DESIGN FEATURES Tiny RS232 Transceivers Run Directly from Alkaline, NiMH or NiCd Batteries by Kevin Wrenner and Troy Seman Introduction Six new devices comprise a family of small-footprint RS-232 transceivers that operate at up to 1Mbps over a supply range of 1.8V to 5.5V. The LTC2801 and LTC2802 are single transceivers available in 4mm × 3mm DFN packages, and the LTC2803 and LTC2804 are dual transceivers available in 5mm × 3mm DFN packages. The LTC2803-1 and LTC2804-1 are dual transceivers offered in 16-pin SSOP packages. The wide supply range permits operation directly from two alkaline, NiCd, or NiMH battery cells, while a separate VL supply pin eliminates interfacing problems in mixed-supply systems. L1 10µH 1.8V TO 5.5V C4 1µF 2V/DIV SW VCC TIN CAP PS OFF ON 5V/DIV MODE TOUT TIN TOUT RIN ROUT 2V/DIV ROUT C1 220nF LTC2802 VL 150pF GND 250pF VEE VDD C2 1µF 400ns/DIV C3 1µF a. b. Figure 1. Operating waveforms at 1.8V and 1Mbps with driver and receiver fully loaded (a) and transmitter loopback mode test circuit (b) Achieving the higher signaling rate—50× the rate provided for in the original standard—necessitates slewing the driver faster than the standard’s 30V/µs limit. The slower parts, the LTC2801 and LTC2803, are fully RS232 compliant. Output levels of all parts are RS232 compliant at their rated data rates even at 1.8V supply. Figure 2 shows the relationship of supply current to supply voltage required to drive 1nF/3kΩ loads at 1Mbps and 250kbps Data Rate All of the devices are capable of driving standard RS232 loads (2.5nF/3kΩ) at 100kbps, and 1nF/3kΩ at 250kbps. The faster parts, the LTC2802, LTC2804 and LTC2804-1, can also drive 250pF/3kΩ at 1Mbps. Waveforms for a single transceiver operating at 1Mbps and 1.8V in a transmitterloopback configuration are shown in Figure 1. various data rates. Figure 3 shows the supply current sensitivity to data rate at 1.8V. More Features Up to four operating modes are available, depending on the part (Table 1). The DFN parts have two power-saving modes. In Shutdown mode, current draw on each supply is reduced below 1µA. Receiver and driver outputs are high impedance, eliminating any problem associated with powering Table 1. Feature summary Drivers and Receivers Package LTC2801 LTC2802 LTC2803 LTC2803-1 LTC2804 LTC2804-1 1+1 1+1 2+2 2+2 2+2 2+2 12-lead 4mm × 3mm DFN 12-lead 4mm × 3mm DFN 16-lead 5mm × 3mm DFN 16-lead SSOP 16-lead 5mm × 3mm DFN 16-lead SSOP 100kbps for RL=3kΩ, CL=2.5nF 250kbps for RL=3kΩ, CL=1nF 1Mbps for RL=3kΩ, CL=250pF 30V/µs Maximum Slew Rate Shutdown Receiver(S) Active Driver Disable 14 Linear Technology Magazine • May 2005 DESIGN FEATURES 25 125kbps 15 10 20.8kbps 5 0 1 2 50 40 30 125kbps 20 3 4 5 SUPPLY VOLTAGE (V) 6 0 20.8kbps 1nF 2 250pF 40 3 4 5 SUPPLY VOLTAGE (V) 0 6 b. Each device in the LTC2801 family drives RS232 compliant output levels over its entire input supply range using an integrated dual regulator (Figure 4) that replaces the charge pump voltage multiplier found in many RS232 integrated circuits. Excellent LTC2804 LTC2802 200 0 600 400 DATA RATE (kbps) 800 1000 Figure 3. Supply current vs data rate (single and dual transceiver) Figure 2. Supply current vs supply voltage for single (a) and dual (b) transceiver Dual Regulator 250pF 20 a. down a part connected to a receiver output. Receiver(s) Active mode is like Shutdown except receivers are biased at low current. With only 15μA current draw, one or two receivers can listen for a wake-up signal. Besides the Normal full-duplex operating mode, a Driver(s) Disabled mode is available to support line sharing and half-duplex operation. These parts have built-in measures that permit reliable operation in the sometimes-harsh environment encountered in RS232 interfaces. All device pins are protected against electrostatic discharge (ESD) events without damage or latch-up. Interface pins have additional protection, tolerating repeated 10kV human body model discharges. Both driver and receiver outputs are current limited. 