Datasheet

Application information LED2000
24/40 DocID023432 Rev 4
Equation 27
Example 2(see Example 1):
V
IN
= 12 V, I
LED
= 700 mA, 
ILED
/I
LED
= 2%, V
FW_LED
= 3.5 V, n
LED
= 2.
A lower inductor value maximizes the inductor current slew rate for better dimming
performance. Equation 26 becomes:
Equation 28
which is satisfied selecting a10 H inductor value.
The output capacitor value must be dimensioned according to Equation 27.
Finally, given the selected inductor value, a 2.2 F ceramic capacitor value keeps the LED
current ripple ratio lower than the 2% of the nominal current. An output ceramic capacitor
type (negligible ESR) is suggested to minimize the ripple contribution given a fixed capacitor
value.
7.1.3 Input capacitor
The input capacitor must be able to support the maximum input operating voltage and the
maximum RMS input current.
Since step-down converters draw current from the input in pulses, the input current is
squared and the height of each pulse is equal to the output current. The input capacitor
must absorb all this switching current, whose RMS value can be up to the load current
divided by two (worst case, with duty cycle of 50%). For this reason, the quality of these
capacitors must be very high to minimize the power dissipation generated by the internal
ESR, thereby improving system reliability and efficiency. The critical parameter is usually the
RMS current rating, which must be higher than the RMS current flowing through the
capacitor. The maximum RMS input current (flowing through the input capacitor) is:
Equation 29
Table 6. Inductor selection
Manufacturer Series Inductor value (µH) Saturation current (A)
Würth Elektronik
WE-HCI 7040 1 to 4.7 20 to 7
WE-HCI 7050 4.9 to 10 20 to 4.0
Coilcraft XPL 7030 2.2 to 10 29 to 7.2
I
RIPPLE
s=j  I
RIPPLE_SPEC
=
I
L
I
LED
----------- 0,5=
I
RMS
I
O
D
2D
2


-------------- -–
D
2

2
------ -+=