Datasheet
MAXLLLED
2
III -
==
-
-
=
OFFO
tV x
ADJ
V
SNS
R5x
PPL
i'
-
L12 x
-
SNS
MAXTMAXL
R
II
==
-
=
SNS
R5x
ADJ
V
CST
V
v
SNS
(t)
V
CST
t
0
t
ON
t
OFF
|
OFFOFF
CR24.1 xx
L1
PPL
i'
-
i
PPL
=
'
-
L1
OFFO
(C
OFF
+ 20 pF)RV xx-
1ln -x
¸
¸
¹
·
¨
¨
©
§
O
V
V24.1
LM3409, LM3409HV, LM3409-Q1
SNVS602J –MARCH 2009–REVISED MAY 2013
www.ti.com
Using the actual t
OFF
equation, the inductor current ripple (Δi
L-PP
) of a buck current regulator operating in CCM is:
(7)
Using the t
OFF
approximation, the equation is reduced to:
(8)
Δi
L-PP
is independent of both V
IN
and V
O
when in CCM!
The Δi
L-PP
approximation only depends on R
OFF
, C
OFF
, and L1, therefore the ripple is essentially constant over
the operating range as long as V
O
>> 1.24V (when the t
OFF
approximation is valid). An exception to the t
OFF
approximation occurs if the IADJ pin is used to analog dim. As the LED/inductor current decreases, the converter
will eventually enter DCM and the ripple will decrease with the peak current threshold. The approximation shows
how the LM3409/09HV achieves constant ripple over a wide operating range, however t
OFF
should be calculated
using the actual equation first presented.
AVERAGE LED CURRENT
For a buck converter, the average LED current is simply the average inductor current.
Figure 25. Sense Voltage v
SNS
(t)
Using the COFT architecture, the peak transistor current (I
T-MAX
) is sensed as shown in Figure 25, which is equal
to the peak inductor current (I
L-MAX
) given by the following equation:
(9)
Because I
L-MAX
is set using peak current control and Δi
L-PP
is set using the controlled off-timer, I
L
and
correspondingly I
LED
can be calculated as follows:
(10)
The threshold voltage V
CST
seen by the high-side sense comparator is affected by the comparator’s input offset
voltage, which causes an error in the calculation of I
L-MAX
and ultimately I
LED
. To mitigate this problem, the
polarity of the comparator inputs is swapped every cycle, which causes the actual I
L-MAX
to alternate between two
peak values (I
L-MAXH
and I
L-MAXL
), equidistant from the theoretical I
L-MAX
as shown in Figure 26. I
LED
remains
accurate through this averaging.
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