P4022 Datasheet, PDF (8/13 Pages) EM Microelectronic - MARIN SA – Multi Frequency Contactless Identification Device Anti-Collision compatible with BTGs Supertag Category Protocols

PRELIMINARY
EM MICROELECTRONIC-MARIN SA P4022
Resonance capacitor
The resonance capacitor CR has a nominal value
of 110 pF and is trimmed to ± 3%. For resonance
at 125 kHz an external 14.7 mH coil is required.
At 13.65 MHz the required coil inductance drops
to 1.2 µH.
Rectifier bridge
Diodes D1-D4 form a full wave rectifier bridge.
They have relatively large forward resistances
(100 -200 Ω ). This is quite sufficient at 128 kHz,
where the output impedance of the tuned circuit
is high, but at 13.5 MHz the diode resistance
becomes significant and external diodes have to
be used to bypass the internal ones. The diode
resistance affects the rate at which the power
capacitor CP can be charged. It also affects the
modulation depth that can be achieved.
Shunt regulator
The shunt regulator has two functions. It limits the
voltage across the logic and in high frequency
applications it limits the voltage across the
external microwave Schottky diodes, which
typically have reverse breakdown voltages less
than 5 V.
The shunt regulator draws less than 500 nA at 1 V.
Its maximum current shunt capability is 50 mA at
3.5 V.
Oscillator
The on-chip RC oscillator has a centre frequency
of 128 kHz and a spread of 30% over the full
temperature and supply range.
Power-on reset (PON)
The reset signal keeps the logic in reset when the
supply voltage is lower than the threshold voltage.
This prevents incorrect operation and spurious
transmissions when the supply voltage is too low
for the oscillator and logic to work properly. It also
ensures that transistor Q2 is off and transistor Q1 is
on during power-up to ensure that the chip starts
up.
Modulation transistor
The N channel transistor Q2 is used to modulate
the transponder coil or antenna. When it is turned
on it loads the antenna or coil, thereby changing
the load seen by the reader antenna or coil, and
effectively changing the amount of energy that is
reflected to the reader. It has an on resistance of
typically less than 40 Ω . The on resistance
affects the depth of modulation, especially at
higher carrier frequencies (> 10 MHz), where the
coil or antenna impedance can be lower than
200 Ω.
Charge preservation transistor
The P channel transistor Q1 is turned off
whenever the modulation transistor Q2 is turned
on to prevent Q2 from discharging the power
storage capacitor. This is done in a non-
overlapping manner, i.e. Q1 is first turned off
before Q2 is turned on, and Q2 is turned off before
Q1 is turned on.
Gap detection
Poly-silicon diode DG is used to detect a gap in
the illuminating field. It is a minimum sized
diode with forward resistance in the order of 2
kΩ. The low pass filter shown diagrammatically as
CG and RG actually consists of a pull-up transistor
(approximately 100 kΩ) in conjunction with the
parasitic capacitance of the GAP input pad
(approximately 2 pF). The effective time constant
is in the order of 0.2 µs.
Through the diode the GAP input will be pulled
low during each negative going cycle of the
carrier. When the carrier is switched off, the GAP
input will be pulled high by the pull-up transistor.
At very high carrier frequencies (> 100 MHz) the
carrier will be filtered out, so that the GAP input
will be low continuously when the carrier is
present. When the carrier disappears, the GAP
input will go high with the time constant of the
low pass filter. At very low frequencies the GAP
input will go high and low at each cycle of the
carrier, and will stay high when the carrier
disappears. To detect the gap, the logic must
check for a high period longer than the maximum
high period of the carrier.
As the rise and fall times of the GAP can be slow,
a Schmitt trigger is used to buffer the GAP input.
Power storage capacitor
A 94 pF power supply capacitor is included in the
layout of the P4022. This is sufficient for 64 kbit/s
applications, but 4 kbit/s applications will
required an additional external storage capacitor.
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