LM3407_09 Datasheet, PDF (12/16 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

For applications with no output capacitor in place, the mag-
nitude of the inductor ripple current should not be more than
20% of the average inductor current, which is equivalent to
the output current, I_OUT. However, in some situations the
physical size of the required inductor may be too large and
thus not allowed. The output capacitor can help absorb this
current ripple to significantly reduce the ripple component
along the LED string. With an output capacitor C_OUT in place,
the magnitude of the inductor ripple current can be relaxed to
80% of the output current. Figure 6 illustrates the relationship
between I_OUT, I_L(peak), and I_L(ripple).
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FIGURE 6. Relationship between I_OUT, I_L(peak) and I_L(ripple)
Table 1 provides the suggested inductance of the inductor for
500 kHz and 1 MHz switching frequency operation with
C_OUT = 4.7µF and I_L(ripple) = 0.8 x I_OUT
TABLE 1. Suggested Inductance Value of the Inductor
VIN / V
Number of LED
1
2
3
4
Inductor selection table for F_SW = 500 kHz, C_OUT = 4.7µF (1µF for 1 LED)
5
22 µH
10
22 µH
22 µH
15
22 µH
22 µH
22 µH
20
22 µH
33 µH
22 µH
22 µH
25
22 µH
33 µH
33 µH
22 µH
30
22 µH
47 µH
33 µH
33 µH
Inductor selection table for F_SW = 1 MHz, C_OUT = 4.7µF (1µF for 1 LED)
5
22 µH
10
22 µH
22 µH
15
22 µH
22 µH
22 µH
20
22 µH
22 µH
22 µH
22 µH
25
22 µH
33 µH
22 µH
22 µH
30
22 µH
33 µH
33 µH
33 µH
5
22 µH
22 µH
33 µH
22 µH
22 µH
22 µH
6
22 µH
22 µH
22 µH
22 µH
7
22 µH
22 µH
FREE-WHEELING DIODE
The LM3407 is a non-synchronous floating buck converter
that requires an external free-wheeling diode to provide a path
for recirculating current from the inductor to the LED array
when the power switch is turned OFF. Selecting the free-
wheeling diode depends on both the output voltage and cur-
rent. The diode must have a rated reverse voltage higher than
the input voltage of the converter and a peak current rating
higher than the expected maximum inductor current. Using a
schottky diode with a low forward voltage drop can reduce
power dissipation and enhance conversion efficiency.
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