R1218X Datasheet, PDF (7/14 Pages) RICOH electronics devices division – Step-up DC/DC converter for Whilte LED Backlight

R1218X
=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, 0.1uF or more capacitor is our recommendation value as C3.
Figure 3. Dimming controle by CE pin input
Figure 4. Dimming control by DC voltage
CE
VFB
R1
Figure 5. Dimming controle by filtered PWM signal
R2
VFB
R1
DC
R2
VFB
R3
R1
VH
C3 _R4
VL
• Selection of Inductors
The peak current of the inductor at normal mode can be calculated as next formula:
ILxpeak=1.25×ILED×VOUT/VIN+0.5×VIN×(VOUT-VIN)/(L×VOUT×fOSC)
When the start-up or dimming control by CE pin, transient current flows, the peak current must be equal or less
than the current limit of the IC. The peak current should not beyond the rating current of the inductor. For
example, for 4 serial LED drive from VIN=3.6V, recommendation value of the inductor is 10uH or more.
• Selection of Capacitors
Set 1µF or more value bypass capacitor between VIN pin and the GND like C1 as in the figure above as close as
posible to the IC.
Set 0.22µF or more capacitor between VOUT and the GND like C2 as in the figure above.
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