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
DD
)
The V
DD
pin bypass capacitor C
DD
should be located as near
as possible to the V
DD
and GND pins.
Switching Loop Areas
The area of the switching loop connecting the input filter
capacitor C
IN
, 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
C
O
, the inductor L1 and the diode D1 together should be kept
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
IN
is selected, it must be
positioned orthogonal with respect to L1. The loop area
formed by C
IN2
, L
IN
and C
IN
should be minimized. The input
lead wires must be twisted together.
Figure 7. Recommended circuit board layout with the HV9921N3/HV9922N3/HV9923N3
COMPONENT SIDE VIEW
C
O
LED +
D1
C
IN2
AC Line
85-264VAC
D2-5
C
DD
U1
C
IN
L1
LED -
VRD1
F1
L
IN
9
NR092005
SUPERTEX [ Supertex, Inc ]
