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Texas Instruments LF HDX System Parameter Calculation for TMS37157 (PaLFI) Application notes
Application Report
SCBA029 – July 2010
LF HDX System Parameter Calculation for TMS37157
(PaLFI)
K. Aslanidis, J. Wyatt
............................................................................................... MCU-RF Systems
ABSTRACT
The Texas Instruments low frequency half-duplex (HDX) transponder technology allows the possibility to
improve the communication distance between the transponder and reader. This application report provides
information for the calculation of the basic system parameters.
The TMS37157 TI transponder IC is based on the TI HDX RFID concept. Using the given formulas, the
performance and component parameters can be simulated and adjusted according to the application
requirements.
1
2
3
Contents
TMS37157 Dual Interface RFID IC ....................................................................................... 2
TMS37157 System Description ........................................................................................... 4
Reference ................................................................................................................... 10
List of Figures
1
2
3
4
5
6
7
8
9
........................................................................................
TMS37157 PaLFI EVM .....................................................................................................
TMS37157 PaLFI Application Example ..................................................................................
Demo Reader Antenna .....................................................................................................
Calculated Reader Field Strength Over Distance ......................................................................
Transponder Antenna Dimension .........................................................................................
Coupling Factor Over Distance ...........................................................................................
Simulation Model for Charge Voltage ....................................................................................
Typical Charge Voltage Over Distance ..................................................................................
TMS37157 PaLFI Block Diagram
2
3
4
5
6
7
8
8
9
List of Tables
1
Abbreviations ................................................................................................................ 3
2
Parameter of Demo Reader Antenna .................................................................................... 5
3
Typical Parameter of a LF Transponder Antenna ...................................................................... 7
4
Parameter for VCL Calculations
SCBA029 – July 2010
..........................................................................................
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
9
1
TMS37157 Dual Interface RFID IC
1
www.ti.com
TMS37157 Dual Interface RFID IC
The TMS37157 TI RFID transponder IC passive low-frequency interface (PaLFI) is designed to work in the
low-frequency band (134.2 kHz) and uses the HDX RFID communication protocol.
The IC provides a dual communication interface: one interface is used for the communication over the RF
interface and one for the communication over the SPI interface.
The IC fully operates as a passive RFID transponder without any need for external power supply. For
additional functionality, the IC can be directly connected to a MSP430 microcontroller via the SPI
interface.
Depending on the system parameters and antennas used on both reader and tag side, the PaLFI can
supply current derived from the magnetic field from the reader to the connected microcontroller over
various distances.
This application report shows how to design the parameters to meet the system performance
requirements (see Figure 1).
TMS37157
RF
EEPROM
CRC
GEN.
ROM
ENCR.
TRP
INTF.
LR
CLKA/M
CR
VBATI
CL
VOLT.
REG.
VCL
SIMO
S
P
I
Control
Unit
VCCD
SOMI
SPI_CLK
BUSY
GND
Q CL
S
CLEAR
VBATION
VBAT
BATBATI
PUSH
VBAT
+
–
BAT
VBATI
CBAT
Figure 1. TMS37157 PaLFI Block Diagram
All trademarks are the property of their respective owners.
2
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
SCBA029 – July 2010
TMS37157 Dual Interface RFID IC
www.ti.com
Figure 2. TMS37157 PaLFI EVM
Table 1. Abbreviations
Abbreviation
Transponder antenna area
CL
Charge capacitor
d
Distance from the reader antenna
dw
Transponder antenna wire diameter
fres
Transponder resonance frequency
ftx
H(d)
H
SCBA029 – July 2010
Description
Atrp
Operating frequency (134.2 kHz)
Reader field strength over distance
Reader field strength
ICL
Medium DC standby current of the IC during charge
Itx
Reader antenna current
k
Coupling factor between reader and transponder antenna
Ltrp
Transponder antenna Inductance
Ltx
Reader antenna inductance
Ntrp
Transponder antenna number of turns
Ntx
Reader antenna number of turns
Qant
Reader antenna quality factor
Qant'
Reader antenna total Q (including Rdms)
QRX
Antenna circuit quality factor during receive
Qtrp
Transponder antenna quality factor
QTX
Antenna circuit quality factor during transmit
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
3
TMS37157 System Description
www.ti.com
Table 1. Abbreviations (continued)
Abbreviation
Description
Rant
Reader antenna resistance
Rant’
Reader antenna total resistance (including Rdms)
Rdmp
Parallel resistor to the antenna (damping)
Rdms
Serial resistor to the antenna (damping)
Ron
Driver on resistance
rtx
Medium radius of reader antenna ((A+B)/2)/2
rtrp
Medium radius of transponder antenna
Usup
Driver supply voltage
Utx
Reader antenna peak to peak voltage
VCL
Charge voltage
VCL_50
2
Charge voltage after 50 ms charge time
TMS37157 System Description
A typical RFID system consists basically of two main components:
• Reader
• Transponder
The right definition and design of the transponder and reader system parameters will provide the best
possible system performance.
