# LED Current Balance Using a Variable Voltage Regulator with Low Dropout \(v_{DS}\) Control
## 4 Design and Experiment  
To verify the proposed control theme, a scaled-down simple layout of 5 \(\times\) 6 LEDs are arranged with five string LEDs connected in parallel, in which each string has six LEDs in series. Five LCRs are employed to drive the five string LEDs, respectively, in which MOSFET IRF110 is used as the current regulator in LCR, and the LED EHP-C04/UT01-P01/TR with a rated power of 1 W is adopted. The operational amplifier TL074 with a slew rate of 13 V/us is used as a comparator. The power LED has a conductance \((i_{D}/v_{D})\) of approximately 0.42 mA/mV. The transconductance \((i_{DS}/v_{GS})\) of the MOSFET is about 2.97 mA/mV under an \(i_{DS}\) varying from 0 mA to 350 mA; correspondingly, \(v_{GS}\) varies from 3.72 V to 4.5 V. The low-dropout \(v_{DS}=V_{Q}\), is 1.5 V, specified at \(i_{DS}\approx\) 200 mA. The circuit schemes for the experiment are as in Figures 7 and 8, in which the digital controller for synthesizing the VVR is as per the loop parameters of the master LCR, with reference to the estimation profile in Figure 5b. In Figure 7, the \(v_{DS}\) is instantly detected by the differential amplifier (A1) and is controlled at a low dropout \(V_{Q}\). The voltage \(v_{k}=f(i_{DSk})\) in (16) is a function of the \(k\)th current \(i_{DSk}\) detected from a source voltage \((v_{R})\), in which the voltage \(v_{k}\) is the summation of \(v_{R}\) and \(Nv_{D}\), where the footnote \"\(k\)\" is used to denote the position of the operation point driven by \(v_{GSk}\). The \(v_{Q}\) and \(v_{k}\) are merged at the inverting input of the error amplifier (A2) and compared with a specified reference  
Figure 8: The algorithm to implement \(VVR\) for supplying the LCRs, executed by Microchip dsPIC33 FJ06 GS202 with reference to Figure 5b.  
voltage \((V_{Q}+V_{k})_{\text{ref}}\). After comparing the error output \(V_{ox}\) from A2 with a reference sawtooth at the PWM controller (A3), an updated variable voltage \(VV_{dc}\) from the VVR is acquired through the loop regulation in the master LCR. With the \(VV_{dc}\), the \(v_{DS}\)'s in the five string LCRs can then be kept at a low dropout \(V_{Q}\) on the five flat \(v_{GS}\)-characteristic curves, respectively. A source resistor \(R_{s}=12\)\(\Omega\) which is, estimated for the current sensor is used, the VVR is supplied by a dc source \(V_{dc}=35\) V, and the dimmer control is implemented by a precision variable resistor. The digitizing controller, as shown in Figure 8, is implemented by Microchip dsPIC33 FJ06 GS202, in which three parameters, including \(v_{D5}\), \(v_{R}\), and \(v_{DS}\), detected from the control loop of the master LCR, are sampled and held for digitizing. Through D/A conversion, the voltage \(v_{k}\) is acquired. After merging the \(v_{k}\) and \(v_{DS}\) to compare with the reference \((V_{Q}+V_{k})_{\text{ref}}\) at the error amplifier (EA), the updated \(VV_{dc}\) from the VVR to supply the five LED strings, is achieved. Consequently, the self-feedback in the control loop of the master LCR for keeping all slave \(v_{DS}\)'s at low dropout is always valid, and indeed the updated \(VV_{dc}\) is able to achieve the current balance in the multiple LED strings.