The design calculations are given in the ZXLD1350 data sheet page 16. The design method is to choose an inductor value to determine the TON time. Note this value must be greater than 200 nano-seconds to avoid excessive switching losses. The explanation for each of the parameters is shown in the left hand column. Once you have the TON time calculate the TOFF time using the second equation. The frequency that the circuit will oscillate at is 1 divided by the sum of the TON and TOFF times. If the frequency is not where you want to operate, try different values until it is the right value. Typically the frequency should be between 50 kHZ and 1 MHZ. Do not exceed 1 MHZ or the switching losses may cause the ZXLD1350 to over heat. It also import to make sure the duty cycle is between 0.3 and 0.7. This is to make sure the average current in the LED is correct and avoids switching problems at the extremes of duty cycles due to propagation delays inside the chip. In this data sheet design example a current of 350 mA and one LED has been used. In our design example the average LED current is 300 mA and 3 LEDs so we must multiply the VLED by the number of LEDS. In practice you will find a typical value for the inductance is between 47 and 220 µH to operate in the frequency range of 50 kHz to 1 MHz.