LM3557 Datasheet, PDF (10/11 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

Application Information (Continued)
junction capacitance magnitudes (C_J or C_T or C_D) are con-
ducive to high efficiency. The chosen diode must have a
reverse breakdown voltage rating (V_R and/or V_RRM) that is
larger than the output voltage (V_OUT). No matter what type of
diode is chosen, Schottky or not, certain selection criteria
must be followed:
1. V_R and V_RRM > V_OUT
2. I_F or I_O ≥ I_LOAD or I_OUT
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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