QT118H 查看數據表（PDF） - Quantum Research Group

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QT118H

CHARGE-TRANSFER TOUCH SENSOR

Quantum Research Group 

Figure 2-5 Eliminating HB Pulses

GATE OR

M ICR O IN P UT

CMOS

Co

1 00pF

2

OUT

3

OPT 1

7

S NS 2

5

GA IN

4

OPT 2

6

S NS 1

metal disc will act as the sensing electrode. Piezo transducer

capacitances typically range from 6nF to 30nF (0.006µF to

0.03µF) in value; at the lower end of this range an additional

capacitor should be added to bring the total Cs across SNS1

and SNS2 to at least 10nF, or more if Cx is large.

The burst acquisition process induces a small but audible

voltage step across the piezo resonator, which occurs when

SNS1 and SNS2 rapidly discharge residual voltage stored on

the resonator. The resulting slight clicking sound can be

used to provide an audible confirmation of functionality if

desired, or, it can be suppressed by placing a non-critical 1M

to 2M ohm bleed resistor in parallel with the resonator. The

resistor acts to slowly discharge the resonator, preempting

the occurrence of the harmonic-rich step (Figure 2-6).

With the resistor in place, an almost inaudible clicking sound

may still be heard, which is caused by the small charge

buildup across the piezo device during each burst.

2.2.6 OUTPUT DRIVE

The QT118H’s `output is active high and it can source 1mA

or sink 5mA of non-inductive current.

Care should be taken when the IC and the load are both

powered from the same supply, and the supply is minimally

regulated. The device derives its internal references from the

power supply, and sensitivity shifts can occur with changes

in Vdd, as happens when loads are switched on. This can

induce detection ‘cycling’, whereby an object is detected, the

load is turned on, the supply sags, the detection is no longer

sensed, the load is turned off, the supply rises and the object

is reacquired, ad infinitum. To prevent this occurrence, the

output should only be lightly loaded if the device is operated

from an unregulated supply, e.g. batteries. Detection

‘stiction’, the opposite effect, can occur if a load is shed

when Out is active.

3 - CIRCUIT GUIDELINES

3.1 SAMPLE CAPACITOR

When used for most applications, the charge sampler Cs

can be virtually any plastic film or ceramic capacitor. The

type should be relatively stable in the anticipated

temperature range. If fast temperature swings are expected,

especially with higher sensitivities, more stable capacitors be

required, for example PPS film, X7R, or NPO/C0G ceramic.

3.2 PIEZO SOUNDER

The use of a piezo sounder in place of Cs is described in the

previous section. Piezo sounders have very high,

uncharacterized thermal coefficients and should not be used

if fast temperature swings are anticipated, especially at high

gains.

3.3 OPTION STRAPPING

The option pins Opt1 and Opt2 should never be left floating.

If they are floated, the device will draw excess power and the

options will not be properly read on powerup. Intentionally,

there are no pullup resistors on these lines, since pullup

resistors add to power drain if tied low.

The Gain input is designed to be floated for sensing one of

the three gain settings. It should never be connected to a

pullup resistor or tied to anything other than Sns1 or Sns2.

Table 2-1 shows the option strap configurations available.

3.4 POWER SUPPLY, PCB LAYOUT

The power supply can range from 2.5 to 5.0 volts. At 3 volts

current drain averages less than 20µA in most cases, but

Figure 2-6 Damping Piezo Clicks with Rx

+2.5 to 5

1

2

V dd

7

OUT

S NS 1

SE NS ING

E LEC T R O DE

3

O PT 1

5

GA IN

4

O PT 2

6

S NS 2

Vss

8

Rx

Cx

can be higher if Cs is large. Large Cx values will actually

decrease power drain. Operation can be from batteries, but

be cautious about loads causing supply droop (see Output

Drive, previous section).

As battery voltage sags with use or fluctuates slowly with

temperature, the IC will track and compensate for these

changes automatically with only minor changes in sensitivity.

If the power supply is shared with another electronic system,

care should be taken to assure that the supply is free of

digital spikes, sags, and surges which can adversely affect

the device. The IC will track slow changes in Vdd, but it can

be affected by rapid voltage steps.

if desired, the supply can be regulated using a conventional

low current regulator, for example CMOS regulators that

have nanoamp quiescent currents. Care should be taken that

the regulator does not have a minimum load specification,

which almost certainly will be violated by the QT118H's low

current requirement.

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6

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