FC9921

2012. 10. 31

8/8

Revision No : 0

See

Figure 7 for a recommended circuit board layout for the

FC9921.

Single Point Grounding

capacitor to the area of copper connected to the GND pin.

Bypass Capacitor (C

The VDD pin bypass capacitor C

near as possible to the VDD and GND pins.

Switching Loop Areas

capacitor C

be kept as small as possible.

C

as small as possible.

Thermal Considerations vs. Radiated EMI

The copper 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

FC9921, (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 FC9921, 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 inp

direct proximity to the inductor L1 in order to avoid magnetic

important when unshielded construction of L1 is used. When

positioned orthogonal with respect to L1. The loop area

formed by C

lead wires must be twisted together.

FC9921 Layout Considerations

Pin # Function Description

1

DRAIN

Drain terminal of the output switching MOSFET and a linear regulator input.

2

GND

Common connection for all circuits.

3

VDD

Power supply pin for internal control circuits. Bypass this pin with a 0.1uF low impedance capacitor.

Pin Description

Figure 7. Recommended circuit board layout with the FC9921

D2-5

L

IN

C

O

L1

U1

VRD1

C

IN

C

N2

D1

C

DD

F1

AC Line

85-264VAC

LED +

LED -

COMPONENT SIDE VIEW