A Low Cost 4Mbps IrDA Receiver in MS8 and SO-8 Packages Design Note 152 Alexander Strong Introduction The need for ever increasing data rates required by a vast array of devices, such as notebook computers, printers, mobile phones, pagers and modems, has been satisfied by the technology of infrared data transmission. The Infrared Data Association (IrDA) standard, which covers data rates from 2400bps to 4Mbps, is the overwhelming choice for infrared data transmission. The LT®1328 is a photodiode receiver that supports IrDA data rates up to 4Mbps, as well as other modulation methods, such as Sharp ASK and TV remote control. The LT1328, in the MS8 and SO-8 packages, contains all the necessary circuitry to convert current pulses from an external photodiode to a digital TTL output while rejecting unwanted lower frequency interference. The LT1328 plus five external components is all that is required to make the IrDA-compatible receiver shown in Figure 1. An IrDA-compatible transmitter can also be implemented with only six components, as shown in C6 1000pF LT1328 LIGHT IN TEMIC BPV22NF 1 D1 IN 2 C1 10nF C4 330pF FILT 3 FILT LO 4 GND VBIAS MODE VCC DATA Figure 2. Power requirements for the LT1328 are minimal: a single 5V supply and 2mA of quiescent current. LT1328 Functional Description Figure 3 is a block diagram of the LT1328. Photodiode current from D1 is transformed into a voltage by feedback resistor RFB. The DC level of the preamp is held at VBIAS by the servo action of the transconductance amplifier’s gm. The servo action only suppresses frequencies below the Rgm/CFILT pole. This highpass filtering attenuates interfering signals, such as sunlight or incandescent or fluorescent lamps, and is selectable at Pin 7 for low or high data rates. For high data rates, Pin 7 should be held low. The highpass filter breakpoint is set by the capacitor C4 at f = 25/(2π • Rgm • C4), where Rgm = 60k. The 330pF capacitor (C4) sets a 200kHz corner frequency and is used for data rates above L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. HIGH: SIR LOW: FIR AND 4ppm 8 VBIAS 7 C2 4.7μF 5 5V PREAMP 1 IN BIAS D1 RIN TRANSMIT INPUT R2 10k RGM gm CELL MODE 7 VCC 6 DATA 5 – 2 VCC D2 HSDL-4220 + DN152 F01 Figure 1. LT1328 IrDA Receiver R1 100Ω C6 1000pF RFB 6 TTL DATA OUT C5 4.7μF 8 C1 10nF 3 FILTER FILTER LO C4 330pF R3 3.9Ω 1/2W Q3 2N7002 4 GND – COMP + DN152 F02 DN152 F03 Figure 2. IrDA Transmitter 04/97/152_conv Figure 3. LT1328 Block Diagram 115kbps. For low data rates (115kbps and below), the capacitance at Pin 2 is increased by taking Pin 7 to a TTL high. This switches C1 in parallel with C4, lowering the highpass filter breakpoint. A 10nF capacitor (C1) produces a 6.6kHz corner. Signals processed by the preamp/gm amplifier combination cause the comparator output to swing low. IrDA SIR The LT1328 circuit in Figure 1 operates over the full 1cm to 1 meter range of the IrDA standard at the stipulated light levels. For IrDA data rates of 115kbps and below, a 1.6μs pulse width is used for a zero and no pulse for a one. Light levels are 40mW/sr (Watts per steradian) to 500mW/sr. Figure 4 shows a scope photo for a transmitter input (bottom trace) and the LT1328 output (top trace). Note that the input to the transmitter is inverted; that is, transmitted light produces a high at the input, which results in a zero at the output of the transmitter. The MODE pin (Pin 7) should be high for these data rates. RECEIVER OUTPUT TRANSMITTER INPUT DN152 F05 Figure 5. IrDA-FIR Modulation 4ppm The last IrDA encoding method is for 4Mbps and uses pulse position modulation, thus its name: 4ppm. Two bits are encoded by the location of a 125ns wide pulse at one of the four positions within a 500ns interval (2 bits • 1/500ns = 4Mbps). Range and input levels are the same as for 1.152Mbps. Figure 6 shows the LT1328 reproduction of this modulation. RECEIVER OUTPUT RECEIVER OUTPUT TRANSMITTER INPUT TRANSMITTER INPUT DN152 F04 Figure 4. IrDA-SIR Modulation IrDA FIR The second fastest tier of the IrDA standard addresses 576kbps and 1.152Mbps data rates, with pulse widths of 1/4 of the bit interval for zero and no pulse for one. The 1.152Mbps rate, for example, uses a pulse width of 217ns; the total bit time is 870ns. Light levels are 100mW/sr to 500mW/sr over the 1cm to 1 meter range. A photo of a transmitted input and LT1328 output is shown in Figure 5. The LT1328 output pulse width will be less than 800ns wide over all of the above conditions at 1.152Mbps. Pin 7 should be held low for these data rates and above. Data Sheet Download www.linear.com Linear Technology Corporation DN152 F06 Figure 6. IrDA-4ppm Modulation Conclusion In summary, the LT1328 can be used to build a low cost receiver compatible with IrDA standards. Its ease of use and flexibility also allow it to provide solutions to numerous other photodiode receiver applications. The tiny MSOP package saves on PC board area. For applications help, call (408) 432-1900 dn152f_conv LT/GP 0497 185K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1997