LM3423MHBSTEVAL National Semiconductor, LM3423MHBSTEVAL Datasheet - Page 19

• Manufacturer Part Number: LM3423MHBSTEVAL
• Description: BOARD, EVALUATION, FOR LM3423MHBST
• Manufacturer: National Semiconductor

Specifications of LM3423MHBSTEVAL

• Kit Contents: LM3423 Assembled Evaluation Board, LM3423 Datasheet, Application Note
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Cycle-by-Cycle Current Limit, LED Ready Flag, Fault Flag, Programmable

Design Considerations
This section describes the application level considerations
when designing with the LM3421/23. For corresponding cal-
culations, 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
the design process, L1 is chosen to provide a desired Δi
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 Δi
equal to the LED ripple current Δi
and buck-boost regulators, there is always an output capaci-
tor which reduces Δi
be larger than in the buck regulator case where output ca-
pacitance is minimal or completely absent.
In general, Δi
less than 40% of the average LED current (I
for the buck regulator with no output capacitance, Δi
should also be less than 40% of I
boost topologies, Δi
output capacitance value. However, Δi
less than 100% of the average inductor current (I
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
LED DYNAMIC RESISTANCE
When the load is a string of LEDs, the output load resistance
is the LED string dynamic resistance plus R
junction diodes, and their dynamic resistance shifts as their
forward current changes. Dividing the forward voltage of a
single LED (V
incorrect calculation of the dynamic resistance of a single LED
(r
r
LED
ON
LED
or t
value.
). The result can be 5 to 10 times higher than the true
OFF
) determines the inductor current ripple (Δi
FIGURE 20. Dynamic Resistance
LED-PP
LED
L-RMS
) by the forward current (I
L-PP
is recommended by manufacturers to be
LED-PP
).
can be much higher depending on the
, therefore the inductor ripple can
LED
LED-PP
. For the boost and buck-
L-PP
. However, for boost
is suggested to be
SNS
LED
L-PP
LED
. LEDs are PN
) leads to an
). Therefore,
L
30067374
is basically
) to limit the
L-PP
L-PP
). In
L-PP
.
19
Obtaining r
manufacturer's LED I-V characteristic. It can be calculated as
the slope at the nominal operating point as shown in
20. For any application with more than 2 series LEDs, R
can be neglected allowing r
ber of LEDs multiplied by r
OUTPUT CAPACITOR
For boost and buck-boost regulators, the output capacitor
(C
(D1) is reverse biased during the first switching subinterval.
An output capacitor in a buck topology will simply reduce the
LED current ripple (Δi
(Δi
PP
mended by manufacturers to be less than 40% of the average
LED current (I
C
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
continuous portions of the switching period. For buck and
buck-boost regulators, C
during t
average input current (I
needs to provide the ripple current due to the direct connec-
tion to the inductor. C
voltage ripple (Δv
gested to be less than 10% of the input voltage (V
An input capacitance at least 100% greater than the calcu-
lated C
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
pin with 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 LM3421/23 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.
O
. As mentioned in the Inductor section, Δi
O
L-PP
should be carefully chosen to account for derating due to
) provides energy to the load when the recirculating diode
). In all cases, C
IN
OFF
value is recommended to account for derating due
, the input voltage source charges up C
LED
LED-PP
IN-PP
is accomplished by refering to the
).
LED-PP
IN
) which can be tolerated. Δv
O
is selected given the maximum input
IN
is sized to provide a desired Δi
IN
IN
). For boost regulators, C
LED
D
) below the inductor current ripple
) provides energy during the dis-
provides energy during t
to be approximated as the num-
.
LED-PP
www.national.com
IN-PP
IN
IN
is recom-
).
with the
ON
IN
is sug-
Figure
only
LED-
and
SNS
IN