Datasheet
I
L-RMS
=
1 +
1
12
x
I
LED
'
I
L-PP
x D'
¸
¹
·
¨
©
§
2
D'
I
LED
x
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
1L
=
)
O
DV
x
-
(
IN
V
SWPP-L
füi
x
GAIN
R
=
2
1
1REF
V45.2
R
x
-
¸
¸
¹
·
¨
¨
©
§
ENDNTC
R
-
BIASENDNTC
RR
+
-REFREF
RR
+
CSH
I
R
REF1
R
BIAS
= R
NTC-BK
x
R
REF2
1.24V
R
HSP
=
RRI
SNSCSHLED
xx
SNS
R =
SNS
V
LED
I
LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
www.ti.com
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. THERMAL FOLDBACK
For all topologies, set the thermal foldback breakpoint (T
BK
) by finding corresponding R
NTC-BK
from manufacturer's
datasheet and solving for R
BIAS
:
(44)
The easiest approach is to set R
REF1
= R
REF2
, therefore setting R
BIAS
= R
NTC-BK
will properly set T
BK
. Remember,
capacitance is recommended at the TSENSE and TREF pins, so ensure C
REF
> C
NTC
to prevent start-up in
foldback.
Then set the thermal foldback endpoint (T
END
) by finding the corresponding R
NTC-END
from manufacturer's
datasheet and solving for R
GAIN
:
(45)
5. INDUCTOR RIPPLE CURRENT
Set the nominal inductor ripple current (Δi
L-PP
) by solving for the appropriate inductor (L1):
Buck
(46)
Boost and Buck-boost
(47)
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
(48)
Boost and Buck-boost
(49)
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