9.2.2.1 Inductor Selection

The device requires typically a 22-µH or 10-µH inductance. When selecting the inductor, the inductor saturation current should be rated as high as the peak inductor current at maximum load, and respectively, maximum LED current. Because of the special control loop design, the inductor saturation current does not need to be rated for the maximum switch current of the converter. The maximum converter switch current usually is not reached even when the LED current is pulsed by applying a PWM signal to the enable pin. The maximum inductor peak current, as well as LED current, is calculated as:

Equation 1.

Equation 2.

Equation 3.

with:

   fs = Switching frequency (1 MHz typical)

   L = Inductor value

   η = Estimated converter efficiency (0.75)

   Isw = Minimum N-channel MOSFET current limit (325 mA)

Using the expected converter efficiency is a simple approach to calculate maximum possible LED current as well as peak inductor current. The efficiency can be estimated by taking the efficiency numbers out of the provided efficiency curves or to use a worst-case assumption for the expected efficiency, for example, 75%.

9.2.2.2 Efficiency

The overall efficiency of the application depends on the specific application conditions and mainly on the selection of the inductor. A physically smaller inductor usually shows lower efficiency due to higher switching losses of the inductor (core losses, proximity losses, skin effect losses). A trade-off between physical inductor size and overall efficiency has to be made. The efficiency can typically vary around ±5% depending on the selected inductor. Figure 2 to Figure 7 can be used as a guideline for the application efficiency. These curves show the typical efficiency with a 22-µH inductor (Murata Electronics LQH32CN220K23). Figure 11 shows a basic setup where the efficiency is taken/measured as:

Equation 5.

Figure 11. Efficiency Measurement Setup

9.2.2.3 Output Capacitor Selection

The device is designed to operate with a fairly wide selection of ceramic output capacitors. The selection of the output capacitor value is a trade-off between output voltage ripple and capacitor cost and form factor. In general, capacitor values of 220 nF up to 4.7 µF can be used. When using a 220-nF output capacitor, it is recommended to use X5R or X7R dielectric material to avoid the output capacitor value falling far below 220 nF over temperature and applied voltage. For systems with wireless or RF sections, EMI is always a concern. To minimize the voltage ripple in the LED string and board traces, the output capacitor needs to be connected directly from the OUT pin of the device to ground rather than across the LEDs. A larger output capacitor value reduces the output voltage ripple. Table 4 shows possible input and/or output capacitors.

9.2.2.4 Input Capacitor Selection

For good input voltage filtering, low ESR ceramic capacitors are recommended. A 1-µF ceramic input capacitor is sufficient for most of the applications. For better input voltage filtering and EMI reduction, this value can be increased. The input capacitor should be placed as close as possible to the input pin of the converter. Table 4 shows possible input and/or output capacitors.