## 3 The Use of the TIBuck DC/DC Converter as a VLC Driver
### Analysis of the Switching Harmonic Components of the Proposed TIBuck DC/DC Converter  
Although V2 is considered constant during a switching cycle in the TIBuck DC/DC converter, it can have switching harmonic components (i.e., output voltage ripple), which come from the auxiliary Buck DC/DC converter. Figure 10 is a representation of the filtering process in both converters and the effect of their switching harmonics on the output voltage of the proposed TIBuck DC/DC converter. Both filters are depicted in Figure 8. The output voltage V2 is the result of the low-pass filtering process of the switching node voltage vswbu(t) at the auxiliary Buck DC/DC converter. Its second-order filter is defined by Lbu and Cbu, setting the cut-off frequency, fc-bu. Its spectral representation, 1 vswbu(t)1, has a DC component, as well as switching harmonics at multiples of the switching frequency of the auxiliary Buck DC/DC converter, fbu. The cut-off frequency fc-bu is chosen according to the switching frequency fbu, attenuating the output switching harmonic components. Figure 9 shows that point; although the frequencies fbu, fsig and fsw are sufficiently separated, part of the attenuated switching frequency harmonics of the auxiliary Buck DC/DC converter arise in V2, and they can lie within the communication signal bandwidth. For the sake of simplicity, this possibility is represented by the kth harmonic of fbu. To minimize this effect, fc-bu must be selected low enough so that the switching frequency fbu, and therefore the ripple on V2, is negligible in the communication bandwidth. This is not really a problem, because the auxiliary Buck DC/DC converter does not need high bandwidth.  
Figure 9: Representation of the output voltage range of the TIBuck DC/DC converter versus the useful voltage range of the HB-LED load and the impact on the resolution for tracking the output voltage when temperature changes occur.  
The spectral representation of the switching node voltage of the proposed TIBuck DC/DC converter, \(\rm{v_{sw}(t)}\), is \(\rm{|v_{sw}(f)|^{2}}\). It has components coming from the \(\rm{V_{2}}\) ripple (as \(\rm{f_{bu}}\) harmonics), the communication signal spectrum (centred in \(\rm{f_{sig}}\)) and the switching harmonic components of the switching frequency of the TIBuck DC/DC converter, \(\rm{f_{fi}}\). Finally, the output filter of the TIBuck DC/DC converter is in charge of attenuating the \(\rm{f_{fi}}\) switching harmonics and letting the communication signal spectrum pass. The result is depicted as \(\rm{|v_{o}(f)|^{2}}\). The cut-off frequency of the output filter of the TIBuck DC/DC converter, \(\rm{f_{fi}}\), and the order of the filters are selected to achieve a certain bandwidth with low-level distortion in \(\rm{v_{o}(f)}\) (i.e., a low level of switching harmonic components in \(\rm{v_{o}(f)}\)). As analysed above, it is desirable to increase the bandwidth while reducing the switching frequency \(\rm{f_{fi}}\). Equation (7) shows that \(\rm{f_{c-ti}}\) must be selected in between the signal spectrum and the switching frequency. How close these frequencies can be is determined by the order of the output filter. The higher the order, the closer the frequencies can be, but the more difficult it is to design.  
To summarize this section, the proposed solution of the VLC driver based on the split power technique using a proposed TIBuck DC/DC converter outperforms the Buck DC/DC converter in terms of efficiency and resolution for tracking the output voltage. Moreover, the modification of the TIBuck DC/DC converter with the auxiliary Buck DC/DC converter reduces the hardware complexity of previous approaches to achieve the split power technique. However, the price to pay is the loss in resolution for tracking the output voltage when the temperature of HB-LEDs increases.