LM3557

SNVS338B – NOVEMBER 2004 – REVISED FEBRUARY 2013

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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

followed:

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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