The PR4401 chip from Prema can be used to drive an LED directly, but not a high-power LED like one of the popular 1-watt types currently available on the market. The circuit shows that the drive signal at the Vout terminal of the PR4401 chip (pin 2) turns a medium-power PNP switching transistor (T1) on and off. When T1 is switched into conduction, inductor L1 is charged. When T1 is switched off, the inductor discharges its stored energy through the LED during flyback with enough current to allow a one-watt LED to light up at nominal brightness.
During the ‘on’ time of transistor T1, the current through inductor L2 ramps up linearly to a peak value as expressed by.
IL2(pk) = [(Vbatt – VCEsat(T1)) ×Ton ] / L2
where VCEsat(T1) is the collector-to-emitter saturation voltage of T1 (here, a type BD140 is suggested). During T1’s ‘off’ time, the inductor voltage reverses, forward-biasing the LED and discharging through it at a constant voltage roughly equal to the forward voltage of the LED, while its current ramps down to zero. Because this cycle repeats at a high rate, the LED appears to be always on, its brightness depending on the device’s average current, which is proportional to the peak value. The LED current is roughly a triangular pulse with a peak current approximately equal to the inductor’s current because of the finite turn-off time of T1. The estimated average current may be calculated from
ILED(avg) = 1/2 × IL2peak × [Tdis / (Ton + Toff)]
Where Tdis is the discharge time of inductor L2 through the LED. The LED’s brightness can be increased or decreased by varying the inductance of L2. In practice, any value between 10 and 56 μH will work just fine. The inductor current increases on each cycle until T1 goes out of saturation, hence a small resistance (R1) is required at the base of T1. Without the ‘stopper resistor’, the final current goes out of control due to the DC gain of T1. A transistor with a high DC current gain and low collector-to-emitter saturation voltage is the best choice if you want to tweak the circuit for efficiency. Regarding L2, make sure the peak current through it is below the saturation level.