Microchip Technology HV2722, Руководство пользователя

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HV2722 ANALOG SWITCH
EVALUATION BOARD
USER’S GUIDE

2019 Microchip Technology Inc. DS50002810A-page 25
Chapter 4. PCB Design and Layout Notes
4.1 PCB LAYOUT TECHNIQUES FOR HV2722
The HV2722 Analog Switch Evaluation Board is equipped with the HV2722, which has
16 analog switches to pass high-voltage, high-current and high-frequency pulses. The PCB
design and layout are important to ensure the success of the implementation.
4.1.1 High-Voltage and High-Speed Grounding and Layout
Techniques
The center pad at the bottom of the HV2722 QFN package is internally connected to
V
NN
, and should be connected to V
NN
externally on the PCB.
The user must pay attention to the connecting traces, since the analog switches pass
the high-voltage and high-speed signals. In particular, controlled impedance of 50

to
the ground plane and more trace spacing needs to be applied in this situation.
High-speed PCB trace design practices are used for the HV2722 Analog Switch Eval-
uation Board PCB layout. The internal circuitry of the HV2722 can operate at quite a
high frequency, with the primary speed limitation being the load capacitance. Because
of this high speed and the high transient currents that result from driving capacitive
loads, the supply voltage bypass capacitors should be as close to the pins as possible.
All the GND pins should have low-inductance feedthrough via connections that are
connected directly to a solid ground plane at the second layer of the PCB.
It is recommended to minimize the trace length to the ground plane and to insert a
ferrite bead in the power supply lead to the capacitor to prevent resonance in the power
supply lines.
It is important to minimize trace lengths and use sufficient trace width to reduce
inductance. Surface mount components are highly recommended.
The use of a solid ground plane, and good power and signal layout practices, will
prevent any possible parasitic capacitance coupling. The user should also ensure that
the circulating ground return current from a capacitive load cannot react with common
inductance to create noise voltages in the input logic circuitry.
4.1.2 Decoupling Capacitors Selection
The V
DD
, V
PP
and V
NN
supply voltage rails are able to provide fast transient current.
Therefore, they should have a low-impedance bypass capacitor close to each of the
power supply pins. Use a surface mount ceramic capacitor of 1.0 to 2.2
μ F capacitance
with an appropriate voltage rating.
It is important to verify what type of ceramic capacitor is selected for these bypass
capacitors. The low-impedance means low-ESR/ESL impedance within the frequency
bandwidth range of ultrasound pulses transmitted, including the very fast dV/dt of the
pulse’s rising and falling edges. A capacitor with low-temperature coefficient and
low-voltage coefficient is also recommended. The type of X7R and X5R, or other more
advanced multilayer ceramic types, should be selected for these purposes.