Datasheet

STLDC08 Application information
Doc ID 18476 Rev 1 17/29
As a first approximation we choose the inductor ripple current, I
L
, equal to approximately 40
% of the output current. Higher ripple current allows for smaller inductors, but it also
increases the output capacitance for a given LED current ripple requirement. Conversely,
lower ripple current can be obtained increasing the value of the inductance, and this enables
a reduction of the output capacitor value. This trade-off can be altered once standard
inductance and capacitance values are chosen.
I
L
is determined by the input and output voltage, the value of the inductance, and T
OFF
.
The minimum value of inductance which guarantees the fixed inductor ripple current can be
determined using the following equation:
Equation 4
where V
d
is the forward drop of the Schottky diode, I
L
is the fixed inductor ripple current, and
T
OFF
is the constant OFF time.
The following equation shows the average inductor current as a function of the output
current and duty cycle.
Equation 5
An inductor that can carry the maximum input DC current which occurs at the minimum
input voltage should be chosen. The peak-to-peak ripple current is set by the inductance
and a good starting point is to choose a ripple current of at least 40 % of its maximum value
of the:
Figure 18. Timing diagram
I
RIPPLE
I
L
t
I
PEAK
I
OUT
T
OFF
T
ON
D1
I
I
OUT
IN
−
=
AM07848v1
I
RIPPLE
I
L
t
I
PEAK
I
OUT
T
OFF
T
ON
D1
I
I
OUT
IN
−
=
AM07848v1
OFF
L
INMINdOUT
T
)I(
)V-V+(V
L ×
Δ
>
D1
I
I
LED
)AVG(L
−
=