LM3407 Datasheet, PDF (15/22 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

LM3407
www.ti.com
SNVS553B – JANUARY 2008 – REVISED MAY 2013
Table 1. Suggested Inductance Value of the Inductor
Number of LED
VIN / V
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 current. 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.
PRINTED CIRCUIT BOARD DESIGN
Since the copper traces of PCBs carry resistance and parasitic inductance, the longer the copper trace, the
higher the resistance and inductance. These factors introduce voltage and current spikes to the switching nodes
and may impair circuit performance. To optimize the performance of the LM3407, the rule of thumb is to keep the
connections between components as short and direct as possible. Since true average current regulation is
achieved by detecting the average switch current, the current setting resistor R_ISNS must be located as close as
possible to the LM3407 to reduce the parasitic inductance of the copper trace and avoid noise pick-up. The
connections between the LX pin, rectifier D1, inductor L1, and output capacitor C_OUT should be kept as short as
possible to reduce the voltage spikes at the LX pin. It is recommended that C_VCC, the output filter capacitor for
the internal linear regulator of the LM3407, be placed close to the VCC pin. The input filter capacitor C_IN should
be located close to L1 and the cathode of D1. If C_IN is connected to the VIN pin by a long trace, a 0.1µF
capacitor should be added close to VIN pin for noise filtering. In normal operation, heat will be generated inside
the LM3407 and may damage the device if no thermal management is applied. For more details on switching
power supply layout considerations see TI Lit NumberSNVA021: Layout Guidelines for Switching Power
Supplies.
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