Texas Instruments LM3409 Demonstration Board LM3409EVAL/NOPB LM3409EVAL/NOPB Datenbogen
Produktcode
LM3409EVAL/NOPB
MAX
L
L
LED
2
I
I
I
-
=
=
-
-
=
OFF
O
t
V x
ADJ
V
SNS
R
5 x
PP
L
i
'
-
L1
2 x
-
SNS
MAX
T
MAX
L
R
I
I
=
=
-
=
SNS
R
5 x
ADJ
V
CST
V
v
SNS
(t)
V
CST
t
0
t
ON
t
OFF
|
OFF
OFF
C
R
24
.
1
x
x
L1
PP
L
i
'
-
i
PP
L
=
'
-
L1
OFF
O
(C
OFF
+ 20 pF)
R
V
x
x
-
1
ln
-
x
¸
¸
¹
¹
·
¨
¨
©
©
§
O
V
V
24
.
1
SNVS602J – MARCH 2009 – REVISED MAY 2013
Using the actual t
OFF
equation, the inductor current ripple (
Δ
i
L-PP
) of a buck current regulator operating in CCM is:
(7)
Using the t
OFF
approximation, the equation is reduced to:
(8)
Δ
i
L-PP
is independent of both V
IN
and V
O
when in CCM!
The
Δ
i
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
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
, 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
Δ
i
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
. I
LED
remains
accurate through this averaging.
12
Copyright © 2009–2013, Texas Instruments Incorporated
Product Folder Links: