The illumination driver of the DLPA3000 is a buck converter with two internal low-ohmic N-channel FETs (see Figure 8-6). The theory of operation of a buck converter is explained in Understanding Buck Power Stages in Switchmode Power Supplies. For proper operation, selection of the external components is very important, especially the inductor L_OUT and the output capacitor C_OUT. For best efficiency and ripple performance, an inductor and capacitor should be chosen with low equivalent series resistance (ESR). Set the voltage rating of the capacitor equal or greater than two times of the applied voltage across the capacitor in the application.

Figure 8-6 Typical Illumination Driver Configuration

Several factors determine the component selection of the buck converter, such as input voltage (SYSPWR), desired output voltage (V_LED) and the allowed output current ripple. Configuration starts with selecting the inductor L_OUT.

The value of the inductance of a buck power stage is selected such that the peak-to-peak ripple current flowing in the inductor stays within a certain range. Here, the target is set to have an inductor current ripple, k_{I_RIPPLE}, less than 0.3 (30%). The minimum inductor value can be calculated given the input and output voltage, output current, switching frequency of the buck converter (ƒ_SWITCH= 600 kHz) and inductor ripple of 0.3 (30%):

Equation 1.

Example: V_IN= 12 V, V_OUT= 4.3 V, I_OUT= 6 A results in an inductor value of L_OUT= 2.7 µH

Once the inductor is selected, the output capacitor C_OUT can be determined. The value is calculated using the fact that the frequency compensation of the illumination loop has been designed for an LC-tank resonance frequency of 15 kHz:

Equation 2.

Example: C_OUT= 41.7 µF given that L_OUT= 2.7 µH. A practical value is 2 × 22 µF. Here a parallel connection of two capacitors is chosen to lower the ESR even further.

The selected inductor and capacitor determine the output voltage ripple. The resulting output voltage ripple V_{LED_RIPPLE} is a function of the inductor ripple k_{I_RIPPLE}, output current I_OUT, switching frequency ƒ_SWITCH and the capacitor value C_OUT:

Equation 3.

Example: k_{I_RIPPLE}= 0.3, I_OUT= 6 A, ƒ_SWITCH= 600 kHz and C_OUT= 44 µF results in an output voltage ripple of V_{LED_RIPPLE}= 8.5 mVpp

As can be seen, this is a relative small ripple.

It is strongly advised to keep the capacitance value low. The larger the capacitor value the more energy is stored. In case of a V_LED going down, stored energy needs to be dissipated. This might result in a large discharge current. For a V_LED step down from V₁ to V₂, while the LED current was I₁. The theoretical peak reverse current is:

Equation 4.

For the single-LED case, it is advised to keep C_OUT at maximum 44 µF.

Two other components need to be selected in the buck converter. The value of the input-capacitor (pin ILLUM_A_VIN) should be equal to or greater than the selected output capacitance C_OUT, in this case >44 µF. The capacitor between ILLUM_A_SWITCH and ILLUM_A_BOOST is a charge pump capacitor to drive the high side FET. The recommended value is 100 nF.