Texas Instruments AN-2255, User Manual, Page: 12 / 26

V
RAIL(peak)
is the highest output voltage
that the power converter needs to
deliver.
I
LEDx
is the forward current of an LED
string.
Determine:
I
LEDx
, V
RAIL(peak)
and V
RAIL(nom)
Prepare a power converter which has a
maximum output power > P
OUT(nom)
and a
maximum output voltage > V
RAIL(peak)
Adjust R
1
and R
2
to reduce the nominal
output voltage of the power converter to
V
RAIL(nom)
Begin of power
supply selection
The power converter must be able to
deliver a power no less than P
RAIL(peak)
even if the V
RAIL
is pushed to the
maximum by the LM3463, V
RAIL(peak)
REMARKS PROCEDURES
Adjust the value of R
1
and R
2
so as to
meet the equation:
V
FB
x
R
2
V
RAIL(nom)
=
Calculate the required maximum output
Power, P
RAIL(peak)
of the power supply
End of power
supply selection
x I
LEDx
No. of output ch. P
RAIL(peak)
= x V
RAIL(peak)
R
1
+ R
2
Response of the DHC Loop
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Figure 7. Procedure of selecting a primary power supply
Going through the above procedures, the value of the R5, R2 and R4 are determined. The values of the
R5, R2 and R4 on the LM3463 evaluation board are 2.94 k
Ω , 150 k Ω and 8.25 k Ω respectively. The
resistors may need replacing as needed to interface the board to a primary power supply.
8 Response of the DHC Loop
The cut-off frequency of the DHC loop f
C(LM3463)
is determined by the value of the external capacitor, C4.
The f
C(LM3463)
is governed by the following equation.
(1)
The default value of the C4 on the board is 1 uF which sets the cut-off frequency of the DHC loop to
0.1Hz.
In order to secure stable operation of the system, the cut-off frequency of the DHC loop of the LM3463
must be set lower than that of the primary power supply. Usually a DHC response of 1/10 of which of the
primary power supply is enough to secure stable operation. In the case where the primary power supply
has an unknown frequency response, the selection of the value of the C4 can be based on estimation.
Use a 1 uF ceramic capacitor as an initial value and reduce the value of C4 to increase the DHC loop
response as needed.
9 Reducing the System Startup Time
The total system startup time is generally dependent on the frequency response of both the primary power
supply and DHC loop of the LM3463. The slower response of the two circuits, the longer time the system
takes to startup. Because the response of the primary is usually not user programmable, the overall
system startup time can be reduced by shortening the time for the V
RAIL
to increase from V
RAIL(nom)
to
V
DHC_READY
, namely the t
ST
. as shown in Figure 6. The t
ST
is adjusted dependent on the value of the C4 and
RISR, which governed by the following equation:
12 AN-2255 LM3463 Evaluation Board SNVA642 – May 2012
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