FEATURE COVER STORY
AUTOMOTIVE LED DRIVER DELIVERS flicker-free PWM dimming
Keith Szolusha, Applications Engineering Section Lead, Power Products at Linear Technology Corporation, explores the options for achieving Flicker Free PWM Dimming with an LED Driver Switch Mode Power Supply for advanced automotive LED lighting solutions using spread spectrum
A
utomotive LED drivers should be compact, efficient and support
flicker-free PWM dimming. They should not produce significant conducted EMI at and around the AM radio band. Unfortunately, low EMI is not in the nature of high power switch mode power supplies. The constant switching frequency produces a significant EMI signature at a number of frequencies, including the power supply’s fundamental operating frequency and its harmonics. Odds are good that something will fall into the AM band. One way to minimise EMI peaks is to
allow the switch mode power supply (SMPS) operating frequency to cover a range of values, namely spread spectrum switching. The desired effect of spread spectrum switching is to push down the EMI peaks that would occur at the SMPS fundamental operating frequency and harmonics, spreading the EMI energy over a range of frequencies instead. LED driver SPMSs have an additional
requirement: the frequency spreading should also be synchronised with the PWM dimming (brightness control) frequency to ensure that there is no resulting LED flicker. To this end, the LT3795 from Linear Technology Corp. generates its own spread spectrum ramp signal and aligns it with the lower frequency PWM dimming input with a patent pending technique. This eliminates the chance that the spread spectrum frequency could combine with the PWM signal to produce visible flicker in the LEDs— even at the highest PWM dimming ratio.
HIGH POWER LED DRIVER The LT3795 is a high power LED driver that uses the same high performance PWM dimming scheme as the company's LT3756/LT3796 family, but with the additional feature of the internal spread spectrum ramp for reduced EMI. It is a 4.5V-to-110V input to 0V-to-110V output single-switch controller IC that can be configured as a boost, SEPIC, buck-boost
mode or buck mode LED driver. It features a 100kHz to 1MHz switching frequency range, open LED protection, short-circuit protection, and can also be operated as a constant voltage regulator with current limit or as a constant current SLA battery or supercapacitor charger. Figure 1 shows a 92% high efficiency 80V, 400mA, 300kHz-450kHz automotive LED headlamp driver with spread spectrum frequency modulation and short-circuit protection.
Figure 1:
80V, 400mA automotive LED driver with internal spread spectrum for low EMI
INTERNAL SPREAD SPECTRUM SIMPLIFIES DESIGN Unlike other high power LED drivers, the LT3795 generates its own spread spectrum ramp to produce 30 percent switching frequency modulation below the programmed switching frequency. This lowers its conducted EMI peaks, reducing the need for costly and bulky EMI input filter capacitors and inductors. Using an external, or separate, spread spectrum clock to produce the switching frequency in an LED driver can produce visible flicker during PWM dimming since the spread spectrum frequency pattern is not synchronised with the PWM period. For this reason, in many high end LED driver applications, implementing spread spectrum is not trivial. Without spread spectrum, designers must rely upon bulky EMI filters, gate resistors that slow down switching edges (but reduce efficiency) and snubbers on the switch and catch diode. Figure 2 shows a comparison of the conducted EMI measurements of the LT3795 LED driver around the AM band when spread spectrum is enabled and disabled. Normal (non-spread spectrum) operation yields high energy peaks at the switching frequency and its harmonics.
Figure 2: Conducted peak EMI around the AM band is reduced by 3dBµV–6dBµV when the LT3795’s spread spectrum frequency modulation is used. The CISPR25 Class 5 AM-band limit is provided for reference
12 JUNE 2015 | ELECTRONICS
Figure 3: Spectrum analyser scan of the LT3795 150kHz– 30MHz peak conducted EMI shows the reduction in peak EMI over a broad frequency range
/ ELECTRONICS
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