Figure 3.7 Override Pulse (Delay Multiplier x1)
SENSE
ELECTRODE
Figure 3.9 Overriding Auto Off
OUT
P
P
P
t
off
Bursts
O
Vm
VDD
RS
3
SNSK
5
VDD
CS
4
SNS
SNSK
C
C
C
C
OUT
1
P - override (reload auto off delay)
O - switch output off (t
off
burst time + 50ms)
C - sensor recalibration
Rm
VDD Vm
VSS
TIME
6
VSS
2
Rop
3.5.5 Configuring the User-programmed Auto-off
Delay
As described in Section 3.5.3, page 8, the QT102 can be
configured to give auto-off delays ranging from minutes to
hours by means of a simple CR network and the delay
multiplier input.
With the delay multiplier set at x1 the auto-off delay is
calculated as follows:
Delay value = integer value of Rt x Ct x 15 seconds.
K
And Rt x Ct = Delay (in seconds) x K
15
Note:
Rt is in k , Ct is in nF.
Tp
Figure 3.8 Override Pulse (Delay Multiplier x24)
SENSE
ELECTRODE
VDD
RS
3
SNSK
5
To ensure correct operation it is recommended that the value
of Rt x Ct is between 4 and 240.
K
Values outside this range may be interpreted as the hard
wired options TIME linked to OUT and TIME linked to ‘off’
respectively, causing the QT102 to use the relevant
predefined auto-off delays.
The charts in Figures 3.10 and 3.11 show typical values of K
versus supply voltage for a QT102 with active high or active
low output.
Rop
VDD
CS
4
SNS
OUT
1
Rm
TIME
6
VSS
2
Example using the formula to calculate Rt and Ct
Requirements:
•
Active high output (Vop connected to VSS)
Auto-off delay 45 minutes
VDD = 3.5V
•
•
VDD Vm
VSS
Tp
To ensure the pulse is detected it must be present for a time
greater than the burst length as shown in Table 3.6.
Proceed as follows:
1. Calculate Auto-off delay in seconds 45 x 60 = 2700
2. Obtain K from Figure 3.10, K= 42
3. Calculate Rt x Ct = 2700 x 42 = 7560
15
4. Decide on a value for Rt or Ct (e.g.Ct = 47nF)
5. Calculate Rt = 7560 = 160k
47
As an alternative to calculation, Figures 3.12 and 3.13 show
charts of typical curves of auto-off delay against resistor and
capacitor values for active high and active low outputs at
various values of VDD (delay multiplier = x1).
Table 3.6 Time Delay Pulse
Pulse Duration
tp > burst time + 10ms
(typical value 25ms)
tp > burst time + 50ms
(typical value 65ms)
Action
Retrigger (reload auto-off delay
counter)
Switch output to off state and
inhibit further touch detection until
Vm returns to original state
While Vm is held in the override state the QT102 inhibits
bursts and waits for Vm to return to its original state. When
Vm returns to its original state the QT102 performs a sensor
recalibration before continuing in its current output state.
Figure 3.9 shows override pulses being applied to a QT102
with delay multiplier set to x1.
lQ
9
QT102_2R3.04_0807
QUANTUM [ QUANTUM RESEARCH GROUP ]
