MC33690DW View Datasheet(PDF) - Motorola => Freescale

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Description

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MC33690DW

Standalone Tag Reader Circuit

Motorola => Freescale 

Freescale SMeCm33i6c9o0nductor, Inc.

Read function

When answering to the base station, a

transponder generates an absorption modulation

of the magnetic field. It results in an amplitude/

phase modulation of the current across the

antenna. This information is picked up at the

antenna tap point between the coil and the

capacitor. An external resistive ladder down

scales this voltage to a level compatible with the

demodulator input voltage range (see parameter

VINRD page 16).

The demodulator (see figure 2) consists of :

- an input stage (emitter follower),

- a sample & hold circuit,

- a voltage follower,

- a low offset voltage comparator.

The sampling time is automatically set to take

into account a phase shift due to the tolerances

of the antenna components (L and C) and of the

oscillator. The allowed phase shift measured at

the input RD ranges from -45° to +45°. Assuming

that the phase reference is the falling edge of the

driving signal TD1, this leads to a sampling time

phase ranging from -78.75° to 90° with discrete

steps of 11.25°. After reset condition, the

sampling time phase is +11.25°.

The antenna phase shift evaluation is only

done :

- after each wake-up command (see pages

10 to 12),

- or after reset (see page 7).

This is necessary to obtain the best demodulator

performances.

In order to ensure a fast demodulator settling

time after wake up, reset or a write sequence, the

external capacitor CEXT is preloaded at its

working voltage.

This preset occurs 256µs after switching the

antenna drivers on and its duration is 128µs.

After wake up or reset, the preset has the same

duration but begins 518µs after clock settling.

After power on reset, VSUP must meet the

minimum specified value, enabling the nominal

operation of VDD, before the start of the preset.

Otherwise the preset must be done by the user

through a standby/wake-up sequence.

Write function

Whatever the selected configuration (see

page 9), the write function is achieved by

switching on/off the output drivers TD1/2.

Once the drivers have been set in high

impedance, the load current flows alternatively

Figure 3 : Current flow when the buffers are switched off

VDD

RA

ILOAD

LA

TD1

R1

VDD

CA

TD2

© Motorola, Inc., 2002.

FMoOr TMOoRrOeLAInSfoEMrmICaOtNioDnUCOTnORTShPisROPDroUdCuTSct,

Go to: www6.freescale.com

revision 4.8, 5 February 2002

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