HV9921/HV9922/HV9923
Thermal Considerations vs. Radiated EMI
HV9921/22/23 Layout Considerations
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.
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 VDD pin bypass caDpDacitor CDD should be located as near
as possible to the VDD and GND pins.
Input Filter Layout Considerations
Switching Loop Areas
The area of the switching loop connecting the input filter
capacitor C , the diode D1 and the HV9921/22/23 together
should be kIeNpt as small as possible.
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 LIN is selected, it must be
positioned orthogonal with respect to L1. The loop area
formed by C , LIN and CIN should be minimized. The input
lead wires mIuNs2 t be twisted together.
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.
Figure 7. Recommended circuit board layout with the HV9921N3/HV9922N3/HV9923N3
COMPONENT SIDE VIEW
CO
VRD1
LIN
LED +
LED -
L1
D1
F1
CIN2
CIN
D2-5
AC Line
85-264VAC
CDD
U1
NR092005
9
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