Datasheet

Application information LED7707
34/47
T
OFF
can be calculated as:
Equation 35
defining D
2
as:
Equation 36
The worst case for the output voltage ripple is when input voltage is lower (V
IN,min
= 10.8 V).
A simple way to select the C
OUT
value is fixing a maximum voltage ripple.
In order to affect as less as possible the current generators, it would be better to fix the
maximum ripple lower than the typical voltage across the generators.
For example considering
ΔV
OUT
lower than 70 mV (i.e. the 10 % of the voltage across the
leading generator), the required capacitance is:
Equation 37
A margin from the calculated value should be taken into account because of the
capacitance drop due to the applied voltage when MLCCs are used.
One 10 µF MLCC (or two 4.7 µF MLCCs) can be a good choice for this application.
In case a dimming duty cycle different from 100% is used, a further contribution to the
capacitor discharge (during the off time of the dimming cycle) should be considered.
6.4.5 Input capacitor choice
The input capacitor of a boost converter is less critical than the output capacitor, due to the
fact that the inductor is in series with the input, and hence, the input current waveform is
continuous.
A low ESR capacitor is always recommended.
A capacitor of 10 µF is tentatively a good choice for most of the applications.
⎩
⎨
⎧
=
=
=⋅=
V2.13V@ns2.618
V8.10V@ns7.569
DTT
max,IN
min,IN
2SWOFF
()
⎩
⎨
⎧
=
=
=
−⋅
⋅⋅⋅
=
V2.13V@408.0
V8.10V@376.0
1MR
MLF2
D
max,IN
min,IN
0
SW
2
(
)
F33.6
V2
TII
C
max,OUT
OFFOUTpeak,L
OUT
μ=
Δ⋅
⋅−
>