EM4022V20WS11 datasheet(4/15 Pages) EMMICRO

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 2002, EM Microelectronic-Marin SA

www.emmicroelectronic.com

EM4022

Power storage capacitor calculation

The global current consumption of the device defines the

external storage capacitor.

When the device modulate, the supply voltage is picked

from the supply capacitor and should never decrease

under the falling edge of the power on reset (VPONF). If

this occurs, the device goes in a reset mode and any

data transmission is aborted. The worst case for the

storage capacitor calculation is when the device is put in

the electromagnetic field. At this moment the supply

reaches the VPONR and start to modulate. During

modulation the power store in the capacitor must be high

enough so that at the end of the modulation the supply is

higher than VPORF.. This means that the voltage reduction

on the capacitor must be less than the hysteresis of the

power on reset (VPHYS).

And this when the chip has a supply voltage of around

the power on reset threshold

The total current consumption from the storage capacitor

is defined by the modulation current IMOD,

This current is the consumption of the power on reset

block, oscillator and the logic which work at a typical

frequency of 125KHz. The GAP current is also included

in this parameter.

The duration where this currents is present for the

capacitor calculation, is dependent of the data rate

Calculation example :

Below we define typical cases combinations :

FOSC = 125 KHz

VPHYS = 120 mV

IMOD = 9

µA

Data rate is 4 KBaud.

CPx

BaudRate

MOD

OSC

HYS

−

* *

128 10

9 10

128 10

125 10 160 10

4 10

14 4

Of course, this value can be adapted to the

electromagnetic power and to the performances that

must be achieved. If a tag is put in a field within a short

time, the emitting power must be high enough to charge

up the capacitor.

The chip integrates a 140pF supply capacitor.

Block Diagram

Shunt

PON

VDD

VSS

GAP

TST

VDD

OSC

COIL1

GAP

XCLK TMC

VSS

VDD

COIL2

LOGIC

VSS VSS VSS

Fig. 5

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