FAN5608_06 Datasheet, PDF (8/13 Pages) Fairchild Semiconductor – Serial / Parallel LED Driver with Current-Regulated, Step-Up DC/DC Converter

Typical Characteristics
T_A=25C, V_BAT = 3.6V, unless otherwise specified.
Figure 11. Peak power efficiency
Startup Response
V_IN=3.6V, L=6.8uH, C_OUT=4.7uF, 3-LEDs/CH
Figure 12. PWM Dimming
Tim e 200µS/Div
Figure 13. Startup Response
Design and Component Selection Guidelines
Inductor Selection
The inductor is one of the main components required by
the boost converter to store energy. The amount of
energy stored in the inductor and transferred to the load
is controlled by the regulator, using pulse-frequency
modulation (PFM) and pulse-skipping techniques. In most
cases, the FAN5608 operates in discontinuous conduction
mode, resulting in higher inductor current ripple.
To ensure proper operation of the current regulator over
the entire range of conditions, select the inductor based
on the maximum output power (P_OUT) and the minimum
input voltage (V_IN):
L ≤ ^{(VIN )2 ×T}
P_OUT
EQ 6
where units of L, V_IN, and P_OUT are in μH, Volt, and Watt,
respectively and T = 0.4μs is a factor depending upon
the FAN5608 internal architecture. The above
relationship is applicable up to P_OUT = 0.8W and
L ≥ 2.2μH . At lower inductor values, the efficiency
© 2006 Fairchild Semiconductor Corporation
FAN5608 Rev. 1.0.5 • 8/8/06
8
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