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U3280M Arkusz danych(PDF) 7 Page - ATMEL Corporation |
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U3280M Arkusz danych(HTML) 7 Page - ATMEL Corporation |
7 / 19 page 7 Atmel U3280M [DATASHEET] 4688E–RFID–05/13 4.3 Power Supply The Atmel® U3280M has a power management that handles two power supply sources. Normally, the IC is supplied by a battery at the VBatt pin. If a magnetic field is applied at the LC-resonant circuit of the device, the field detection circuit switches automatically from VBatt to field supply. The VDD 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 VDD 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 (Vcoil) from the coil inputs becomes higher than the field-on-detection voltage (VFDon), even if no battery voltage is available (0 < VBatt < 1.8V). It switches back to battery if the coil voltage becomes lower than the field-off-detection voltage (VFDoff). The field detection stage of the power management has low pass characteristics to suppress noise. An applied field needs a time delay tBFS (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 VDDC (2.9V). During field supply, the battery is switched off and VDD changes to VDDC. 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 battery 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 communicate 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. 4.3.3 Buffer Capacitor CB The buffer capacitor connected at VDD is used to buffer the supply voltage for the microcontroller and the EEPROM during field supply. It smooths the rectified AC from the coil and buffers the supply voltage during modulation and gaps in the field. The size of this capacitor depends on the application. It must be of a dimension so that during modulation and gaps the ripple on the supply voltage is in the range of 100mV to 300mV. During gaps and damping the capacitor is used to supply the device, which means the size of the capacitor depends on the length of the gaps and damping cycles. V Coil > VFDon for t > tBFS V Coil < VFDon for t > tBFS Battery Supply (V Batt) Field Supply Table 4-1. Example for a 350µA Supply Current, 200mV Ripple at VDD No Field Supply During Necessary CB 250µs 470nF 500µs 1000nF |
Podobny numer części - U3280M_11 |
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Podobny opis - U3280M_11 |
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