LM3424 Datasheet, PDF (19/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

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 (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 capaci-
tor which reduces Δi_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, 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 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.
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 (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 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 (Δv_IN-PP) which can be tolerated. Δv_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.
30085774
FIGURE 19. Dynamic Resistance
19
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