MXT1664T3-ABESSQTP datasheet(62/98 Pages) MICROCHIP

62 / 98 page

MXT1664T3 2.0

DS40002054A-page 62

 2018 Microchip Technology Inc.

11.4.1

SINGLE SUPPLY OPERATION

When designing a PCB for an application using a single LDO, extra care should be taken to ensure short, low inductance

traces between the supply and the touch controller supply input pins. Ideally, tracking for the individual supplies should

be arranged in a star configuration, with the LDO at the junction of the star. This will ensure that supply current variations

or noise in one supply rail will have minimum effect on the other supplies. In applications where a ground plane is not

practical, this same star layout should also apply to the power supply ground returns.

Only regulators with a 300 mA or greater rating can be used in a single-supply design.

Refer to the following application note for more information on routing with a single LDO:

• Application Note: MXTAN0208 – Design Guide for PCB Layouts for maXTouch Touch Controllers

11.4.2

MULTIPLE VOLTAGE REGULATOR SUPPLY

The AVdd supply stability is critical for the device because this supply interacts directly with the analog front end. If noise

problems exist when using a single LDO regulator, Microchip recommends that AVdd is supplied by a regulator that is

separate from the digital supply and high voltage regulators. This reduces the amount of noise injected into the

sensitive, low signal level parts of the design.

11.5

Crystal Oscillator

The internal 16 MHz oscillator must be used when the device is operating in USB mode. An external crystal with a

minimum accuracy of 100 ppm must be used.

The crystal placement is critical to the performance of the design. The connecting leads between the device and the

crystal should be as short as possible. These tracks, together with the crystal itself, should be placed above a suitable

ground plane. It is also important that no other signal tracks are placed close to, or under, these tracks. The crystal input

pins are at a relatively high impedance and cross-talk from other signals will seriously affect oscillator stability and

accuracy. The crystal case should also be connected to ground if possible.

If an external oscillator module is used, care still needs to be taken when tracking to the device. The clock signal should

be kept as short as possible, with a solid ground return underneath the clock output.

11.6

Analog I/O

In general, tracking for the analog I/O signals from the device should be kept as short as possible. These normally go

to a connector which interfaces directly to the touchscreen.

Ensure that adequate ground-planes are used. An analog ground plane should be used in addition to a digital one. Care

should be taken to ensure that both ground planes are kept separate and are connected together only at the point of

entry for the power to the PCB. This is usually at the input connector.

11.7

Component Placement and Tracking

It is important to orient all devices so that the tracking for important signals (such as power and clocks) are kept as short

as possible.

11.7.1

DIGITAL SIGNALS

In general, when tracking digital signals, it is advisable to avoid sharp directional changes on sensitive signal tracks

(such as analog I/O) and any clock or crystal tracking.

A good ground return path for all signals should be provided, where possible, to ensure that there are no discontinuities.

11.8

EMC and Other Observations

The following recommendations are not mandatory, but may help in situations where particularly difficult EMC or other

problems are present:

• A small common mode choke is recommended on the differential USB data pair. This should be placed directly at

the USB connector, between the connector and the relevant pins. Tracking lengths for the USB data pair should

be kept as short as possible.

NOTE

The oscillator is not needed in I2C mode.

MXT1664T3-ABESSQTP Datasheet(PDF) 62 Page - Microchip Technology

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