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NR092005

HV9921/HV9922/HV9923

HV9921/22/23 Layout Considerations

See Figure 7 for a recommended circuit board layout for the

HV9921/22/23.

Single Point Grounding

Use a single point ground connection from the input filter

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

Bypass Capacitor (C

The V

as possible to the V

Switching Loop Areas

The area of the switching loop connecting the input filter

capacitor C

should be kept as small as possible.

The switching loop area connecting the output filter capacitor

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

HV9921N8, HV9922N8, and HV9923N8 (SOT-89 package)

are 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 HV9921/22/23, 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 circuits 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 construction of L1 is used. When

an axial input EMI filter inductor L

positioned orthogonal with respect to L1. The loop area

formed by C

lead wires must be twisted together.

Figure 7. Recommended circuit board layout with the HV9921N3/HV9922N3/HV9923N3

D2-5

L

IN

C

O

L1

U1

VRD1

C

IN

C

IN2

D1

C

DD

F1

AC Line

85-264VAC

LED +

LED -

COMPONENT SIDE VIEW