National Semiconductor LM3424, Руководство пользователя

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Design Considerations
This section describes the application level considerations
when designing with the LM3424. For corresponding calcu-
lations, refer to the Design Guide section.
INDUCTOR
The inductor (L1) is the main energy storage device in a
switching regulator. Depending on the topology, energy is
stored in the inductor and transfered to the load in different
ways (as an example, buck-boost operation is detailed in the
Current Regulators section). The size of the inductor, the volt-
age across it, and the length of the switching subinterval
(t
ON
or t
OFF
) determines the inductor current ripple (
Δ
i
L-PP
). In
the design process, L1 is chosen to provide a desired
Δ
i
L-PP
.
For a buck regulator the inductor has a direct connection to
the load, which is good for a current regulator. This requires
little to no output capacitance therefore
Δ
i
L-PP
is basically
equal to the LED ripple current
Δ
i
LED-PP
. However, for boost
and buck-boost regulators, there is always an output capaci-
tor which reduces
Δ
i
LED-PP
, therefore the inductor ripple can
be larger than in the buck regulator case where output ca-
pacitance is minimal or completely absent.
In general,
Δ
i
LED-PP
is recommended by manufacturers to be
less than 40% of the average LED current (I
LED
). Therefore,
for the buck regulator with no output capacitance,
Δ
i
L-PP
should also be less than 40% of I
LED
. For the boost and buck-
boost topologies,
Δ
i
L-PP
can be much higher depending on the
output capacitance value. However,
Δ
i
L-PP
is suggested to be
less than 100% of the average inductor current (I
L
) to limit the
RMS inductor current.
L1 is also suggested to have an RMS current rating at least
25% higher than the calculated minimum allowable RMS in-
ductor current (I
L-RMS
).
LED DYNAMIC RESISTANCE
When the load is a string of LEDs, the output load resistance
is the LED string dynamic resistance plus R
SNS
. LEDs are PN
junction diodes, and their dynamic resistance shifts as their
forward current changes. Dividing the forward voltage of a
single LED (V
LED
) by the forward current (I
LED
) leads to an
incorrect calculation of the dynamic resistance of a single LED
(r
LED
). The result can be 5 to 10 times higher than the true
r
LED
value.
30085774
FIGURE 19. Dynamic Resistance
Obtaining r
LED
is accomplished by refering to the
manufacturer's LED I-V characteristic. It can be calculated as
the slope at the nominal operating point as shown in
Figure
19 . For any application with more than 2 series LEDs, R
SNS
can be neglected allowing r
D
to be approximated as the num-
ber of LEDs multiplied by r
LED
.
OUTPUT CAPACITOR
For boost and buck-boost regulators, the output capacitor
(C
O
) provides energy to the load when the recirculating diode
(D1) is reverse biased during the first switching subinterval.
An output capacitor in a buck topology will simply reduce the
LED current ripple (
Δ
i
LED-PP
) below the inductor current ripple
(
Δ
i
L-PP
). In all cases, C
O
is sized to provide a desired
Δ
i
LED-
PP
. As mentioned in the Inductor section,
Δ
i
LED-PP
is recom-
mended by manufacturers to be less than 40% of the average
LED current (I
LED-PP
).
C
O
should be carefully chosen to account for derating due to
temperature and operating voltage. It must also have the nec-
essary RMS current rating. Ceramic capacitors are the best
choice due to their high ripple current rating, long lifetime, and
good temperature performance. An X7R dieletric rating is
suggested.
INPUT CAPACITORS
The input capacitance (C
IN
) provides energy during the dis-
continuous portions of the switching period. For buck and
buck-boost regulators, C
IN
provides energy during t
ON
and
during t
OFF
, the input voltage source charges up C
IN
with the
average input current (I
IN
). For boost regulators, C
IN
only
needs to provide the ripple current due to the direct connec-
tion to the inductor. C
IN
is selected given the maximum input
voltage ripple (
Δ
v
IN-PP
) which can be tolerated.
Δ
v
IN-PP
is sug-
gested to be less than 10% of the input voltage (V
IN
).
An input capacitance at least 100% greater than the calcu-
lated C
IN
value is recommended to account for derating due
to temperature and operating voltage. When PWM dimming,
even more capacitance can be helpful to minimize the large
current draw from the input voltage source during the rising
transistion of the LED current waveform.
The chosen input capacitors must also have the necessary
RMS current rating. Ceramic capacitors are again the best
choice due to their high ripple current rating, long lifetime, and
good temperature performance. An X7R dieletric rating is
suggested.
For most applications, it is recommended to bypass the V
IN
pin will an 0.1 µF ceramic capacitor placed as close as pos-
sible to the pin. In situations where the bulk input capacitance
may be far from the LM3424 device, a 10
Ω
series resistor
can be placed between the bulk input capacitance and the
bypass capacitor, creating a 150 kHz filter to eliminate unde-
sired high frequency noise.
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LM3424