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 (V
PONF
). 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 V
PONR
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 V
PORF.
. This means that the voltage reduction
on the capacitor must be less than the hysteresis of the
power on reset (V
PHYS
).
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 I
MOD
,
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 :
F
OSC
= 125 KHz
V
PHYS
= 120 mV
I
MOD
= 9
µ
A
Data rate is 4 KBaud.
I
MOD
* 128 * 10
3
CPx
=
F
OSC
*
V
HYS
*
BaudRate
9 * 10
−
6
* 128 * 10
3
=
=
14.4
nF
125 * 10
3
* 160 * 10
−
3
* 4 * 10
3
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
M
VDD
VDD
P
R
PON
COIL1
D2
Q1
LOGIC
D4
CR
D3
Shunt
Q2
N
GAP
TST
C
VSS
OSC
CP
COIL2
D1
DG
VDD
VDD
RG
VSS VSS VSS
VSS
GAP
CG
SI
XCLK TMC
Fig. 5
Copyright
2002, EM Microelectronic-Marin SA
4
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EMMICRO [ EM MICROELECTRONIC - MARIN SA ]
