DA1641.004
1 February, 2019
APPLICATION INFORMATION (continued)
EXAMPLE 2: Capacitance increase sensing
fCLK = 10Hz, fREF = 100kHz, CREF = 3.2pF, CMON = 3.7pF i.e. C=CMON - CREF = 3.7pF - 3.2pF = +0.5pF
CALARM = (3.2pF+3.7pF)/2=3.45pF
RCLK = 1/(8192*6.1pF*10Hz)/2=2M
RCOM = 1/(2*100kHz*3.45pF)=1.4493M
RDEC = 0 (shorted)
RINC = 1/(2*100kHz*3.2pF)-1.4493M=113.22k
Important note: In the above RCOM and RINC calculations there must be used non-rounded calculated resistor
values. The resistor value rounding to standard series values can be done only in the next step.
RCOM (E24 5% series) = 1.4493M 1.5M
RINC (E24 5% series) = 113.22k 110k
Alarm threshold check: CALARM=3.2pF*(110k/1.5M+1)=3.43pF OK! (very close to target 3.45pF)
Capacitor andta resistor values for charge pump and piezo driver
The external CIN, CF1, CF2 and COUT capacitors must be low loss (low ESR) ceramic capacitors.
Recommended resistor and capacitor values are shown in the table 2.
Table 2. Recommended capacitor and resistor values
Capacitor
Nominal value
Voltage rating
CIN
min 6.3V
min 6.3V
min 16V
min 16V
10F (min 1F)
100nF
100nF
CF1, CF2
COUT
CS
470pF
RB
1.5M
RF
200k
Note: Some capacitor dielectric materials such as Y5V have strong voltage dependence. The actual capacitance
value may reduce remarkably when operating near rated voltage. In such case the nominal capacitor value should
be chosen larger to compensate the voltage dependence.
The input (CIN), flying (CF1, CF2) and output (COUT) capacitor value selections have influence on output (VOUT)
ripple and disturbances at supply voltage input (VIN). Table 3 shows alternative capacitor value selections in
different applications. In battery operated applications it is recommended to use CIN=10F which keeps the start-
up inrush current low.
Table 3. Capacitor value selections in different applications
CIN
CF1-2
COUT
Application
1µF
10µF
10µF
100nF
100nF
100nF
100nF
100nF
1µF
Minimum size layout
Low input disturbances
Low input disturbances and low output ripple
Note: the voltage ripple at VOUT output is approximately proportional to ratio of piezo load capacitance (CPIEZO
)
and charge pump output capacitor (COUT). Then the output ripple can be reduced by choosing output capacitor
value which is much larger relative to piezo capacitance value. However, note that large output capacitor also
lengthens output voltage rise time and increases inrush current during start-up.
7 (14)
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