Datasheet

LED5000 Application notes - buck conversion
Doc ID 023951 Rev 1 25/51
Equation 32
which is satisfied selecting a10 μH inductor value.
The output capacitor value has to be dimensioned according to
Equation 31
Finally, given the selected inductor value, a 1 μF 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.
5.9.3 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 has
to 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
has to 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 33
Where η is the expected system efficiency, D is the duty cycle and I
O
is the output DC
current. Considering η = 1 this function reaches its maximum value at D = 0.5 and the
equivalent RMS current is equal to I
O
divided by 2. The maximum and minimum duty cycles
are:
Equation 34
and
Table 7. Inductor selection
Manufacturer Series Inductor value (µH) Saturation current (A)
Wurth 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
ΔI
L
I
LED
----------- 0.5=
I
RMS
I
O
D
2D
2
⋅
η
-----------------–
D
2
η
2
-------+⋅=
D
MAX
V
OUT
V
F
+
V
INMIN
V
SW
–
------------------------------------ -=