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MLX10803KDC-AAA-000-TU

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P25
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
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:
%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
≈
++
=
CoilffLED
fLED
VVV
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
≈
++++
=
RSENSERDSonCoilffLED
fLED
VVVVV
V
n
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:
%733.07.0
21
≈⋅+⋅= nnn
example1;
%87≈
example2;
IC specification
MLX10803
High power LED driver
3901010803 Page 17/25
Data Sheet
Rev026 Jun/2012
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:
)44.4][/(2.222][ +Ω= kRoscMHzFosc
.
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
t
Iavg2
Iavg1
Imax2
Imax1
I
t
Iavg1
Iavg2
Imax
T1
T2
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
t
Imax
Iavg
tmon_off
By applying a PWM signal on VS/PWM, the LED can be dimmed from 0% to 100%.
VS/PWM = L LED permanent OFF
VS/PWM = PWM LED dimmed with PWM between 0% to 100%
VS/PWM = H LED permanent ON
IC specification
MLX10803
High power LED driver
3901010803 Page 18/25
Data Sheet
Rev026 Jun/2012
Dimming is achieved by applying a PWM directly to the module supply or by changing the reference voltage on pin
VREF or the resistor’s value on IREF2 pin.
IC settling times must always be considered in PWM mode. Please refer also to chapter 1.1.3 for
additional PWM frequency considerations.
Limitation of the ON time prevents from exceeding the allowed average current when the power supply voltage is
not sufficient for the current to reach its peak value and restricts in this case duty cycle of switching to 68%.
I
t
Imax
Iavg
tmon_offtmon_on
Imax/2
A pseudo random generator is applied to the monoflop time. The pseudo random generator runs with the clock
derived out of the monoflop time and adds a random distribution on these 3 LSBs. Therefore, the monoflop time
gets a random variation from its value. The EMI behaviour of the complete module is improved due to the variation
of the otherwise fixed switching frequency.
I
t
Imax
Iavg
tmon
tmon'
tmon'
The inductance L of a coil describes the amount of magnetic energy that can be stored in it.
Consequently, high inductive coils will be discharged less than low inductive coils in a given time.
Generally the coil can be driven in two different ways:
1) The coil is discharged partially only. That means the coil still carries a significant amount of energy
when going from discharging to charging. In that moment the charging current rises immediately to
the coil current that was flowing just before switching. This is connected with large dI/dt transients on the
RSENSE pin that have a negative impact on EMI. This is mostly preferred way of regulation because of low
influence of supply voltage and coil value on output current. Fast flyback diode is recommended and extra
important in this case.
2) The coil discharged completely. Thus, at the end of a discharging cycle, the coil doesn’t carry energy
anymore. With the next charging cycle, current increases steadily from around zero. This way, large dI/dt
transients are completely avoided.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P25

MLX10803KDC-AAA-000-TU

Mfr. #:
Buy MLX10803KDC-AAA-000-TU
Manufacturer:
Melexis
Description:
LED Lighting Drivers High Power LED driver
Lifecycle:
New from this manufacturer.
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