MLX10803_06 Datasheet, PDF (16/25 Pages) List of Unclassifed Manufacturers – High power LED driver

IC specification MLX10803
High power LED driver
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
The MLX10803 avoids these disadvantages as shown by the following calculation with L=220µH, R_SENSE = 0.1 Ω:
Supposed:
V_bat = 13.8V
V_fLED ≈ 3.4V example 1; 8V example2;
I_fLED ≈ 4A
V_f1 ≈ 0.9V (reverse polarity diode)
V_f2 ≈ 0.9V (free wheel diode)
V_RSENSE ≈ 0.4V (@I_fLED, R_SENSE=0.1 Ω)
V_{RDS ON} ≈ 0.04V (@I_fLED)
V_Coil ≈ 0.2V (@I_fLED)
Efficiency using a simple resistor or load dump regulation:
Efficiency n:
n = ^{V fLED} ≈ 29% example1; ≈ 58% example2;
V_BAT
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:
n₁
=
V _fLED
V _fLED
+ V _{f 2} + V_Coil
≈ 75%
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:
n₂
=
V _fLED
V _fLED
+ V _{f 1} + V_Coil + V_RDSon + V_RSENSE
≈ 69% example1;
≈ 84%
example 2;
3) ON and OFF times are in ratio of roughly 30:70 for example 1 and 65:35 for example 2:
Efficiency n: n = n₁ ⋅ 0.7 + n₂ ⋅ 0.3 ≈ 73% example1; ≈ 87% example2;
3901010803
Rev 024
Page 16/25
Data Sheet
7/DEC/06