FC9920

2012. 10. 31

7/7

Revision No : 0

FC9920 Layout Considerations

See Figure 7 for a recommended circuit board layout for the FC9920.

Single Point Grounding

Use a single point ground connection from the input filter capacitor to the area of copper connected to the

GND pin.

Switching Loop Areas

together should be kept as small as possible.

together should be kept as small as possible.

Thermal Considerations vs. Radiated EMI

The cooper area where GND pin is connected acts not only as a single point ground, but also as a heat

sink.

This area should be maximized for good heat sinking, especially when FC9920 (SOT-89 package), is

used.

The same applies to the cathode of the free-wheeling diode D1. Both nodes are quiet and therefore, will

not cause radiated RF emission. The switching node copper area connected to the DRAIN pin of the

FC9920, the anode of D1 and the inductor L1 needs to be minimized. A large switching node area can

increase high frequency radiated EMI.

Input Filter Layout Considerations

The input circuit of the EMI filter must not be placed in the direct proximity to the inductor L1 in order to

avoid magnetic coupling of its leakage fields. This consideration is especially important when unshielded

input lead wires must be twisted together.

L1

VRD1

F1

Figure7. Recommended circuit board layout with the FC9920 (SOT-89 package)

AC Line

85-135VAC

D1

D2-5

U1

LED +

LED -

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