Optoelectronics Figure 3. Boost converter.


Figure 4. Boost converter example: LT8356-1.


Step-up (boost) converters Step-up (or boost) LED drivers regulate the current in an LED string from a voltage that is lower than the total LED string voltage. This is useful in many automotive systems, where many LEDs need to conduct in a single string. Typical 12V automotive systems have operational ranges from 6V to 18V - requiring that the LED driver runs down to 6V, providing large step-up ratios for the LEDs to


Benefits to Boost LED Drivers


Grounded chassis return


Typically, smallest total solution size


Good EMI performance


Direct battery to LED conversion


Trade-Offs to Step-Up LED Drivers


Input voltage must be higher than LED voltage


Lower bandwidth (<1/20 of fSW)


Higher inductor current rating Cannot short output to GND Table 2. Advantages and trade-offs of using boost converters as LED drivers


remain illuminated. Figure 3 and an example schematic in Figure 4 show basic system diagrams with the controller modulating the low-side switch for current control.


Boost-buck using a boost converter Some step-up (or boost) LED drivers may be configured to return the LED cathode to the supply. This configuration is referred to as buck-boost. The total output voltage is VIN (VBATTERY), which is added to the total LED string voltage. The benefit of this topology is being able to drive an LED string that is higher, lower, or equal to the supply voltage.


Benefits to Boost-Buck LED Drivers


Direct battery to LED conversion


LED voltage may be higher or lower than supply


Good EMI performance


May use matrix to short entire string


Trade-Offs to Boost-Buck LED Drivers


Lower efficiency


Lower bandwidth (<1/20 of fSW)


Higher inductor current rating Cannot short output to GND Applications High beam/low beam Turn signal Daytime running lights


Multiple strings on the same output


Table 3. Advantages and trade-offs of using boost-buck converters as LED drivers www.cieonline.co.uk. Components in Electronics Figure 5. Boost-buck converter. Applications High beam/low beam Heads-up displays Backlighting Figure 6. Boost-buck converter: LT8386


Figure 7. Buck-mode converter.


The limitations of this topology are only bounded by the converter - on the low end by the minimum supply voltage of the controller IC and on the high end by the controller IC’s maximum output voltage.


Buck mode using a boost converter Some step-up (or boost) LED drivers may be configured to step-down from the supply (rather than ground referenced, as in a standard buck) - creating a buck-mode configuration. This configuration has the same limitations as a buck, where the total LED string voltage must be less than the input supply.


continues page 57 September 2023 55


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