Datasheet
OFFONSWLEDRMSIN
ttfII
xxx
=
-
MININ
C
-
= =
OFFLED
t
1
I
¨
¨
©
§
-x
¸
¸
¹
·
SW
f
PPIN
v
-
'
ONLED
tI x
PPIN
v
-
'
C
Z
MINO
1
C
=
-
SW
f2 xxSx
C
Z
=
D
r x
PPLED
i
-
'
PPLEDPPL
ii
--
'-'
SNS
R
=
MAXL
I5
-
x
ADJ
V
2
I
I
LED
MAXL-
+
=
i
PPL-
'
L1
=
OFFO
tV
x
PPL
i
-
'
=
OFF
R
-- 1
¸
¸
¹
·
¨
¨
©
§
xK
IN
V
O
V
¸
¸
¹
·
-1
¨
¨
©
§
xx
SW
lnf(C
OFF
+ 20 pF)
V24.1
O
V
LM3409, LM3409HV, LM3409-Q1
www.ti.com
SNVS602J –MARCH 2009–REVISED MAY 2013
(28)
2. INDUCTOR RIPPLE CURRENT
Set the inductor ripple current (Δi
L-PP
) by solving for the appropriate inductor (L1):
(29)
3. AVERAGE LED CURRENT
Set the average LED current (I
LED
) by first solving for the peak inductor current (I
L-MAX
):
(30)
Peak inductor current is detected across the sense resistor (R
SNS
). In most cases, assume the maximum value
(V
ADJ
= 1.24V) at the IADJ pin and solve for R
SNS
:
(31)
If the calculated R
SNS
is far from a standard value, the beginning of the process can be iterated to choose a new
R
OFF
, L1, and R
SNS
value that is a closer fit. The easiest way to approach the iterative process is to change the
nominal f
SW
target knowing that the switching frequency varies with operating conditions anyways.
Another method for finding a standard R
SNS
value is to change the V
ADJ
value. However, this would require an
external voltage source or a resistor from the IADJ pin to GND as explained in the Theory of Operation section of
this datasheet.
4. OUTPUT CAPACITANCE
A minimum output capacitance (C
O-MIN
) may be necessary to reduce Δi
LED-PP
below Δi
L-PP
. With the specified
Δi
LED-PP
and the known dynamic resistance (r
D
) of the LED string, solve for the required impedance (Z
C
) for C
O-
MIN
:
(32)
Solve for C
O-MIN
:
(33)
5. INPUT CAPACITANCE
Set the input voltage ripple (Δv
IN-PP
) by solving for the required minimum capacitance (C
IN-MIN
):
(34)
The necessary RMS input current rating (I
IN-RMS
) is:
(35)
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