1-Watt LED Driver (PR4401)

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.

 

Circuit diagram :

PR4401 1-Watt LED Driver-Circuit-Diagram

 

PR4401 1-Watt LED Driver Circuit Diagram

 

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 ┬ÁH 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.

 

Author :  T.A. Babu - Copyright : Elektor