1 Overview

1.1 Introduction

QT240 devices are burst mode digital charge-transfer (QT)

sensor ICs designed specifically for touch controls; they

include all hardware and signal processing functions

necessary to provide stable sensing under a wide variety of

conditions. Only a single low cost capacitor per channel is

required for operation.

Figures 1.1 and 1.2 show basic circuits for these device s.

See Table 1.1 for device pin listings.

The devices employ bursts of charge-transfer cycles to

acquire signals. Burst mode permits low power operation,

dramatically reduces RF emissions, lowers susceptibility to

RF fields, and yet permits excellent speed. Internally, signals

are digitally processed to reject impulse noise using a

'consensus' filter that requires six consecutive confirmations

of detection.

The QT switches and charge measurement hardware

functions are all internal to the device. A single-slope

switched capacitor ADC includes the QT charge and transfer

switches in a configuration that provides direct ADC

conversion; an external Cs capacitor accumulates the charge

from sense-plate Cx, which is then measured.

Larger values of Cx cause the charge transferred into Cs to

rise more rapidly, reducing available resolution; as a

minimum resolution is required for proper operation, this can

result in dramatically reduced gain. Larger values of Cs

reduce the rise of differential voltage across it, increasing

available resolution by permitting longer QT bursts. The

value of Cs can thus be increased to allow larger values of

Cx to be tolerated. The IC is responsive to both Cx and Cs,

and changes in either can result in substantial changes in

sensor gain.

Unused channels: If a channel is not used, a dummy sense

capacitor (nominal value 1nF ) of any type plus a 2.2K series

resistor must be connected between unused SNS pin pairs,

to ensure correct operation.

1.2 Operating Modes

The QT240 features spread-spectrum acquisition

capability, external synchronization of acquire

bursts, and fast and slow acquisition modes. These

modes are enabled via high-value resistors

connected to the SNS pins to ground or Vdd. These

resistors are required in every circuit.

There are two basic modes as shown in Figures 1.1

and 1.2.

Low-power Sync mode: In this mode the device

operates with about a 100ms response time and

very low current (about 90µA average at 4.0V). This

mode allows the device to be synchronized to an

external clock source, which can be used to either

suppress external interference (such as from

50/60Hz wiring) or to decrease response time

(which will also increase power consumption).

Spread-spectrum operation is not directly supported

in this mode. Sync usage is optional; the Sync pin

should be grounded if unused.

Fast, Spread-Spectrum mode: In this mode the device

operates with ~40ms response times but higher current drain

(~1.5mA @ 4.0V). This mode also supports spread-spectrum

operation via a few optional passive parts (if desired ). Sync

operation is not supported in this mode.

1.3 Electrode Drive; Wiring

The QT240 has four completely independent sensing

channels. The conversion process treats Cs on each channel

as a floating transfer capacitor; as a direct result, sense

electrodes can be connected to either SNS pin and the

sensitivity and basic function will be the same; however

electrodes should be connected to SNSnK lines to reduce

EMI susceptibility.

The PCB traces, wiring, and any components associated

with or in contact with either SNS pin will become touch

sensitive and should be treated with caution to limit the touch

area to the desired location.

3

QT240R R1.11/1006

Figure 1.1 Low Power, Synchronized Circuit

S1

S3

S2

VDD

10nF

CS1

62K

R4

10nF

CS3

10nF

CS4

2.2K

RS3

VDD

1M

R2

22K

RSNS1

2.2K

RS1

22K

RSNS4

2.2K

RS4

OPT1

OUT1

SYNC

OUT4

OUT2

OUT3

10nF

CS2

RS2

2.2K

1M

R1

VDD

1M

R3

10 second

timeout shown

SPEED

OPT

22K

RSNS2

OPT2

22K

RSNS3

QT240-ISS

Sense pin (to Cs3, electrode); OPT1

SNS3K

20

Sense pin (to Rs3 + Cs3)

SNS3

19

Sense pin (to Cs4, electrode); OPT2

SNS4K

18

Sense pin (to Rs4 + Cs4)

SNS4

17

Ground or no connect

VSS

16

Oscillator bias in

OSC

15

Power: +4.0 to +5V locally regulated

VDD

14

Reset pin, active low. Can usually tie to Vdd.

/RES

13

Output, key 4

OUT4

12

Unbonded internally

n.c.

11

Unbonded internally

n.c.

10

Sync in and/or spread-spectrum drive

SYNC/SS

9

Ground

VSS

8

Output, key 3

OUT3

7

Output, key 2

OUT2

6

Output, key 1

OUT1

5

Sense pin (to Cs1, electrode); speed option

SNS1K

4

Sense pin (to Rs1 + Cs1)

SNS1

3

Sense pin (to Cs2, electrode)

SNS2K

2

Sense pin (to Rs2 + Cs2)

SNS2

1

Description

Name

Pin

Table 1.1 Pin Listing - QT240-ISS