FSK

Features • Integrated PLL Loop Filter • ESD Protection also at ANT1/ANT2 (4 kV HBM/200V MM; Except Pin 2: 4 kV HBM/100V MM) • High Output Power (5.5 dBm) with Low Supply Current (8.5 mA) • Modulation Scheme ASK/FSK – FSK Modulation is Achieved by Connecting an Additional Capacitor Between the XTAL Load Capacitor and the Open Drain Output of the Modulating Microcontroller Easy to Design-in Due to Excellent Isolation of the PLL from the PA and Power Supply Single Li-cell for Power Supply Supply Voltage 2.0V to 4.0V in the Temperature Range of –40°C to +85°C/+125°C Package TSSOP8L Single-ended Antenna Output with High Efficient Power Amplifier CLK Output for Clocking the Microcontroller One-chip Solution with Minimum External Circuitry 125°C Operation for Tire Pressure Systems • • • • • • • • UHF ASK/FSK Transmitter T5750 1. Description The T5750 is a PLL transmitter IC which has been developed for the demands of RF low-cost transmission systems at data rates up to 32 kBaud. The transmitting frequency range is 868 MHz to 928 MHz. It can be used in both FSK and ASK systems. Figure 1-1. 1 Li cell System Block Diagram UHF ASK/FSK Remote control transmitter T5750 T5760/ T5761 Demod Control UHF ASK/FSK Remote control receiver 1 to 3 Microcontroller Keys Encoder ATARx9x PLL Antenna XTO VCO Antenna PLL XTO LNA LNA VCO 4546F–RKE–12/08 2. Pin Configuration Figure 2-1. Pinning TSSOP8L CLK PA_ENABLE ANT2 ANT1 1 2 3 4 8 7 6 5 ENABLE GND VS XTAL Table 2-1. Pin Pin Description Symbol Function Configuration VS 1 CLK Clock output signal for micro con roller The clock output frequency is set by the crystal to fXTAL/4 100Ω CLK 100Ω PA_ENABLE 50 kΩ UREF = 1.1V 2 PA_ENABLE Switches on power amplifier, used for ASK modulation 20 µA ANT1 3 4 ANT2 ANT1 Emitter of antenna output stage Open collector antenna output ANT2 2 T5750 4546F–RKE–12/08 T5750 Table 2-1. Pin Pin Description (Continued) Symbol Function Configuration VS VS 1.5 kΩ 1.2 kΩ 5 XTAL Connection for crystal XTAL 182 µA 6 7 VS GND Supply voltage Ground See ESD protection circuitry (see Figure 4-5 on page 9) See ESD protection circuitry (see Figure 4-5 on page 9) ENABLE 200 kΩ 8 ENABLE Enable input Figure 2-2. Block Diagram T5750 Power up/down f 1 4 8 CLK ENABLE f 64 PA_ENABLE 2 PDF 7 GND CP ANT2 3 LF 6 VS ANT1 4 PA PLL VCO XTO 5 XTAL 3 4546F–RKE–12/08 3. General Description This fully integrated PLL transmitter allows particularly simple, low-cost RF miniature transmitters to be assembled. The VCO is locked to 64 × fXTAL hence a 13.5672 MHz crystal is needed for a 868.3 MHz transmitter and a 14.2969 MHz crystal for a 915 MHz transmitter. All other PLL and VCO peripheral elements are integrated. The XTO is a series resonance oscillator so that only one capacitor together with a crystal connected in series to GND are needed as external elements. The crystal oscillator together with the PLL needs typically < 1 ms until the PLL is locked and the CLK output is stable. There is a wait time of ≥ 4 ms must be used until the CLK is used for the microcontroller and the PA is switched on. The power amplifier is an open-collector output delivering a current pulse which is nearly independent from the load impedance. The delivered output power is hence controllable via the connected load impedance. This output configuration enables a simple matching to any kind of antenna or to 50Ω. A high power efficiency of η= Pout/(IS,PA × VS) of 24% for the power amplifier at 868.3 MHz results when an optimized load impedance of ZLoad = (166 + j226)Ω is used at 3V supply voltage. 