EM4006

Copyright

 2001, EM Microelectronic-Marin SA

6

www.emmicroelectronic.com

Modulator Switch

Due to the low impedance of the antenna and resonant

capacitor the Modulator Switch has to present low RF

impedance when switched ON (about 100 ohms).

The minimum time period with the Modulator Switch ON

is 38 µs. At lower data rates this time is even much

longer. The current consumption of divider chain running

at 13 MHz is near 60 µA. Putting together this two figures

it is clear that it is not possible to supply the IC during the

time the Modulator Switch is ON from the integrated

Supply Buffer Capacitor which value is approximately 140

pF. The IC has to get power from the RF field also during

the time the Modulator Switch is ON.

This problem is solved by putting the Modulator Switch on

the output of the Rectifier (between VDD and VSS) and

regulating its ON resistance in function of supply voltage.

When the supply voltage is high the ON impedance is low.

When the supply voltage drops near the region where the

operation of the IC at 13.56 MHz is not guaranteed the

ON impedance is increased in order to prevent further

drop.

NRZ-L

STREAM

DM-M

CODED

1

1

1

1

1

1

0

0

0

0

0

Bit i-1

Bit i

x

1

no transition at the beginning of Bit i,

transition in the middle of Bit i

0

0

transition at the beginning of Bit i,

no transition in the middle of Bit i

1

0

no transition at the beginning of Bit i,

no transition in the middle of Bit i

Fig. 6

Power Supply Management

For a correct operation, the device must be initialised.

When the transponder is put in the RF field, the supply

voltage increases until it achieves Vr limit (see Figure 7).

During this time and for an additionnal 64 bit period, the

modulator switch is on and the device initialises its

internal logic.

At this point, the data transmission starts and runs while

the supply voltage is higher than Vmin.

If the supply

voltage decreases under this limit, the device is again in

an initialising state and the modulator is on.