LED DRIVER PR4401/PR4402

Operation of LED with smoothed current (rectifier)

With a diode (preferably a Schottky diode) and a smoothing capacitor the voltage at the

LED can be buffered if necessary. The capacitance must be small enough so that the

voltage at the capacitor will exceed a voltage of 2.5V in the first cycle, otherwise the circuit

may not start up. In most cases, values between 100nF and 1µF are appropriate.

Due to different load characteristics, output current and efficiency are typically higher than

without capacitor, especially in the high-current range.

Vcc

Gnd

Vout

PR4401/02

Vbat =1.2V or 1.5V

L1

White

LED

S

Recommended configuration for higher LEDs currents

While for lower LED currents it is possible to operate PR4401 with the minimum number

of components, it is recommended to provide both rectifier circuit a the output and buffer

capacitor at the input at high LED currents, to achieve the best performance.

Vcc

Gnd

Vout

PR4401/02

Vbat =1.2V or 1.5V

L1

White

LED

S

Whether the extra components are necessary or not depends largely on the performance

of the components used, most importantly the peak current of the LED, the internal

resistance of the battery and the resistance of the battery cables. If by adding the extra

components the mean LED current increases significantly, it is usually advisable to add

them permanently to achieve a high efficiency.

As a rule of thumb, with a 22µH inductor the extra components will usually not increase

the current significantly, while for inductors below 10µH they will usually improve the

performance noticeably.

Buffer capacitor and rectifier circuit are independent measures. For powerful LEDs, or two

or more LEDs in parallel at the output, the rectifier may not be necessary, but the buffer

capacitor will still prevent high voltage drops along the supply wire.

For the buffer capacitor, values between 220nF and 1µF are common.

© PREMA Semiconductor GmbH 2006-2008

Page 8/12

Rev 0908