QT102-ISG View Datasheet(PDF) - Quantum Research Group

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Description

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QT102-ISG

QTOUCH™ TOGGLE-MODE CHARGE-TRANSFER IC WITH POWER MANAGEMENT FUNCTIONS

Quantum Research Group 

3.4 Output Drive

The OUT pin is active high and can sink or source up to 2mA.

When a large value of Cs (>20nF) is used the OUT current

should be limited to <1mA to prevent gain-shifting side effects,

which happen when the load current creates voltage drops on

the die and bonding wires; these small shifts can materially

influence the signal level to cause detection instability.

3.5 Auto Off Delay

3.5.1 Introduction

In addition to toggling the output on/off with key touch, the

QT102 can automatically switch the output off after a specific

time. This feature can be used to save power in situations

where the switched device could be left on inadvertently.

The QT102 has:

• three predefined delay times (Section 3.5.2)

• the ability to set a user-programmed delay

(Section 3.5.3)

• the ability to override the auto off delay (Section 3.5.4)

The TIME and SNS pins are used to configure the Auto Off

delay and must always be connected in one of the ways

described in Sections 3.5.2, 3.5.3 and 3.5.4.

3.5.2 Auto Off - Predefined Delay

To configure the predefined delay the TIME pin is hard wired

to Vss, Vdd or OUT as shown in Tables 3.2 and 3.3. This

provides nominal values of 15 minutes, 60 minutes or infinity

(remains on until toggled off).

A single 1M resistor (Rm) is connected between the SNS pin

and the logic level Vm to provide three auto off functions:

delay multiplication, delay override and delay retriggering. On

power-up the logic level at Vm is assessed and the delay

multiplication factor is set to x1 or x24 accordingly (see

Figure 3.5 and Table 3.4). At the end of each acquisition

cycle the logic level of Vm is monitored to see if an Auto off

delay override is required (see Section 3.5.4).

Setting the delay multiplier to x24 will decrease the key

sensitivity. To compensate, it may be necessary to increase

the value of Cs.

Figure 3.5 Predefined Delay

SENSE

ELECTRODE

VDD

RS

5

VDD

3 SNSK

CS

4 SNS

OUT 1

Table 3.2 Predefined Auto-off Delay

(Active High Output)

Vt

Auto-off delay (to)

Vss

Infinity (remain on until toggled to off)

Vdd

15 minutes

OUT

60 minutes

Table 3.3 Predefined Auto-off Delay

(Active Low Output)

Vt

Auto-off delay (to)

Vss

15 minutes

VDD

Infinity (remain on until toggled to off)

OUT

60 minutes

Table 3.4 Auto-off Delay Multiplier

Vm

Auto-off delay multiplier

Vss

to x 1

VDD

to x 24

3.5.3 Auto Off - User-programmed Delay

If a user-programmed delay is required a resistor and capacitor

can be used to set the auto-off delay (see Table 3.5 and

Figure 3.6 The delay time is dependent on the RC time constant

(Rt x Ct) the output polarity and the supply voltage. Section 3.5.5

gives full details of how to configure the QT102 to have auto-off

delay times ranging from 1 minute to up to 24 hours.

Table 3.5 Programmable Auto Off Delay

(Vm = Vss (delay multiplier = 1), VDD = 3.5V)

Output Type Auto Off Delay (seconds)

Active high (Rt x Ct x 15) / 42

Active low (Rt x Ct x 15) / 14.3

K values (42 and 14.3) are obtained from Figures 3.10

and 3.11.

Note: Rt is in k, Ct is in nF.

Figure 3.6 Programmable Delay

SENSE

ELECTRODE

VDD

RS

5

VDD

3 SNSK

CS

4 SNS

OUT 1

Rt

TIME 6

Rm

VSS

Ct

Rop

2

Vm

Rm

Vm

TIME 6 Vt

VSS

Rop

2

3.5.4 Auto Off - Overriding the Auto Off Delay

In normal operation the QT102 output is turned off

automatically after the auto-off delay. In some applications it

may be useful to extend the auto-off delay (‘sustain’ function)

or to switch the output off immediately ('cancel' function). This

can be achieved by pulsing the voltage on the delay multiplier

resistor Rm as shown in Figures 3.7 and 3.8.

lQ

8

QT102_2R3.04_0807

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