Datasheet

LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009 www.ti.com
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 transferred 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 voltage across it, and the length
of the switching subinterval (t
ON
or t
OFF
) determines the inductor current ripple (Δi
L-PP
). In the design process, L1
is chosen to provide a desired Δi
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 Δi
L-PP
is basically equal to
the LED ripple current Δi
LED-PP
. However, for boost and buck-boost regulators, there is always an output
capacitor which reduces Δi
LED-PP
, therefore the inductor ripple can be larger than in the buck regulator case
where output capacitance is minimal or completely absent.
In general, Δi
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, Δi
L-PP
should also be less than 40% of I
LED
. For the
boost and buck-boost topologies, Δi
L-PP
can be much higher depending on the output capacitance value.
However, Δi
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 inductor 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.
Figure 37. Dynamic Resistance
Obtaining r
LED
is accomplished by referring to the manufacturer's LED I-V characteristic. It can be calculated as
the slope at the nominal operating point as shown in Figure 37. For any application with more than 2 series
LEDs, R
SNS
can be neglected allowing r
D
to be approximated as the number 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 (Δi
LED-PP
) below the inductor current ripple (Δi
L-PP
). In all cases, C
O
is sized
to provide a desired Δi
LED-PP
. As mentioned in the INDUCTOR section, Δi
LED-PP
is recommended 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 necessary 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.
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