Microchip Technology HV2918, Руководство пользователя

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HV2918 ANALOG SWITCH
EVALUATION BOARD
USER’S GUIDE

2020 Microchip Technology Inc. DS50003006A-page 25
Chapter 4. PCB Design and Layout Notes
4.1 PCB LAYOUT TECHNIQUES FOR HV2918
The HV2918 Analog Switch Evaluation Board has an analog switch 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 HV2918 VQFN package is internally connected to
the IC’s substrate (V
SUB
). This pad should be connected to GND, 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, a 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 HV2918 Analog Switch Eval-
uation Board PCB layout. The internal circuitry of the HV2918 device can operate at a
high frequency, with the primary speed limitation being the load capacitance. Because
of these high-speed and 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 feed through connections that are
connected directly to a solid ground plane at the second layer of the PCB. It is recom-
mended 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 prevent
any possible parasitic capacitance coupling. The user should also ensure that the
circulating ground return current from a capacitive load does not react with common
inductance to create noise voltages in the input logic circuitry.
4.1.2 Decoupling Capacitors Selection
The V
LL
and V
DD
supply voltage rails are able to provide fast transient current. There-
fore, they should have a low-impedance bypass capacitor close to each of the power
supply pins. Use a surface-mounted ceramic capacitor of 0.1 to 2.2 µF 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.