R1218XXX1A RICOH [RICOH electronics devices division], R1218XXX1A Datasheet - Page 7

• Manufacturer Part Number: R1218XXX1A
• Description: Step-up DC/DC converter for Whilte LED Backlight
• Manufacturer: RICOH [RICOH electronics devices division]

=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.
by the duty cycle, our recommendation external components values are, R1=10 , R2=5.1k , R3=51k , R4=5.1k
around.
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 5. Dimming controle by filtered PWM signal
The peak current of the inductor at normal mode can be calculated as next formula:
ILxpeak=1.25 I
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 V
Set 1 F 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 0.22 F or more capacitor between V
I
LEDMIN
I
For example, supposed that the PWM signal level is set as 2.5V/0V, to adjust the LED current range from 0mA to 20mA
C3 should be set large enough to regard the PWM signal as adjustable DC voltage by the filter. In this method, higher
Selection of Inductors
Selection of Capacitors
LEDMAX
=(0.2-R2*(VH-0.2)/(R3+R4))/R1
V
FB
=(0.2-R2*(VL-0.2)/(R3+R4))/R1
C3
LED
CE
R3
R4
R2
V
OUT
V
/V
FB
R1
IN
+0.5 V
IN
IN
R1
(V
=3.6V, recommendation value of the inductor is 10uH or more.
OUT
OUT
and the GND like C2 as in the figure above.
-V
VH
VL
IN
)/(L V
OUT
Figure 4. Dimming control by DC voltage
f
OSC
)
V
FB
R1
R2
DC
R1218X
or
7