IC specification

MLX10803

High power LED driver

3901010803

Page 16/25

Data Sheet

Rev026

Jun/2012

7.

LED driving principle

7.1.

General

The LED is driven by a switched mode power supply using an inductor as the energy storage element. This method

has several advantages. The supply voltage has to be set down to the forward bias voltage of the LED. In ordinary

applications this is achieved by a resistor with the following drawbacks:

-

A resistor dissipates power which is transformed to heat

-

Efficiency is reduced drastically

-

The light output of the LED is dependent on the supply and the temperature of the resistor

Supposed:

Efficiency using a simple resistor or load dump regulation:

Efficiency n:

%

29

≈

=

BAT

fLED

V

V

n

example1

;

%

58

≈

example2

;

Efficiency using the MLX10803:

The following calculation is an approximation only, due to the fact that coil current is not constant. It is therefore

calculated with average currents.

1) During OFF time, the coil acts as the storage element and delivers its energy to the flyback diode

and the LED:

%

75

2

1

≈

+

+

=

Coil

f

fLED

fLED

V

V

V

V

n

example1

;

%

88

≈

example 2;

2) During ON time, current flows through the reverse polarity diode, LED, coil , FET driver and RSENSE,

which causes the following voltage drops:

%

69

1

2

≈

+

+

+

+

=

RSENSE

RDSon

Coil

f

fLED

fLED

V

V

V

V

V

V

n

example1

;

%

84

≈

3) ON and OFF times are in ratio of roughly 30:70 for example 1 and 65:35 for example 2:

Efficiency n:

%

73

3

.

0

7

.

0

2

1

≈

⋅

+

⋅

=

n

n

n

example1

;

%

87

≈

example2

;