1nF 60 LTC2804 LTC2803 1 ALL DRIVERS SWITCHING VCC = VL = 1.8V RL = 3kΩ 80 250kbps 10 LTC2802 LTC2801 100 ALL DRIVERS SWITCHING VCC = VL RL = 3kΩ CL = 1nF 60 SUPPLY CURRENT (mA) SUPPLY CURRENT (mA) 30 20 70 ALL DRIVERS SWITCHING VCC = VL RL = 3kΩ 250kbps CL = 1nF SUPPLY CURRENT (mA) 35 C1 220nF L1 10µH 1.8V TO 5.5V C4 1µF VCC CAP SW VDD C2 1µF VL 1.8V TO 5.5V BOOST REGULATOR C5* 220nF VEE C3 1µF *OMIT IF VL IS CONNECTED TO VCC Figure 4. Dual regulator and recommended biasing line and load regulation is achieved with a constant frequency (1.2MHz typical) boost regulator that generates a positive supply of 7V and a coupled inverting charge pump that generates a negative supply of –6.3V. Like its charge pump voltage multiplier counterpart, regulator switching varies according to the driver loading. The regulator operates in a pulse skipping mode when driver activity/loading is low. Because all its Schottky diodes C1 220nF L1 10µH 2 ALKALINE, NiCd, OR NiMH CELLS + C4 1µF – * DC-DC VCC SW CAP 1.8V VL VCC * C5 220nF µP PPx PPy TXD PPz RXD LTC2804 PS MODE T2IN T2OUT T1IN T1OUT R2OUT R2IN R1OUT R1IN GND VDD CTS RX UART RTS TX VEE C2 1µF C3 1µF *ADDITIONAL BYPASS CAP AS NEEDED Figure 5. Example board layout with 5mm × 3mm DFN package Linear Technology Magazine • May 2005 Figure 6. Diagnostic port operating directly off unregulated battery 15 DESIGN FEATURES 1.8V TO 5.5V 2.5V TO 5.5V L1 10µH VCC VL LTC2802 R T VCC C4 2µF PS SW LTC2803 VL C1 470nF CAP VCC SW LTC2803 VL CAP MODE TOUT TIN 3.3k RIN ROUT GND T2IN T2OUT T2IN T2OUT T1IN T1OUT T1IN T1OUT R2OUT R2IN R2OUT R2IN R1OUT R1IN R1OUT R1IN VEE GND GND VDD VDD VEE Figure 7. Half-duplex mode on RS232 interface. The logic interface shares a single wire, too. are integrated, the regulator requires only five external components: one small inductor and four tiny ceramic capacitors (Figure 5). Battery-Operated Microcontroller Interface The advantage of the VL interface logic supply feature can be seen in Figure 6, which shows a battery-operated RS232 interface to a diagnostic port on a 1.8V microprocessor. For maximum efficiency, the LTC2804 is operated directly off the battery voltage. The VL pin is connected to the microprocessor’s regulated 1.8V supply, setting the RxOUT high level and the TxIN and control input threshold voltages, which are automatically scaled. This configuration can extend battery life while eliminating the need for level translators. Half-Duplex on Shared Line RS232 transceivers are often used in configurations outside the scope of the original standard. Figure 7 shows an LTC2802 configured to signal half-duplex over a single RS232 interface wire. The logic interface, too, shares a single wire between driver and receiver. With PS kept high, the MODE input serves as a low-latency driver enable that can switch between transmit and receive modes within 2μs. Using a switchable terminator in the remote device can help avoid degrading output levels and increasing power consumption. C2 2µF C3 2µF ANY COMBINATION LTC2801/LTC2802/LTC2803/LTC2804 Figure 8. Quad transceiver with reduced component count Quad Transceiver Adjustable Level Translator Dual transceivers are commonly used to provide a bidirectional interface that includes a data line and a hardware handshaking control signal. If two such ports are needed, two dual transceiver devices can share one device’s regulator (Figure 8). Tie both device’s CAP pins together, connecting in parallel the inverting charge pump Schottky diodes from both devices. The negative supply level is improved due to a reduction in the combined diode’s forward voltage. The second device’s unused SW pin should be grounded. This configuration eliminates one set of external components. Any RS232 transceiver is a bidirectional level translator. With the regulator and drivers disabled, the receiver(s) can provide simple unidirectional level translation with the output high level defined by the VL supply (Figure 9). This makes a useful 3V-to-5V or 5V-to-1.8V inverting translator capable of 1Mbps. A static dual translator consumes 120μA current. If hysteresis is not required, the MODE and PS pin connections can be reversed to obtain a lower power version (15μA static) capable of 100kbps. 1.8V TO 5.5V C5 220nF VL LTC2803 3V TO 25V –25V TO 0V OFF ON R2IN R2OUT R2IN R1OUT PS MODE VCC T2IN SW T1IN GND VDD VL 0V Conclusion The LTC2801 family’s wide input range of 1.8V to 5.5V enables these parts to provide RS232 interfaces with fully compliant output levels using a broad range of power sources. The small footprint required by each part and its external components (Figure 5), independent logic interface supply, and power saving features, make this family of parts an attractive choice for designing low cost standardized signaling interfaces into modern consumer electronics. VEE Authors can be contacted at (408) 432-1900 Figure 9. Inverting level translator 16 Linear Technology Magazine • May 2005