LED Driver PR4401 / PR4402

Application Notes

INFLUENCE OF THE LED FORWARD VOLATAGE

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

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 %

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 22 µH.

• PR4402 is best operated with inductors

between 4.7 and 10 µH.

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 currents.

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

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 10 µH and less, or mean output

currents of 23 mA 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.

INDUCTUR 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.

CONNECTION FROM BATTERY

Due to high peak currents, it is important to

connect the PR4401/PR4402 to the battery with

short, low resistance wires, to achieve the best

performance. A voltage drop along the wire

affects LED current, efficiency and minimum

startup and operating voltage. This is most

critical for applications with low inductivity and

high current. In cases where a longer wire from

the battery cannot be avoided, a capacitor

should be placed close to the Vcc and Gnd pin of

PR4401. Typically capacitors between 220 nF

and 1 µF are used. With PR4402 operated at

higher currents, a blocking capacitor is usually

necessary even if wires or board layout are

optimized.

© PREMA Semiconductor GmbH 2006-2014 I rev. 1014

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