R1218N021A Datasheet, PDF (7/21 Pages) RICOH electronics devices division – Step-up DC/DC converter for White LED Backlight

R1218x
(2) Dimming control by DC voltage
LED dimming control is also possible by using the DC voltage to VFB pin. LED current is adjustable by DC
voltage and resisters, R1 and R2 in the following figure.
ILED=(DC − 0.2) / R2 − 0.2 / R1
R2
VFB
DC
R1
Dimming control by DC voltage
(3) Dimming control by feedback voltage and filtered PWM signal
LED dimming control is also possible by using the feedback voltage and filtered PWM signal.
LED current is adjustable according to the "H" level (VH) and "L" level(VL) of PWM signal and resisters,
R2
VFB
R1
R3
VH
C3
R4
VL
Dimming controle by filtered PWM signal
R1, R2, R3, and R4 in the figure5. Duty=0% to 100% PWM signal duty cycle can be used up to the
maximum LED current and minimum LED current as in the next formulas.
ILEDMIN=(0.2 − R2 × (VH − 0.2) / (R3 + R4)) / R1
ILEDMAX=(0.2 − R2 × (VL − 0.2) / (R3 + R4)) / R1
For example, supposed that the PWM signal level is set as 2.5V/0V, to adjust the LED current range from 0mA
to 20mA by the duty cycle, our recommendation external components values are, R1=10Ω, R2=5.1kΩ,
R3=51kΩ, R4=5.1kΩ or around.
C3 should be set large enough to regard the PWM signal as adjustable DC voltage by the filter. In this method,
higher frequency control than the frequency against the CE pin can be used for dimming control.
For example, if the frequency is 40kHz, 0.1µF or more capacitor is our recommendation value as C3.
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