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LM3557 Fiches technique(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No de pièce LM3557
Description  Step-Up Converter for White LED Applications
Download  11 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3557 Fiches technique(HTML) 10 Page - National Semiconductor (TI)

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Application Information (Continued)
junction capacitance magnitudes (C
J or CT or CD) are con-
ducive to high efficiency. The chosen diode must have a
reverse breakdown voltage rating (V
R and/or VRRM) 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 VRRM > VOUT
2.
I
F or IO
≥ I
LOAD or IOUT
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
10


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