LED DRIVER PR4401/PR4402

Influence of the LED Forward Voltage

measured with 1 LED / 2 LEDs in parallel; Vcc = 1.5V

Circuit

LED Peak

Forward Voltage

LED Mean Current

Mean Supply

Current

Efficiency

10 µH, 1 LED

5.8 V

20 mA

70 mA

77 %

10 µH, 2 LEDs ||

4.6 V

24 mA

70 mA

80 %

22 µH, 1 LED

4.2 V

13 mA

33 mA

82 %

22 µH, 2 LEDs ||

3.5 V

14 mA

32 mA

82 %

Peak forward voltages above approx. 5V are often considered as an overload condition

and may lead to a lower LED efficiency.

Application Notes

Selection of PR4401 and PR4402

The circuit type should be selected according to the required LED current.

PR4401 is best operated with inductors between 10 and 22µH.

PR4402 is best operated with inductors between 4.7 and 10µH.

Using lower inductances may lead to erratic behaviour, especially at low supply voltages

and should be avoided.

Operating with higher inductances is possible and will lead to lower supply and LED

current.

However the quiescent current which is independent of the inductance will lead to a lower

overall efficiency. Since PR4402 has about twice the quiescent current of PR4401, it is not

recommended for small LED currents.

LED compatibility

It must be considered that the peak current through the LED is a factor of up to 7 higher

than the mean current. LED lifetime may be affected if operated outside the range

specified by the LED vendor.

Since the emission spectrum of white LEDs usually depends on the current, the light color

may shift to blueish white. High peak currents may also saturate the LED and reduce the

light efficiency of the LED.

If the rated LED peak current is exceeded, it is recommended to use a smoothing

capacitor and diode to provide a continuous output current (see below). With most

standard LEDs, this will improve the overall performance with inductors of 10µH and less,

or mean output currents of 23mA and more.

Note that with especially with the higher currents of PR4402 the current rating of most

standard LEDs is exceeded, and more powerful or multiple LEDs must be used.

Inductor compatibility

While the series resistance of the coil has a small impact on the LED current, it is

important that the saturation current is higher than the maximum peak current over the

supply voltage range. Inductors optimized for DC-DC converters are mostly suitable.

© PREMA Semiconductor GmbH 2006-2008

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