FAN5606 Datasheet, PDF (6/9 Pages) Fairchild Semiconductor – Serial LED Driver with Current-Regulated, Step-Up DC/DC Converter

FAN5606
PRODUCT SPECIFICATION
PWM Control (Continued)
3. A and B are PWM. I_LED (Average) = δ x 20mA, where δ is Duty Cycle.
A Input (PWM)
ss
30%
Duty Cycle
1KHz
ss
ss
70%
Duty Cycle
1KHz
B Input (PWM) _ss
30%
Duty Cycle
1KHz
ss
ss
70%
Duty Cycle
1KHz
I_LED (Average) = 0.7 x 20mA = 14mA
I_LED (Average) = 0.3 x 20mA = 6mA
LED CURRENT _0mA
ss
OFF
Notes:
4. Proportionally select the duty cycle to achieve a typical LED current between 1mA to 4mA.
5. Maximum PWM frequency can be up to 30KHz.
6. Proportionally select the duty cycle to achieve a typical LED current between 1mA and 19mA.
Applications Information
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 PWM and pulse skipping techniques. In most
cases, the FAN5606 operates the inductor in discontinuous
conduction mode.
To ensure proper operation of the current regulator over the
entire range of conditions, the inductor should be selected
based on the maximum required power (P_OUT) and the mini-
mum input voltage (V_IN).
L < ⁽---V----^I--^N----⁾---²---^×-----F--
P_OUT
where units of L, V_IN, and P_OUT are in µH, Volt, and Watt,
respectively and F = 0.4 is a factor depending upon the
FAN5606 architecture.
The above relation is applicable up to P_OUT = 0.6W and
L = 4.3µΗ , or greater. The peak current in the inductor is:
I= -T---^O----^N----^_--^M-----^a--^x----^×-----V----^I--^N----^_--^M-----^a--^x-
L
where T_{ON_Max} = 1.25µS. The recommended inductor for
driving upto 6 LEDs, type QTLP670CiW is L = 6.8µH rated
at minimum 0.5A.
Low ESR capacitors should be used to minimize the input
and output ripple voltage. Use of C_IN = 4.7µF/6.3V and
C_OUT = 4.7µF/25V type X5R/X7R multi layer ceramic
capacitor are recommended.
A larger value input capacitor placed as close as possible to
FAN5606 may be needed to reduce the input voltage ripple
in noise sensitive applications. An additional LC filter
between the battery and the FAN5606 input can help to fur-
ther reduces the battery ripple to the level required by a par-
ticular application.
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REV. 1.0.3 1/7/05