9.1.1 Maximum Output Current

The overcurrent limit in a boost converter limits the maximum input current and thus maximum input power for a given input voltage. Maximum output power is less than maximum input power due to power conversion losses. Therefore, the current limit setting, input voltage, output voltage and efficiency can all change maximum current output. The current limit clamps the peak inductor current; therefore, the ripple has to be subtracted to derive maximum dc current. The ripple current is a function of switching frequency, inductor value and duty cycle. The following equations take into account of all the above factors for maximum output current calculation.

Equation 1.

where

• I_p = inductor peak to peak ripple
• L = inductor value
• V_f = Schottky diode forward voltage
• F_s = switching frequency
• V_out = output voltage of the boost converter. It is equal to the sum of VFB and the voltage drop across LEDs.

Equation 2.

where

• I_{out_max} = Maximum output current of the boost converter
• I_lim = overcurrent limit
• η = efficiency

For instance, when V_IN is 3 V, 8 LEDs output equivalent to V_OUT of 26 V, the inductor is 22 μH, the Schottky forward voltage is 0.2 V, the maximum output current is then 110 mA in typical condition. When V_IN is 5 V, 10 LEDs output equivalent to V_OUT of 32 V, the inductor is 22 μH, the Schottky forward voltage is 0.2 V, the maximum output current is 150 mA in typical condition.