4. Functional Description If ENABLE = L and the PA_ENABLE = L, the circuit is in standby mode consuming only a very small amount of current, so that a lithium cell used as power supply can work for several years. With ENABLE = H the XTO, PLL and the CLK driver are switched on. If PA_ENABLE remains L only the PLL and the XTO is running and the CLK signal is delivered to the microcontroller. The VCO locks to 64 times the XTO frequency. With ENABLE = H and PA_ENABLE = H the PLL, XTO, CLK driver and the power amplifier are on. With PA_ENABLE the power amplifier can be switched on and off, which is used to perform the ASK modulation. 4.1 ASK Transmission The T5750 is activated by ENABLE = H. PA_ENABLE must remain L for t ≥ 4 ms, then the CLK signal can be taken to clock the microcontroller and the output power can be modulated by means of pin PA_ENABLE. After transmission PA_ENABLE is switched to L and the microcontroller switches back to internal clocking. The T5750 is switched back to standby mode with ENABLE = L. 4.2 FSK Transmission The T5750 is activated by ENABLE = H. PA_ENABLE must remain L for t ≥ 4 ms, then the CLK signal can be taken to clock the microcontroller and the power amplifier is switched on with PA_ENABLE = H. The chip is then ready for FSK modulation. The microcontroller starts to switch on and off the capacitor between the XTAL load capacitor and GND with an open-drain output port, thus changing the reference frequency of the PLL. If the switch is closed, the output frequency is lower than if the switch is open. After transmission PA_ENABLE is switched to L and the microcontroller switches back to internal clocking. The T5750 is switched back to standby mode with ENABLE = L. The accuracy of the frequency deviation with XTAL pulling method is about ±25% when the following tolerances are considered. 4 T5750 4546F–RKE–12/08 T5750 Figure 4-1. Tolerances of Frequency Modulation VS CStray1 LM XTAL CM RS CStray2 C4 C0 C5 CSwitch Using C4 = 9.2 pF ±2%, C5 = 6.8 pF ±5%, a switch port with CSwitch = 3 pF ±10%, stray capacitances on each side of the crystal of CStray1 = CStray2 = 1 pF ±10%, a parallel capacitance of the crystal of C0 = 3.2 pF ±10% and a crystal with CM = 13 fF ±10%, an FSK deviation of ±21.5 kHz typical with worst case tolerances of ±16.8 kHz to ±28.0 kHz results. 4.3 CLK Output An output CLK signal is provided for a connected microcontroller, the delivered signal is CMOS compatible if the load capacitance is lower than 10 pF. 4.3.1 Clock Pulse Take-over The clock of the crystal oscillator can be used for clocking the microcontroller. Atmel ® ’s ATARx9x has the special feature of starting with an integrated RC-oscillator to switch on the T5750 with ENABLE = H, and after 4 ms to assume the clock signal of the transmission IC, so that the message can be sent with crystal accuracy. Output Matching and Power Setting The output power is set by the load impedance of the antenna. The maximum output power is achieved with a load impedance of ZLoad,opt = (166 + j226)Ω at 868.3 MHz. There must be a low resistive path to VS to deliver the DC current. The delivered current pulse of the power amplifier is 7.7 mA and the maximum output power is delivered to a resistive load of 475Ω if the 0.53 pF output capacitance of the power amplifier is compensated by the load impedance. An optimum load impedance of: Z Load = 475 Ω || j/(2 × p × f × 0.53 pF) = (166 + j226) Ω t hus results for the maximum output power of 5.5 dBm. The load impedance is defined as the impedance seen from the T5750’s ANT1, ANT2 into the matching network. Do not confuse this large signal load