LED Technology
Figure 5. Waveforms show smooth switchover between high and low, low, and DRL LED strings for the LT8391A multibeam application in Figure 5
Figure 3. LT8391A demonstration circuit DC2575A passes CISPR 25 Class 5 automotive radiated EMI
Figure 4. LT8391A demonstration circuit DC2575A passes CISPR 25 Class 5 automotive conducted EMI
Spread spectrum frequency modulation (SSFM) reduces EMI and also runs flicker- free simultaneously with PWM dimming. Its small size is highlighted by its small inductor and especially small input and output EMI filters. Large LC filters are not needed for 2 MHz converters and only small ferrite beads are used for high frequency EMI reduction. Automotive EMI requirements are not easily met by high power converters. High power switches and inductors placed on large PCBs next to large capacitors can create undesirable hot loops, especially when a large sense resistor is included. The unique LT8391A buck-boost architecture removes the sense resistor from both the buck and boost switch-pair hot loops, enabling low EMI. Figure 3 and Figure 4 show the
measured EMI of the 24 W LED driver of Figure 1. Despite this controller’s 2 MHz operating frequency and 24 W of power, this buck-boost passes CISPR 25 Class 5 radiated and conducted EMI. Class 5 is the most stringent requirement and the goal of most automotive EMI testing. Converters that cannot pass Class 5 EMI either get designed out of automotive circuits or must be encased in large metallic EMI shields. Even if the bulkiness of the shield does not create assembly issues, adding them is costly.
36 February 2019
Buck-Boost for multibeam applications
LED headlight clusters can be both innovative and artistically creative. High beams and low beams can be wrapped up with nifty and distinctive daytime running lights (DRLs). Because the daytime running lights are only needed when high and low beams are off, a single LED driver can be used to power either the high and low beam LEDs or the daytime running lights. This only works if the LED driver has a flexible input-to-output ratio and can both step-up and step-down the input-to- output voltage. A buck-boost design satisfies this requirement. The multibeam LT8391A buck-boost LED driver in Figure 5 can drive LED string voltages ranging from 3 V to 34 V. This enables it to drive both a low beam string and create a high beam by adding LEDs to the low beam string. The same driver switches over and drives a higher voltage, yet lower current, DRL. Switching from low beam-only LEDs to a low/high beam combo string generates no spike on the output voltage or LED current. The LT8391A can smoothly transition between boost, 4-switch buck-boost, and buck regions of operation. Changing from a small number of LEDs to a high number of LEDs without an LED spike can be challenging for a converter, but this multibeam circuit does this with ease.
Components in Electronics
Switching back from high and low beams to just low beams is also very clean, without any harmful LED spikes.
FE and QFN packages fit tight spots The LT8391A is available in a 4 mm × 5 mm, 28-lead QFN to meet small size requirements and a 28-lead TSSOP FE package for automotive designs. Both packages have thermally enhanced GND pads for power dissipation of the internal INTVCC LDO from higher voltages. The internal LDO INTVCC regulator of these converters can handle driving four
Figure 6. Compact solution: 2 MHz demonstration circuit DC2575A, featuring LT8391A, drives 16 V LEDs at 1.5 A
synchronous MOSFETs at 2 MHz with about 15 nC gate charge.
Conclusion The LT8391A 2 MHz, 60 V buck-boost LED driver controller powers LED strings in automotive headlights. Its features include its low EMI 4-switch architecture and spread spectrum frequency modulation for meeting CISPR 25 Class 5 EMI requirements. The unique, high switching frequency allows it to operate above the AM band, requiring very little EMI filtering. Its small size and versatility enable use in headlight cluster LED strings of a variety of voltages and currents.
Figure 7. PWM dimming using internal and external PWM options; 1% and 0.05%, respectively
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