junction capacitance magnitudes (C

ducive to high efficiency. The chosen diode must have a

reverse breakdown voltage rating (V

larger than the output voltage (V

diode is chosen, Schottky or not, certain selection criteria

must be followed:

1.

V

2.

I

≥ I

3.

I

FRM

≥ I

Lpeak

Some recommended diode manufacturers are as follows:

Vishay [www.vishay.com]

Diodes, Inc [www.diodes.com]

On Semiconductor [www.onsemi.com]

LAYOUT CONSIDERATIONS

All components, except for the white LEDs, must be placed

as close as possible to the LM3557. The die attach pad

(DAP) must be soldered to the ground plane.

The input capacitor, Cin, must be placed close to the

LM3557. Placing Cin close to the device will reduce the

metal trace resistance effect on input voltage ripple. The

feedback current setting resistor R2 must be placed close to

the Fb and Sw2 pins. The output capacitor, Cout, must be

placed close to the Ovp and Gnd pin connections. Trace

connections to the inductor should be short and wide to

reduce power dissipation, increase overall efficiency, and

reduce EMI radiation. The diode, like the inductor, should

have trace connections that are short and wide to reduce

power dissipation and increase overall efficiency. For more

details regarding layout guidelines for switching regulators

refer to Applications Note AN-1149.

www.national.com

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