## 1 Introduction
This imposes a limit on the average light emitted by the HB-LED. Additionally, since \(\rm V_{\Omega}\) does not depend on temperature, it means that the maximum peak-to-peak value of signal \(\rm V_{pp}\) is also constant. The addition of the communication signal has some implications for the HB-LED that are worth mentioning. Its linear behaviour plays an important role in communication tasks. Some types of pre- and post-equalization have been proposed to counteract non-linearities, making the HB-LED able to reach higher peak-to-peak values [10; 11; 12], but those depend on the modulation scheme used and they are applied directly on the generation of the communication signal, with no impact on VLC driver design. Another important consideration is the dimming capability of the VLC driver. The illumination level is controlled by the average current, \(\rm I_{bias}\). But, if the communication signal is maximized, this means that \(\rm I_{bias}\) will have a fixed value and cannot be modified (i.e., no dimming). In a VLC driver, the dimming capability is strongly linked to the communication modulation scheme and these must all be implemented together [5; 6; 13]. The criteria chosen in this work is a worst-case scenario in terms of efficiency. Maximizing the communication signal means that the power processed at high frequency is also maximized, making the design more challenging in terms of reducing switching-power losses, but meaning that the communication covers a greater distance. HB-LEDs also impose a limitation on the bandwidth of the VLC driver. In general, a blue gallium nitride HB-LED in combination with a yellow inorganic phosphor is the preferred approach for obtaining white light in SSL. However, this phosphor limits the HB-LED bandwidth to a few MHz (3-5 MHz) [14; 15]. All these constraints for HB-LEDs must be considered in the design of VLC drivers. Most VLC driver topologies are based on the use of regular HB-LED drivers for the bias tasks connected in parallel to a bias T scheme with an RF linear power amplifier (i.e., class A or B) [16; 17]. This solution leads to simple implementations with high bit rates. The main disadvantage is decreased efficiency due to using RF linear power amplifiers with theoretical maximum efficiencies of 50% and 78%, respectively, for class A and B. Moreover, efficiencies fall sharply when high bit rates and more complex modulation schemes are used (between 10% and 40%). The main advantage of RF linear power amplifiers is the wider bandwidth they can achieve, but this advantage cannot be exploited with current HB-LED lighting technology. Another approach is based on merging the two tasks, bias and communication, into the HB-LED driver, making a DC/DC converter based on conventional Pulse-Width Modulation (PWM), also able to generate the communication signal, but, in this case additional signal distortion is introduced by the switching frequency. The capability of the fast-response and high-frequency DC/DC converters to track their output voltage according to a communication signal at high efficiency has previously been reported and exploited in communication applications such as Envelope Tracking (ET) and Envelope Elimination and Restoration (EET) [18; 19; 20]. These DC/DC converters can achieve bandwidths around tens of MHz with high efficiency. They have been proposed as VLC drivers, achieving efficiencies around 90% and high bit rates [21]. One of the disadvantages of using the same DC/DC converter for both biasing and communication tasks is that the bias power is processed at high switching frequency. In a VLC driver, the biasing power is much higher than the communication power, and there is no need to use a high-frequency converter for the biasing process. This context is where the technique of splitting the power between two DC/DC converters is proposed, to further improve the efficiency of the VLC driver [22].