LM3407EVAL National Semiconductor, LM3407EVAL Datasheet - Page 12

• Manufacturer Part Number: LM3407EVAL
• Description: BOARD EVALUATION FOR LM3407
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3407EVAL

• Current - Output / Channel: 350mA
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 4.5V
• Features: Dimmable
• Voltage - Input: 4.5 ~ 30V
• Utilized Ic / Part: LM3407
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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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
Table 1 provides the suggested inductance of the inductor for
500 kHz and 1 MHz switching frequency operation with
C
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-
Inductor selection table for F
Inductor selection table for F
OUT
VIN / V
= 4.7µF and I
10
15
20
25
30
10
15
20
25
30
5
5
L(ripple)
22 µH
22 µH
22 µH
22 µH
22 µH
22 µH
22 µH
22 µH
22 µH
22 µH
22 µH
22 µH
1
= 0.8 x I
SW
SW
= 500 kHz, C
= 1 MHz, C
OUT
FIGURE 6. Relationship between I
TABLE 1. Suggested Inductance Value of the Inductor
22 µH
22 µH
33 µH
33 µH
47 µH
22 µH
22 µH
22 µH
33 µH
33 µH
2
OUT
OUT
= 4.7µF (1µF for 1 LED)
= 4.7µF (1µF for 1 LED)
22 µH
22 µH
33 µH
33 µH
22 µH
22 µH
22 µH
33 µH
3
12
Number of LED
the output current, I
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
the magnitude of the inductor ripple current can be relaxed to
80% of the output current. Figure 6 illustrates the relationship
between I
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.
22 µH
22 µH
33 µH
22 µH
22 µH
33 µH
OUT
4
, I
L(peak)
OUT
, I
and I
L(peak)
22 µH
22 µH
33 µH
22 µH
22 µH
22 µH
L(ripple)
5
, and I
OUT
. However, in some situations the
30046625
L(ripple)
.
22 µH
22 µH
22 µH
22 µH
6
OUT
22 µH
22 µH
7
in place,