U3280M Datasheet, PDF (5/18 Pages) ATMEL Corporation – TRANSPONDER INTERFACE FOR MICROCONTROLLER

Digital Output of the
Gap-detection Stage
(NGAP)
Wake-up Signal
Power Supply
Automatic Power
Management
U3280M
NGAP = 0: gap detected/no field
V_coil-peak = V_FDoff
NGAP = 1: field detected
V_coil-peak = V_FDon
Note: No amplifier is used in the gap-detection stage. A digital Schmitt trigger evaluates the
rectified and smoothed coil voltage.
If a field is applied at the coil of the transponder interface, the microcontroller can be
woken up with the wake-up signal at the NGAP pin. For that purpose, the NGAP pin
must be connected to an interrupt input of the microcontroller. A high level at the NGAP
output indicates an applied field and can be used as a wake-up signal for the microcon-
troller via an interrupt. The wake-up signal is generated if power management switches
to field supply. The field-detection stage of the power management has lowpass charac-
teristics to avoid generating wake-up signals and unnecessary switching between
battery and field supply in case of interferences at the coil inputs.
The U3280M has a power management that handles two power supply sources. Nor-
mally, the IC is supplied by a battery at the V_Batt pin. If a magnetic field is applied at the
LC-resonant circuit of the device, the field detection circuit switches automatically from
V_Batt to field supply.
The V_DD pin is used to connect a capacitor to smooth the voltage from the rectifier and
to buffer the power while the field is modulated by gaps and damping. The EEPROM
and the connected controller always operate with the voltage at the V_DD pin.
Note: During field supply the maximum energy from the field is used if a high level is applied at
the MOD input.
There are different conditions that cause a switch from the battery to field and back from
field to the battery.
The power management switches from battery to field if the rectified voltage (V_coil) from
the coil inputs becomes higher than the field-on-detection voltage (V_FDon), even if no bat-
tery voltage is available (0 < V_Batt < 1.8 V). It switches back to battery if the coil voltage
becomes lower than the field-off-detection voltage (V_FDoff).
The field detection stage of the power management has low pass characteristics to sup-
press noise. An applied field needs a time delay t_BFS (battery-to-field switch delay) to
change the power supply. If the field is removed from the coil, the power management
will generate a reset that can be connected to the microcontroller.
Figure 5. Switch Conditions for Power Management
V_Coil > V_FDon for t > t_BFS
Battery
supply
(V_Batt)
Field
supply
V_Coil < V_FDon for t > t_BFS
Note: The rectified supply voltage from the coil is limited to V_DDC (2.9 V). During field supply,
the battery is switched off and V_DD changes to V_DDC.
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4688B–RFID–12/04