7
4688D–RFID–03/07
U3280M
4.3 Power Supply
The U3280M has a power management that handles two power supply sources. Normally, 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.
4.3.1 Automatic Power Management
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 battery voltage is
available (0 < V
Batt
< 1.8V). 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 suppress
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 4-3. Switch Conditions for Power Management
Note: The rectified supply voltage from the coil is limited to V
DDC
(2.9V). During field supply, the battery
is switched off and V
DD
changes to V
DDC
.
4.3.2 Controlling Power Management via the Serial Interface
The automatic mode of the power management can be switched off and on by a command from
the microcontroller. If the automatic mode is switched off, the IC is always supplied by the bat-
tery up to the next power-on reset or to a switch-on command. The power management’s on and
off command must be transferred via the serial interface.
If the power management is switched off and the device is supplied from the battery, it can com-
municate via the field without loading the field. This mode can be used to realize applications
with battery supply if the field is too weak to supply the IC with power.
Battery
supply
Field
supply
V
Coil
> V
FDon
for t > t
BFS
V
Coil
< V
FDon
for t > t
BFS
(V
Batt
)
