MCP2036-ESLASHP datasheet(4/30 Pages) MICROCHIP

4 / 30 page

MCP2036

DS22186B-page 4

1.1

Coil Driver

The coil driver produces the excitation current for the

sensor coils.

The coil driver input is derived from the digital clock

supplied to the CLK input. The digital signal is first

filtered through a low-pass filter, composed of RIN and

CIN, and passed to the DRVIN input. The driver will

create a triangular current in phase and proportional

with the input voltage. Because the digital drive into the

RIN-CIN filter has a 50% duty cycle, the voltage on the

DRVIN input will be centered at VDD/2. The relationship

between voltage, current, inductance and frequency is

shown in Equation 1-1.

EQUATION 1-1:

1.2

Synchronous Detector and Output

Amplifier

The Synchronous Detector has two inputs, LREF and

LBTN, selectable by REFSEL. This routes either signal

into the frequency mixer of the detector. The frequency

mixer then converts the AC waveform into a pulsed DC

signal which is amplified and filtered.

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

external resistor, RGAIN (see Equation 1-2).

EQUATION 1-2:

An ADC plus firmware algorithm then digitizes the

detector output voltage and uses the resulting data to

detect a key press event.

1.3

Virtual Ground Voltage Reference

Circuit

To create both an inverting and non-inverting amplifier

topology, a pseudo split supply design is required. To

generate the dual supplies required, a rail splitter is

included, which generates the virtual ground by creat-

ing a voltage output at VDD/2. The output is used by the

external passive network of the Detector/Amplifier sec-

tion as a reference on the non-inverting input. A bypass

capacitor of 0.1uF is required to ensure the stability of

the output. For reference accuracy, no more than 3mA

should be supplied to, or drawn from the reference

output pin.

ΔV

OUT

ΔI

DRV

COIL

•

DRV

•

()

OUT

Pulsed Output Voltage

ΔI

DR V

AC Drive Current Amplitude

DRV

AC Drive Current Frequency

COIL

Inductance of the Sensor Coil

Note:

These equations assume a 50% duty cycle.

Note:

The

output

amplifier/filter

uses

differential connection, so its output is

centered to VREF (VDD/2). The amplitude

of the detected signal should be calculated

as the difference between voltages at the

output of the detector and the reference

voltage.

Gain

GAIN 10kOhm

⁄

∼

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