adding SSFM significantly reduces EMI both within the AM band as well as other regions.

When activated, SSFM drops the LT8391’s 50W LED driver EMI below both the peak and average EMI requirements of CISPR25 (Class 5) in the AM band. CISPR25 average EMI limits are 20dBμV lower in some places than peak limits, and harder to pass for switchers. For this reason, LT8391’s novel SSFM reduces average EMI more than its peak. There is 18dBμV or more reduction of average EMI and about 5dBμV of peak EMI reduction.

In some LED converters, SSFM and PWM dimming do not work together without causing flicker. SSFM, a source of changing switching frequency, may appear as noise to the outside world in order to spread EMI lower than its non-spread peak values, but it can work with PWM dimming for flicker-free operation. Linear’s patent pending PWM dimming and SSFM operation runs both functions simultaneously with flicker-free operation, even at high dimming ratios. At 1000:1 PWM dimming with external PWM and at 128:1 internally generated PWM, SSFM operates with flicker-free LED current.

Figure 3. SSFM Reduces LT8391 Peak and Average EMI Below CISPR25 Class 5 Limits. Average EMI Has Even Greater Reduction Than Peak EMI.

 Figure 4. Infinite-Persist Scope Capture Shows
Flicker-Free Dimming with PWM and SSFM Working Together.


Download the Design Note 548 (PDF)

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