MLX10803_06 Datasheet, PDF (17/25 Pages) List of Unclassifed Manufacturers – High power LED driver

IC specification MLX10803
High power LED driver
7.2. The principle in detail
After powering on the MLX10803 the switch becomes open and the current through the LED starts to rise. The rate
of current raise is limited by the value of the coil. When the current through the LED reaches half of a maximum
value, the ON timer is started, and if during 58.5 clocks of the internal oscillator the maximum current value through
the LED is not reached, the driver switches off. This maximum current is adjusted by the resistors on the IREF2,
IREF1 or voltage applied to VREF pins (voltage on these pins is divided by 5). The minimum of these voltages is
taken as a reference. The driver is switched off for a monoflop time, which is equal to 9…16 pulses of oscillator.
The frequency of the oscillator can be set by the customer using the Rosc value using such formula:
Fosc[MHz] = 222.2 /(Rosc[kΩ] + 4.44) .
Both parameters, the peak current threshold voltage and the monoflop time, create an ON/OFF period to form an
average current through the LED. By adjusting these parameters, an adjustment of the average load current is
possible in a wide range.
I
Imax2
Iavg2
Imax1
Iavg1
t
I
Imax
Iavg1
Iavg2
T1
T2
t
Note: The current sense comparator has a typical debouncing time of 300ns as shown in the block diagram. This
delay time prevents the driver from being switched off due to short term switching oscillations. When working with
very short monoflop times, this time has to be taken into account for calculations.
I
Imax
Iavg
tmon_off
t
By applying a PWM signal on VS/PWM, the LED can be dimmed from 0% to 100%.
VS/PWM = L
VS/PWM = PWM
VS/PWM = H
LED permanent OFF
LED dimmed with PWM between 0% to 100%
LED permanent ON
3901010803
Rev 024
Page 17/25
Data Sheet
7/DEC/06