AMG-LL9910B Datasheet, PDF (9/15 Pages) alpha microelectronics gmbh – Universal High Voltage Single Channel LED Driver

AMG-LL9910B
Universal High Voltage Single Channel LED Driver
b) LED Current
The maximum LED current is programmed with an external resistor RCS.
If peak current for highest light intensity is desired, the resistor value should be calculated as
follows:
RCS = 250mV/ILED peak
If an average current for highest efficacy and longest lifetime is desired, the resistor value
should be calculated as follows:
RCS = 250mV/[ILED avg + (0.5*ΔIL)]
ΔIL = Iripple
c) Inductor Value
The inductor value depends on the input voltage, output voltage, PWM frequency and the
desired current ripple.
First , we calculate the ratio between input and output voltage and the voltage over the
inductor. The input voltage is the average supply voltage for the IC while the output voltage is
the number of LEDs on the string times the LED forward voltage.
Vout = n * VF
VL = Vin - Vout
Please note: that the forward LED voltage is a function of the LED current. The LED data
sheet should provide the corresponding forward voltage for a given LED current.
D = Vout/Vin
For a buck design, D must be lower than 1.
Then, the ripple of the LED currents needs to be determined, which will then provide the
actual inductor value.
For the maximum peak current (highest intensity), a current ripple higher than 50% (0.5) is
desirable.
ΔIL = ripple * ILED peak
Inductor values should be rounded down in this case.
For a stable average current (highest efficacy), a current ripple lower than 20% (0.2) is
desirable.
ΔIL = ripple * ILED avg
Inductor values should be rounded up for this case.
L= (D/fosc) * (VL/ΔIL)
AMG-LL9910B Revision: B
09/13/12
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