The TMS37157 operates as a typical RFID system, but offers additional functionality that can be executed
using the MSP430 microcontroller connected directly to the PaLFI via the SPI interface.
A typical application can be seen in Figure 3.
Digital or Analog
Sensor
MSP430
TMS3705
LF Base Station
134, 2 kHz
Energy
TMS37157
LF Data
LF Transponder
With EEPROM
Base Station
Sensor System
Figure 3. TMS37157 PaLFI Application Example
4
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
SCBA029 – July 2010
TMS37157 System Description
www.ti.com
2.1
2.1.1
RFID Reader
Reader Field Strength
The typically reader (demo reader) using the ASIC TMS3705A [2] uses a circular air coil antenna with the
following typical parameter (see Table 2 and Figure 4):
Table 2. Parameter of Demo Reader Antenna
Parameter
Symbol
Value (typ)
Unit
Inner diameter of transmit antenna
A
36
mm
Outer diameter of transmit antenna
B
39
mm
Medium radius of transmit antenna ((A+B)/2)/2
rtx
0.01875
m
Inductance
Ltx
442
µH
Ntx
72
Qant
25
Turns
Antenna quality factor
START
–Number of turns: 72
–Cupper wire: AWG 36
–Inductance (average): 442mH
–Typical Q–factor: 25
END
26.1
3
Ø4
Figure 4. Demo Reader Antenna
2.1.2
Antenna Current and Voltage Calculation
The antenna current (Itx) needed for field strength calculation can be measured with either a current probe
or be calculated after a measurement of antenna peak-to-peak voltage (Utx) and antenna quality factor
(Qant):
Itx =
1
1
Utx
Utx
×
=
×
æ 1
ö 2 × 2 (Rant + 2 × p × ftx × Ltx ) 2 × 2
2 × p × ftx × Ltx × ç
+ 1÷
è Qant
ø
(1)
2 × p × ftx × Lant
Rant =
Qant
(2)
SCBA029 – July 2010
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
5
TMS37157 System Description
www.ti.com
The maximum peak-to-peak antenna voltage Utx is:
Utx [Vpp] = Itx × 2 × 2 × (Rant + (2 × p × ftx × Lant ))
(3)
2.1.3
Reader Field Strength Calculation
The field strength H generated by the reader in the distance d in case of a certain antenna current Itx, is
calculated with the following formula:
H [d ] =
Ntx × rtx 2 × Itx
1.5
2 × d 2 + rtx 2
(
)
(4)
H [dBµV / m] = 20 × log H [ A / m] × 106
(
) + 51.5
(5)
Figure 5 shows a typical calculated and measured field strength over distance.
