LM3407_09 Datasheet, PDF (13/16 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

PRINTED CIRCUIT BOARD DESIGN
Since the copper traces of PCBs carry resistance and para-
sitic 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 cir-
cuit 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 aver-
age current regulation is achieved by detecting the average
switch current, the current setting resistor R_ISNS must be lo-
cated 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 rec-
ommended that C_VCC, the output filter capacitor for the inter-
nal 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 Application
Note AN-1149: Layout Guidelines for Switching Power Sup-
plies.
FIGURE 7. Typical Application Schematic for 6 LEDs
30046626
FIGURE 8. Typical Application Schematic for 1 LED
30046627
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