LM3409MYX National Semiconductor Corporation, LM3409MYX Datasheet - Page 11

• Manufacturer Part Number: LM3409MYX
• Description: Pfet Buck Controller For High Power Led Drives
• Manufacturer: National Semiconductor Corporation

AVERAGE LED CURRENT
For a buck converter, the average LED current is simply the
average inductor current.
Using the COFT architecture, the peak transistor current (I
MAX
peak inductor current (I
Because I
set using the controlled off-timer, I
I
The threshold voltage V
parator is affected by the comparator’s input offset voltage,
which causes an error in the calculation of I
mately I
parator inputs is swapped every cycle, which causes the
actual I
and I
in Figure 6. I
LED
FIGURE 6. Inductor Current i
) is sensed as shown in Figure 5, which is equal to the
can be calculated as follows:
L-MAXL
L-MAX
LED
L-MAX
. To mitigate this problem, the polarity of the com-
), equidistant from the theoretical I
LED
FIGURE 5. Sense Voltage v
to alternate between two peak values (I
is set using peak current control and Δi
remains accurate through this averaging.
L-MAX
CST
seen by the high-side sense com-
) given by the following equation:
L
(t) Showing I
L
and correspondingly
SNS
(t)
L-MAX
L-MAX
L-MAX
as shown
and ulti-
Offset
30085612
L-MAXH
L-PP
30085611
T-
is
11
INDUCTOR CURRENT RIPPLE
Because the LM3409/09HV swaps the polarity of the differ-
ential current sense comparator every cycle, a minimum in-
ductor current ripple (Δi
accurate I
terminated at the higher of the two polarity-swapped thresh-
olds (corresponding to I
decreases until the second t
as the second t
current threshold (corresponding to I
t
tion. The minimum inductor current ripple (Δi
adhere to the following equation in order to ensure accurate
I
SWITCHING FREQUENCY
The switching frequency is dependent upon the actual oper-
ating point (V
a given application, therefore the switching frequency will vary
with V
switching frequency (f
selected based on the tradeoffs between efficiency (better at
low frequency) and solution size/cost (smaller at high fre-
quency). The off-time of the LM3409/09HV can be pro-
grammed for switching frequencies up to 5 MHz (theoretical
limit imposed by minimum t
quencies higher than 1MHz may be difficult to obtain due to
gate drive limitations, high input voltage, and thermal consid-
erations.
At CCM operating points, f
At DCM operating points, f
In the CCM equation, it is apparent that the efficiency (η) fac-
tors into the switching frequency calculation. Efficiency is hard
to estimate and, since switching frequency varies with input
voltage, accuracy in setting the nominal switching frequency
is not critical. Therefore, a general rule of thumb for the
LM3409/09HV is to assume an efficiency between 85% and
100%. When approximating efficiency to target a nominal
switching frequency, the following condition must be met:
ON
LED
pulse will follow. This will result in degraded I
regulation:
IN
(frequency increases as V
LED
IN
regulation. Referring to Figure 6, the first t
ON
and V
begins, i
O
SW
). V
L-MAXH
) at the nominal operating point is
O
L
SW
SW
L-PP
ON
will remain relatively constant for
will still be above the lower peak
ON
is defined as:
is defined as:
) is necessary to maintain
). During the following t
begins. If t
). In practice, switching fre-
IN
L-MAXL
increases). The target
OFF
) and a minimum
is too short, then
L-PP-MIN
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LED
) should
regula-
OFF
ON
, i
is
L