EM4022
Calculation example :
Power storage capacitor calculation
Below we define typical cases combinations :
The global current consumption of the device defines the
external storage capacitor.
FOSC = 125 KHz
VPHYS = 120 mV
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).
I
MOD = 9 µA
Data rate is 4 KBaud.
I
MOD *128*103
CPx =
F
OSC *VHYS * BaudRate
9*10−6 *128*103
=
= 14.4nF
125*103 *160*10−3 *4*103
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.
And this when the chip has a supply voltage of around
the power on reset threshold
The total current consumption from the storage capacitor
The chip integrates a 140pF supply 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
Block Diagram
M
VDD
VDD
P
N
R
C
PON
COIL1
D2
Q1
Q2
LOGIC
CP
D4
D3
Shunt
CR
GAP TST
VSS
OSC
COIL2
VSS
D1
VDD
VDD
DG
CG
RG
GAP
VSS VSS VSS
SI XCLK TMC
Fig. 5
4
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