HV9921 datasheet(11/19 Pages) MICROCHIP

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 2014 Microchip Technology Inc.

DS20005311A-page 11

HV9921/HV9922/HV9923

5.0

LAYOUT CONSIDERATIONS

For a recommended circuit board layout for the

HV9921/22/23, see Figure 5-1.

5.1

Single Point Grounding

Use a single point ground connection from the input fil-

ter capacitor to the area of copper connected to the

GND pin.

5.2

Bypass Capacitor (CDD)

The VDD pin bypass capacitor CDD should be located

as near as possible to the VDD and GND pins.

5.3

Switching Loop Areas

The area of the switching loop connecting the input fil-

ter capacitor CIN, the diode D1 and the HV9921/22/23

together should be kept as small as possible.

The switching loop area connecting the output filter

capacitor CO, the inductor L1 and the diode D1

together should be kept as small as possible.

5.4

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 the SOT-89 package is used. The

same applies to the cathode of the free-wheeling diode

D1. Both nodes are quiet; therefore, they 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 mini-

mized. A large switching node area can increase high

frequency radiated EMI.

5.5

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 consider-

ation is especially important when unshielded con-

struction of L1 is used. When an axial input EMI filter

inductor LIN is selected, it must be positioned orthogo-

nal with respect to L1. The loop area formed by CIN2,

LIN and CIN should be minimized. The input lead wires

must be twisted together.

FIGURE 5-1:

RECOMMENDED CIRCUIT BOARD LAYOUT WITH HV9921/22/23

D2-5

VRD1

IN2

AC Line

85-264VAC

LED +

LED -

COMPONENT SIDE VIEW

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