Datasheet

Application notes - buck conversion LED5000
24/51 Doc ID 023951 Rev 1
The LED ripple current can be calculated as the inductor ripple current ratio flowing into the
output impedance using the Laplace transform (see
Figure 11 ):
Equation 27
where the term 8/
π
2
represents the main harmonic of the inductor current ripple (which has
a triangular shape) and
ΔI
L
is the inductor current ripple.
Equation 28
so L value can be calculated as:
Equation 29
where T
OFF
is the OFF time of the embedded high switch, given by 1-D.
As a consequence the lower is the inductor value (so higher the current ripple), the higher
would be the C
OUT
value to meet the specification.
A general rule to dimension L value is:
Equation 30
Finally the required output capacitor value can be calculated equalizing the LED current
ripple specification with the module of the Fourier transformer (see
Equation 27 ) calculated
at f
SW
frequency.
Equation 31
Example (see
Chapter 5.6 ):
V
IN
=48 V, I
LED
=700 mA, Δ
ILED
/I
LED
=2%, V
FW_LED
=3.7 V, n
LED
=10
A lower inductor value maximizes the inductor current slew rate for better dimming
performance.
Equation 30 becomes:
I
RIPPLE
s()Δ
8
π
2
-----
ΔI
L
1s ESR C
OUT
⋅⋅+()⋅⋅
1s R
S
ESR n
LED
R
LED
⋅++()C
OUT
⋅⋅+
-------------------------------------------------------------------------------------------------------------------=
ΔI
L
V
OUT
L
--------------
T
OFF
⋅
n
LED
V
FW_LED
200mV+⋅
L
---------------------------------------------------------------------
T
OFF
⋅==
ΔI
L
I
LED
-----------
0.5≤
I
RIPPLE
s=j ω⋅()Δ I
RIPPLE_SPEC
Δ=