Datasheet

DocID023432 Rev 4 23/40
LED2000 Application information
40
Figure 15. Equivalent circuit
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 23
where the term 8/

2
represents the main harmonic of the inductor current ripple (which has
a triangular shape) and
I
L
is the inductor current ripple.
Equation 24
so L value can be calculated as:
Equation 25
where T
OFF
is the OFF-time of the embedded high switch, given by 1-D.
As a consequence, the lower the inductor value (so the higher the current ripple), the higher
the C
OUT
value would be to meet the specification.
A general rule to dimension L value is:
Equation 26
Finally, the required output capacitor value can be calculated equalizing the LED current
ripple specification with the module of the Fourier transformer (see Equation 23) calculated
at F
SW
frequency.
'&5 '&5
&287
5V
9,1
/
(65
5G
9,1
//
(65
'OHG
 
''
&287


''
5V
5GQ
'OHGQ
$0Y
I
RIPPLE
s
8

2
----- - I
L
1 s ESR C
OUT
+
1sR
S
ESR n
LED
R
LED
++C
OUT
+
-----------------------------------------------------------------------------------------------------------=
I
L
V
OUT
L
-------------- T
OFF

n
LED
V
FW_LED
100mV+
L
------------------------------------------------------------------ T
OFF
==
L
n
LED
V
FW_LED
100mV+
I
L
------------------------------------------------------------------ T
OFF

n
LED
V
FW_LED
100mV+
I
L
------------------------------------------------------------------ 1
n
LED
V
FW_LED
100mV+
V
IN
------------------------------------------------------------------–


==
I
L
I
LED
----------- 0,5