LM3557 Datasheet, PDF (12/19 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

LM3557
SNVS338B – NOVEMBER 2004 – REVISED FEBRUARY 2013
www.ti.com
OUTPUT CAPACITOR SELECTION
The output capacitor serves as an energy reservoir for the white LED load when the internal power FET switch
(Figure 3: N1) is ON or conducting current. The requirements for the output capacitor must include worst case
operation such as when the load opens up and the LM3557 operates in over-voltage protection (OVP) mode
operation. A minimum capacitance of 0.5 µF is required to ensure normal operation. Consult the capacitor
manufacturer’s data curves to verify whether the minimum capacitance requirement is going to be achieved for a
particular application.
Some recommended capacitor manufacturers are:
TDK
[www.tdk.com]
Murata
[www.murata.com]
Vishay
[www.vishay.com]
DIODE SELECTION
To maintain high efficiency it is recommended that the average current rating (I_F or I_O) of the selected diode
should be larger than the peak inductor current (I_Lpeak). To maintain diode integrity the peak repetitive forward
current (I_FRM) must be greater than or equal to the peak inductor current (I_Lpeak). Diodes with low forward voltage
ratings (V_F) and low junction capacitance magnitudes (C_J or C_T or C_D) are conducive 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 (SNVA021).
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