MCP2036-ESLASHP datasheet(10/30 Pages) MICROCHIP

10 / 30 page

MCP2036

DS22186B-page 10

The peak-to-peak amplitude of the resulting triangular

waveform, at the coil driver input, is shown in

Equation 3-5:

EQUATION 3-5:

From the previous equation, the designer should

choose values for VPKPK and RIN. Using the equation

above, the value of CIN will be:

EQUATION 3-6:

The amplitude of the pulsed current applied to key

inductors will be:

EQUATION 3-7:

This current produces a pulsed voltage to key inductors

ends. The amplitude of this voltage will be:

EQUATION 3-8:

The total voltage across both the reference and sensor

coils would be double (two series inductors). For a

specific power supply voltage, half of this power supply,

relative to the voltage reference, is available for output

amplifier/detector. Assuming a 30% margin, the

desired gain for the detector should be about:

EQUATION 3-9:

The gain of the amplifier is user-settable, using an

external resistor, RGAIN. The value of that resistor will

be determined using the following equation:

EQUATION 3-10:

With a 10-bit ADC, using oversampling and averaging

techniques, the effective resolution is close to 11 bits.

As shown in AN1239, “Inductive Touch Sensor

Design

”, the typical shift in sensor impedance is typi-

cally 3-4%, so the actual number of counts per press is

typically between 20 and 40 counts. In this way, the

microcontroller firmware could easily detect press

event.

Note:

VPKPK should not exceed specified value

(600mV) for best performance.

Note:

Assuming a power supply of 5V and

VPKPK=500mV, for RIN=3.9KΩ, CIN should

have about 320pF. A 330pF capacitor will

be used.

Note:

For a PWM frequency of 2 MHz and

inductor value of 2.7µH, the amplitude of

pulsed voltage will be:

PK PK

ΔV

PKPK

–

INCIN

------------------

⎝⎠

⎛⎞

exp

–

INCIN

------------------

⎝⎠

⎛⎞

exp

-------------------------------------------

•

PKPK

–

PKPK

----------------------------------------

⎝⎠

⎜⎟

⎛⎞

•

-------------------------------------------------------------------

PKPK

–

PKPK

----------------------------------------

⎝⎠

⎜⎟

⎛⎞

•

-------------------------------------------------------------------------------------

ΔIV

PK PK

DRV

•

DRV

- Gain of Coil Driver

ΔUL ΔI

Δt

------

•

PK PK

DRV

•

- PWM Frequency

- Inductance of Key Inductor

ΔU 10.8mV

Note:

For a power supply of 5V and

ΔU = 10mV,

the resulted gain is 81. To obtain this gain,

RGAIN = 820kOhm should be used.

Gain

70%

-----------

⎝⎠

⎛⎞

•

ΔU

•

---------------------------------

Gain

GAIN/10kOhm

∼

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