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MPC-WORKSHOP JULI 2012
Table 1: Characteristic data of the band gap circuit.
Output voltage 1.2 ± 0.02 V
Load Current ≤ 75 µA
PSRR @ 10KHz -26 dB
Current Consumption @1.9V 4 µA
Figure 9: Low drop out rectifier (LDO).
So, the current of X is N times more than the current
of the N BJT’s connected in parallel which are identical
to X. The value of R2 depends upon the value of R1
chosen.
Now in order to keep the current consumption small,
the value of R1 is set to 60 kΩ and the value of R2 is
600 kΩ in order to get a factor of 10.
For a successful bandgap design there must be a
compromise made between the current and the number
of passive elements. Also the PSRR is important for a
bandgap, because in an RFID application there are big
changes in the available supply voltage. Further the
PSRR must suppress the RF variations, because there
is no space for filtering the input. The bandgap concept
of figure 8 is preferred due to its high PSRR
value in comparison to the other circuits, see [7].
Some characteristic data of the bandgap circuit is
listed in table 1. Current consumption is only 4 µA at
1.9 V supply. The circuit can deliver a load of maximal
75 µA to the demodulation circuit. The output
voltage is stable over temperature range of -40 °C to
125 °C with a minimum VDD supply of 1.9 V to a
maximum VDD of 3.6 V.
C. Low Drop Out Regulator
The low drop out regulator (LDO) provides a stable
voltage supply for the external circuitry connected to
the frontend. Figure 9 shows the standard configuration
used for the LDO with external components separated
by the dotted line. The voltage Vin is taken from
the output of the power rectifier, and the reference
voltage is taken from the bandgap reference. The LDO
used is made with a PMOS pass transistor with a minimum
dropout voltage of 150 mV. The output voltage
is controlled by the resistor divider R1 and R2,
Table 2: Low drop out circuit.
Figure 10: Load modulation circuit.
which are discrete devices to stay flexible for different
applications. The external Schottky-diode is only used
with semi-passive supply, where an external voltage is
available.
The output voltage of the LDO can be adjusted between
1.2 V to 2.2 V by selecting the values of the
resistors R1 and R2.
Usually, the values of R1 and R2 are in mega ohms,
so that the quiescent current of the LDO is kept as
small as possible.
The PSRR of the error amplifier must be better than
32 dB in order to suppress any noise in the supply
voltage [8]. The output voltage Vout is given by the
equation
(4)
2
Vout = Vref
⋅ ( 1+
)
R1
The external capacitor Cout is needed to bridge and
filter the output. The external Schottky diode prevents
any reverse current flow in case Vout is greater than Vin.
This may happen when a battery or a double layer
capacitor (“gold cap”) is used at the output for intermediate
storage. Some characteristics data of the LDO
are listed in table 2.
V. COMMUNICATION
The communication circuits are modulator, field detector
circuit and demodulator.
A. Modulator
Output Voltage Range 1.2 to 2.2 V
Quiescent Current 1 µA
Max Load Current 4 mA
Drop Out Voltage 150 mV
PSRR @ 4mA Load and
10KHz
-32 dB
R
The modulator circuit used is a load modulation (onoff
amplitude shift keying) circuit for sending information
back to the reader as shown in figure 10. The
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