impedance with a small signal input impedance delivered as input characteristic of RF amplifiers and measured from the application into the IC instead of from the IC into the application for a power amplifier. Less output power is achieved by lowering the real parallel part of 475Ω where the parallel imaginary part should be kept constant. Output power measurement can be done with the circuit of Figure 4-2 on page 6. Note that the component values must be changed to compensate the individual board parasitics until the T5750 has the right load impedance ZLoad,opt = (166 + j226)Ω at 868.3 MHz. Also the damping of the cable used to measure the output power must be calibrated out. 4.3.2 5 4546F–RKE–12/08 Figure 4-2. Output Power Measurement VS C1 1 nF L1 ANT1 ZLopt ANT2 10 nH C2 1.5 pF C3 Z = 50Ω Power meter Rin 2.7 pF 50Ω 4.4 Application Circuit For the supply-voltage blocking capacitor C3 a value of 68 nF/X7R is recommended (see Figure 4-3 on page 7 and Figure 4-4 on page 8). C1 and C2 are used to match the loop antenna to the power amplifier where C1 typically is 3.9 pF/NP0 and C2 is 1 pF/NP0; for C2 two capacitors in series should be used to achieve a better tolerance value and to have the possibility to realize the ZLoad,opt by using standard valued capacitors. C1 forms together with the pins of T5750 and the PCB board wires a series resonance loop that suppresses the 1st harmonic, hence the position of C1 on the PCB is important. Normally the best suppression is achieved when C1 is placed as close as possible to the pins ANT1 and ANT2. The loop antenna should not exceed a width of 1.5 mm, otherwise the Q-factor of the loop antenna is too high. L1 (≈ 50 nH to 100 nH) can be printed on PCB. C4 should be selected so that the XTO runs on the load resonance frequency of the crystal. Normally, a value of 12 pF results for a 15 pF load-capacitance crystal. 6 T5750 4546F–RKE–12/08 T5750 Figure 4-3. ASK Application Circuit S1 BPXY ATARx9x 1 VDD VS VSS 20 S2 BPXY BPXY OSC1 7 BPXY T5750 Power up/down CLK 1 f 4 8 ENABLE f 64 PA_ENABLE 2 PDF 7 GND C3 CP ANT2 3 Loop Antenna C1 LF 6 VS VS C2 ANT1 4 L1 PA PLL VCO XTO 5 XTAL XTAL C4 VS 7 4546F–RKE–12/08 Figure 4-4. FSK Application Circuit S1 BPXY ATARx9x 1 VDD VS VSS 20 S2 BPXY BPXY 18 OSC1 7 BP42/T2O BPXY T5750 Power up/down CLK 1 f 4 8 ENABLE f 64 PA_ENABLE 2 PDF 7 GND C3 CP ANT2 3 Loop Antenna C1 LF 6 VS C5 ANT1 4 L1 PA PLL VCO XTO 5 C4 XTAL XTAL VS C2 VS 8 T5750 4546F–RKE–12/08 T5750 Figure 4-5. ESD Protection Circuit VS ANT1 CLK PA_ENABLE ANT2 XTAL ENABLE GND 5. Absolute Maximum Ratings Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Parameters Supply voltage Power dissipation Junction temperature Storage temperature Ambient temperature Input voltage Note: Symbol VS Ptot Tj Tstg Tamb VmaxPA_ENABLE –55 –55 –0.3 Minimum Maximum 5 100 150 125 125 (VS + 0.3) (1) Unit V mW °C °C °C V 1. If VS + 0.3 is higher than 3.7V, the maximum voltage will be reduced to 3.7V. 6. Thermal Resistance Parameters Junction ambient Symbol RthJA Value 170 Unit K/W 7. Electrical Characteristics VS = 2.0V to 4.0V, Tamb = –40°C to 125°C unless otherwise specified. Typical values are given at VS = 3.0V and Tamb = 25°C. All parameters are referred to GND (pin 7). Parameters Test Conditions Power down, VENABLE < 0.25V, –40°C to 85°C VPA_ENABLE < 0.25V, –85°C to +125°C VPA_ENABLE < 0.25V, 25°C (100% correlation tested) Power up, PA off, VS = 3V, VENABLE > 1.7V, VPA_ENABLE < 0.25V Power up, VS = 3.0, VENABLE > 1.7V, VPA_ENABLE > 1.7V VS = 3.0V, Tamb = 25°C, f = 868.3 MHz, ZLoad = (166 + j226)Ω Symbol Min. Typ. Max. 350 7