Datasheet

I
L-RMS
= I
LED
x
1 +
1
12
x
I
LED
'
I
L-PP
¸
¹
·
¨
©
§
2
L1=
IN
DV x
i
ü
SWPP-L
fx
30
1L
=
)
O
DV
x
-
(
IN
V
SWPP-L
f
ü
i
x
1.24V
R
HSP
=
RRI
SNSCSHLED
xx
SNS
R =
SNS
V
LED
I
T
25
R =
TSW
Cf x
( )
T
25
R
=
2
OOIN
VVV
-
xx
2
INTSW
VCf xx
R
T
=
25 x V
IN
- V
O
f
SW
x C
T
X V
IN
( )
LM3421, LM3421-Q1
LM3423, LM3423-Q1
SNVS574E –JULY 2008–REVISED MAY 2013 www.ti.com
Buck (Constant Ripple vs. V
IN
)
(39)
(40)
Boost and Buck-boost
(41)
3. AVERAGE LED CURRENT
For all topologies, set the average LED current (I
LED
) knowing the desired current sense voltage (V
SNS
) and
solving for R
SNS
:
(42)
If the calculated R
SNS
is too far from a desired standard value, then V
SNS
will have to be adjusted to obtain a
standard value.
Setup the suggested signal current of 100 µA by assuming R
CSH
= 12.4 kΩ and solving for R
HSP
:
(43)
If the calculated R
HSP
is too far from a desired standard value, then R
CSH
can be adjusted to obtain a standard
value.
4. INDUCTOR RIPPLE CURRENT
Set the nominal inductor ripple current (Δi
L-PP
) by solving for the appropriate inductor (L1):
Buck
(44)
Boost and Buck-boost
(45)
To set the worst case inductor ripple current, use V
IN-MAX
and D
MIN
when solving for L1.
The minimum allowable inductor RMS current rating (I
L-RMS
) can be calculated as:
Buck
(46)
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