MXT1664T3-ATSQTP datasheet(60/98 Pages) MICROCHIP

60 / 98 page

MXT1664T3 2.0

DS40002054A-page 60

 2018 Microchip Technology Inc.

11.0

PCB DESIGN CONSIDERATIONS

11.1

Introduction

The following sections give the design considerations that should be adhered to when designing a PCB layout for use

with the mXT1664T3. Of these, power supply and ground tracking considerations are the most critical.

By observing the following design rules, and with careful preparation for the PCB layout exercise, designers will be

assured of a far better chance of success and a correctly functioning product.

11.2

Printed Circuit Board

Microchip recommends the use of a four-layer printed circuit board for mXT1664T3 applications. This, together with

careful layout, will ensure that the board meets relevant EMC requirements for both noise radiation and susceptibility,

as laid down by the various national and international standards agencies.

11.2.1

PCB CLEANLINESS

Modern no-clean-flux is generally compatible with capacitive sensing circuits.

11.3

Power Supply

11.3.1

SUPPLY QUALITY

While the device has good Power Supply Rejection Ratio properties, poorly regulated and/or noisy power supplies can

significantly reduce performance.

Particular care should be taken of the AVdd supply, as it supplies the sensitive analog stages in the device.

11.3.2

SUPPLY RAILS AND GROUND TRACKING

Power supply and clock distribution are the most critical parts of any board layout. Because of this, it is advisable that

these be completed before any other tracking is undertaken. After these, supply decoupling, and analog and high speed

digital signals should be addressed. Track widths for all signals, especially power rails should be kept as wide as

possible in order to reduce inductance.

The Power and Ground planes themselves can form a useful capacitor. Flood filling for either or both of these supply

rails, therefore, should be used where possible. It is important to ensure that there are no floating copper areas

remaining on the board: all such areas should be connected to the ground plane. The flood filling should be done on the

outside layers of the board.

In applications where the USB bus supplies power to the board, care should be taken to ensure that suitable capacitive

decoupling is provided close to the USB connector. The tracking to the on-board regulators should also be kept as short

as possible.

It should also be remembered that the screen of the USB cable is not intended to be connected to the ground or 0V

supply of a remote device. It should either be left open circuit (being connected only at the host computer end) or

decoupled with a suitable high voltage capacitor (typically 4.7 nF, 250 V) and a parallel resistor (typically 1 M

). Note

that these components may not be required when the USB cabling is internal and permanently wired, and is routed away

from the noisier parts of the system.

11.3.3

POWER SUPPLY DECOUPLING

Decoupling capacitors should be fitted as specified in Section 2.5 “Schematic Notes”.

The decoupling capacitors must be placed as close as possible to the pin being decoupled. The traces from these

capacitors to the respective device pins should be wide and take a straight route. They should be routed over a ground

plane as much as possible. The capacitor ground pins should also be connected directly to a ground plane.

Surface mounting capacitors are preferred over wire-leaded types due to their lower ESR and ESL. It is often possible

to fit these decoupling capacitors underneath and on the opposite side of the PCB to the digital ICs. This will provide

the shortest tracking, and most effective decoupling possible.

CAUTION!

If a PCB is reworked to correct soldering faults relating to any device, or to any associated traces or

components, be sure that you fully understand the nature of the flux used during the rework process.

Leakage currents from hygroscopic ionic residues can stop capacitive sensors from functioning. If you

have any doubts, a thorough cleaning after rework may be the only safe option.

MXT1664T3-ATSQTP Datasheet(PDF) 60 Page - Microchip Technology

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