[Geben Sie hier die Überschrift ein] - MPC

MPC-WORKSHOP JULI 2012
Figure 4: Model to calculate coupling factor between reader
and tag antenna, both aligned.
The high power part consists of a power rectifier and
a LDO regulator, both get activated when enough RF
energy is available. The LDO provides V_Out, which
can be adjusted by an external feedback resistor divider
(not shown), connected to the analog input FB
(feedback).
The antenna inputs are directly connected to the over
voltage protection circuit. This circuit is very critical
because there are absolute limits for the voltage at the
pads defined by the technology (breakdown voltages
of the gates, about 3.6 V for the referenced technology).
This circuit shall not be active under normal operation;
it is only used in case of overvoltage input,
which may happen because of the undamped resonance
of the antenna, if the load at the output is
switched off unintentionally.
The communication part consists of the passive rectifier,
low pass filter, Schmitt-trigger, high pass filter,
comparator and modulator. For clock recovery a RS
flip flop is used. The circuit blocks are further described
later in detail.
III. PROTECTION CIRCUIT
The protection circuit consists of the ESD protection
of the pad cells (library) and additional circuits. The
operation of a transponder in the magnetic field depends
mainly upon two factors which are,
• Coupling factor (K)
• Quality factor (Q)
The coupling factor between the reader antenna and
the tag antenna depends on geometrical parameters as
shown in figure 4. The coupling factor K can be calculated
by equation 1,
µ 0π
⋅ N1N
K(
d)
=
2 L . L
1
2
2
⋅
2
B
2
B
r
r
⋅ r
2
T
2
+ d
(1)
Figure 5: Overvoltage protection circuit.
Equation 1 shows that the coupling factor K depends
on the antenna diameter of the reader rB and tag rT as
well as the distance d in between them. Normally the
value of K varies from 0.01 to 0.1. The higher the
coupling factor is, the stronger is the field. The tag has
to operate from field strength of 150 mA/m to 5 A/m,
with 8 A/m maximum sustainable field strength (ISO
15693). This is not a problem, if the energy is consumed
by a load.
Resonance of the antenna leads to an increase in
amplitude at the input pins which is proportional to the
quality factor (figure of merit) Q of the inductance/capacitance/load-circuit.
The quality factor Q is given by the relationship
RL
Q =
2πf ⋅ L
(2)
2
where RL is the equivalent load resistance of the transponder
and L is the equivalent inductance of the
transponder antenna. The read range increases with
the Q of the antenna. It is recommended to be adjusted
to ca. Q = 15 for good sensitivity. The voltage is directly
proportional to the Q factor of the circuit [5].
Figure 5 shows the protection circuit. There are three
stages of staggered transistors. The input voltage is
divided by a resistive divider, providing the gate voltages
for each stage. It is designed in such a way so
that when the voltage on the antenna is beyond the
maximum allowed limit, the transistors will switch on
and hence draw power from the input, so the antenna
is damped and the input voltage clamped to a safe
value below the critical breakdown voltage of the
transistors (ca. 3.7 V).
To protect the rest of the circuit, these transistors
must be able to carry milliamps of current. To distribute
the voltage load, the clamp transistors are placed
in separate wells, so each transistor carries only a third
of the input voltage. The switching thresholds are
taken from a voltage divider, which is supplied by the
power rectifier.
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