Reader Fieldstrength
225.0
220.0
215.0
Calculated
Measured
210.0
Fieldstrength [dB uV/m]
205.0
200.0
195.0
190.0
185.0
180.0
175.0
170.0
165.0
160.0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
Distance [m]
Figure 5. Calculated Reader Field Strength Over Distance
6
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
SCBA029 – July 2010
TMS37157 System Description
www.ti.com
2.2
2.2.1
Transponder Parameter
Typical LF Transponder Antenna
A typical LF transponder antenna has the following typical parameter (see Table 3):
Table 3. Typical Parameter of a LF Transponder Antenna
Parameter
Symbol
Value (typ)
Unit
Magnetic field constant
µ0
1.25664*10-6
Vs/Am
Height of transponder antenna
htrp
0.04
m
Width of transponder antenna
wtrp
0.07
m
Equivalent radius
rtrp
0.03
m
Area of loop
Atrp
2.827e-3
m2
Inductance
Ltrp
2.66
mH
Typical transponder quality factor
Qtrp
35
Turns
Ntrp
146
Diameter of wire
dw
0.2
mm
r = 3 cm
dw = 0.02 cm
12 layer
l = 0.24 cm
12 windings
b = 0.24 cm
Figure 6. Transponder Antenna Dimension
Calculation of the resulting inductance of the antenna:
Ltrp[ µH ] =
2
0.8 × (rtrp × Ntrp )
(6 × rtrp + 9 × l + 10 × b ) × 2.56
(6)
Calculation of the number of windings if the antenna inductance is known:
Ntrp =
Ltrp × (6 × rtrp + 9 × l + 10 × b ) × 2.56
0.8 × rtrp2
(7)
2.3
Coupling Between Reader and Transponder
The coupling factor (k) over distance (d) between the reader and the transponder antenna is calculated by
the following formula:
k=
µ0 × Ntx × Ntrp × rtx 2 × Atrp
1.5
2 × d 2 + rtx 2
× Ltx × Ltrp
(
)
(8)
SCBA029 – July 2010
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
7
TMS37157 System Description
www.ti.com
Coupling Factor Over Distance
1.E+00
Coupling Factor (k)
1.E-01
1.E-02
1.E-03
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
0.16
Distance [m]
Figure 7. Coupling Factor Over Distance
2.4
Calculation and Simulation of Charge Voltage for HDX Systems
Figure 8 shows the simulation model for determination of charge voltage VCL.
Qtrp
Itx
RL
RF
Ltrp
Ltx
CRES
Utx
K
ICL
VCL
CL
VCL
AFE
LIMITER
GND
CED80_SIM_MODEL_1.VSD
Figure 8. Simulation Model for Charge Voltage
8
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
SCBA029 – July 2010
TMS37157 System Description
www.ti.com
The final charge voltage VCL can be calculated with the following formula:
VCL = Itx × k × Ltx × Ltrp - ICL × 2 × Ltrp × 2 × p × fres × Qtrp × 2 +
(
)
ICL
Ltrp
×
- 0.5
Ltx
Itx × k × 2
(AssumingKfres = ftx = 134.2 kHz )
(9)
ICL is the medium DC standby current of the IC during charge and Qtrp is the total transponder quality
factor, represented by RL in the simulation model:
Qtrp =
2 × p × fRES × Ltrp
RL
(10)
The charge voltage without limitation after 50 ms can be calculated by:
-0.05
æ
ö
ç
÷
4
×
p
×
fres
×
Qtrp
×
Ltrp
×
CL
VCL _ 50 = VCL × ç 1 - e
÷
çç
÷÷
è
ø
(11)
For the chart shown in Figure 9, the parameters shown in Table 4 are assumed.
Table 4. Parameter for VCL Calculations
Parameter
Symbol
Value (typ)
Transponder Quality Factor
Qtrp
35
Standby Current During Charge
ICL
5
µA
CL+CBAT
100
µF
Charge Capacitor
Unit
Figure 9 shows the typical charge voltage (limited) over distance for a 50 ms charge burst.
Charge Voltage Over Distance @ 50 ms Charge Time
6.0
VCL_50lim
VCL_50 ms Limited [V]
5.0
4.0
3.0
2.0
1.0
0.0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
0.16
Distance [m]
Figure 9. Typical Charge Voltage Over Distance
SCBA029 – July 2010
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
9
Reference
3
www.ti.com
Reference
1. TMS37157 Passive Low Frequency Interface Device With EEPROM and 134.2 kHz Transponder
Interface Data Sheet (SWRS083)
2. TMS3705 Transponder Base Station IC Data Sheet (SCBS881)
3. UCC27423, UCC27424, UCC27425 Dual 4-A High Speed Low-Side MOSFET Drivers With Enable
Data Sheet (SLUS545)
10
LF HDX System Parameter Calculation for TMS37157 (PaLFI)
Copyright © 2010, Texas Instruments Incorporated
SCBA029 – July 2010
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