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 can be higher if
Cs is large. Interestingly, 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 QT110's low
current requirement.
Since the IC operates in a burst mode, almost all the power
is consumed during the course of each burst. During the
time between bursts the sensor is quiescent.
For proper operation a 100nF (0.1uF) ceramic bypass
capacitor should be used between Vdd and Vss; the bypass
cap should be placed very close to the device’s power pins.
3.4.1 MEASURING SUPPLY CURRENT
Measuring average power consumption is a fairly difficult
task, due to the burst nature of the device’s operation. Even
a good quality RMS DMM will have difficulty tracking the
relatively slow burst rate.
The simplest method for measuring average current is to
replace the power supply with a large value low-leakage
electrolytic capacitor, for example 2,700µF. 'Soak' the
capacitor by connecting it to a bench supply at the desired
operating voltage for 24 hours to form the electrolyte and
reduce leakage to a minimum. Connect the capacitor to the
circuit at T=0, making sure there will be no detections during
the measurement interval; at T=30 seconds measure the
capacitor's voltage with a DMM. Repeat the test without a
load to measure the capacitor's internal leakage, and
subtract the internal leakage result from the voltage droop
measured during the QT110 load test. Be sure the DMM is
connected only at the end of each test, to prevent the DMM's
impedance from contributing to the capacitor's discharge.
Supply drain can be calculated from the adjusted voltage
droop using the basic charge equation:
i =
✁VC
t
where C is the large supply cap value, t is the elapsed
measurement time in seconds, and ∆V is the adjusted
voltage droop on C.
3.4.2 ESD PROTECTION
In cases where the electrode is placed behind a dielectric
panel, the IC will be protected from direct static discharge.
However, even with a panel, transients can still flow into the
electrode via induction, or in extreme cases, via dielectric
breakdown. Porous materials may allow a spark to tunnel
right through the material; partially conducting materials like
'pink poly' will conduct the ESD right to the electrode. Testing
is required to reveal any problems. The device does have
diode protection on its terminals which can absorb and
protect the device from most induced discharges, up to
20mA; the usefulness of the internal clamping will depending
on the dielectric properties, panel thickness, and rise time of
the ESD transients.
ESD dissipation can be aided further with an added diode
protection network as shown in Figure 2-7, in extreme cases.
Because the charge and transfer times of the QT110 are
relatively long, the circuit can tolerate very large values of
Re, more than 100k ohms in most cases where electrode Cx
is small. The added diodes shown (1N4150, BAV99 or
equivalent low-C diodes) will shunt the ESD transients away
from the part, and Re1 will current limit the rest into the
QT110's own internal clamp diodes. C1 should be around
10µF if it is to absorb positive transients from a human body
model standpoint without rising in value by more than 1 volt.
If desired C1 can be replaced with an appropriate zener
diode.
Directly placing semiconductor transient protection
devices or MOV's on the sense lead is not advised; these devices
have extremely large amounts of parasitic C which will swamp the
capacitance of the electrode.
Re1 should be as large as possible given the load value of
Cx and the diode capacitances of D1 and D2. Re1 should be
low enough to permit at least 6 timeconstants of RC to occur
during the charge and transfer phases.
- 8 -
Figure 2-7 ESD Protection
3
46
5
1
+2.5 to 5
72
OUT
OPT1
OPT2
GAIN
SNS1
SNS2
Vss
Vdd
8
R
C
D
D
R
R
e3
s
2
1
e2
e1
S ENSING
ELEC TR ODE
10µF
+
C1
