3 April 2000
5.5 DC SPECIFICATIONS VDD = 3.0V, CS = 10NF, CX = 10PF, TA = RECOMMENDED RANGE
Parameter
Notes
Description
Min
Typ
Max
Units
IDD
IDD
S
Supply current, master mode
Supply current, slave mode
Sensitivity range
60
700
-
µA
µA
fF
Cs = 10nF to 100nF
Cs = 10nF to 100nF
Typical, see figs 1, 2; Note 1, 2
1,000
28
Note 1: All percentage thresholds have been eliminated and replaced with a fixed threshold w.r.t. reference level
Note 2: Sensitivity depends on value of Cx and Cs. Refer to Charts 1, 2.
Piezo Driver Note
The piezo acoustic driver has been removed, as the duration required to operate the beeper would interfere with
the sensing interval and slow down the device.
Sensitivity Adjustment Note
The device has a fixed threshold point of 6 counts of deviation. Gain pin adjustment (pin 5) has been eliminated
and replaced with a strap option for Master / Slave mode:
Table 1-1 Master/Slave Strap Options
Mode
Master
Slave
Tie Pin 5 to:
Vdd
Gnd
The sensitivity of the circuit is governed by the relative sizes of Cs and Cx. A detection is made if the signal
rises by 6 counts from the reference level; this amount, unlike the QT110, is not ratiometric to the signal level
and therefore the sensitivity can be altered by simply changing Cs. To provide a consistent level of sensitivity,
only stable types of capacitors are recommended for Cs, such as NPO, C0G, PPS film, and certain types of
polycarbonate when used over normal room temperature ranges.
Larger values of Cs will make the sensor more sensitive, while larger amounts of Cx will desensitize it (see
Charts 1, 2). Minimizing stray Cx is crucial if high levels of sensitivity are desired. By using values of Cs around
0.47uF (470nF), proximity distances of several centimeters can easily be obtained from small electrodes.
NOTE:
It is extremely important to maintain stable levels of Vdd, as the supply is used as a
reference. Minor fluctuations in Vdd WILL cause false triggers or rapid swings in gain. DO NOT use
bench power supplies or supply circuits shared with other digital functions. Ordinary 78L05 class
regulators are fine in almost all cases. The QT115 is an extremely sensitive device... do not take
power supply issues lightly.
Pin Functions
The QT115 pins are defined as shown:
Table 2-1 QT115 Pin Functions
PIN
Function
Vdd
Description
Power, +3V to +5V
Active-low output
1
2
3
4
Out
Sync Pulse Out (master or slave mode)
Sync Pulse In (slave mode only)
20us nominal negative sync pulse
>10us, <50us negative pulse input to trigger in slave mode
In Master Mode: Connect to either Gnd or Vcc.
5
6
7
8
Master/Slave select
Vdd = Master mode, Gnd = Slave mode (strap option)
SNS1
SNS2
Gnd
QT Sense pin 1
QT Sense pin 2
Ground, 0V
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