LTC1485 Differential Bus Transceiver DESCRIPTIO

Transcription

1 LTC85 Differential us Transceiver FETRE ED Protection over ±kv Low Power: I CC =.8m Typ 8ns Typical Driver Propagation Delays with ns kew Designed for R85 or R pplications ingle upply 7V to V us Common-Mode Range Permits ±7V Ground Difference etween Devices on the us Thermal hutdown Protection Power-p/Down Glitch-Free Driver Outputs Driver Maintains High Impedance in Three-tate or with the Power Off Combined Impedance of a Driver Output and Receiver llows up to Transceivers on the us 6mV Typical Input Hysteresis Pin Compatible with the N7576, D7576, and N75LC76 PPLICTI O Low Power R85/R Transceiver Level Translator DECRIPTIO The LTC 85 is a low power differential bus/line transceiver designed for multipoint data transmission standard R85 applications with extended common-mode range (V to 7V). It also meets the requirements of R. The CMO with chottky design offers significant power savings over its bipolar counterpart without sacrificing ruggedness against overload or ED damage. The driver and receiver feature three-state outputs, with the driver outputs maintaining high impedance over the entire common-mode range. Excessive power dissipation caused by bus contention or faults is prevented by a thermal shutdown circuit which forces the driver outputs into a high impedance state. I/O pins are protected against multiple ED strikes of over ±kv. The receiver has a fail-safe feature which guarantees a high output state when the inputs are left open. oth C and DC specifications are guaranteed from C to 85 C and.7 to 5. supply voltage range., LTC and LT are registered trademarks of Linear Technology Corporation. TYPICL PPLICTI O DE 8 8 DE LTC LTC85 DI Ω Ω DI 7 FT GGE TITED PIR 7 RO RO 5 5 RE RE 85 T

2 LTC85 OLTE XI RTI G (Note ) upply Voltage (V CC )... V Control Input Voltages.... to V CC +. Control Input Currents... 5m to 5m Driver Input Voltages.... to V CC +. Driver Input Currents... 5m to 5m Driver Output Voltages... ±V Receiver Input Voltages... ±V Receiver Output Voltages.... to V CC +. Operating Temperature Range LTC85C... C to 7 C LTC85I... C to 85 C torage Temperature Range C to 5 C Lead Temperature (oldering, sec.)... C PCKGE/ORDER I FOR TOP VIE RO 8 V CC R RE DE DI D GND N8 PCKGE 8-LED PLTIC DIP 8 PCKGE 8-LED PLTIC OIC T JMX = 5 C, θ J = C/ (N) T JMX = 5 C, θ J = 5 C/ () Consult factory for Military grade parts. TIO ORDER PRT NMER LTC85CN8 LTC85IN8 LTC85C8 LTC85I8 8 PRT MRKING 85 85I DC ELECTRICL CHRCTERITIC V CC = (Notes, ), unless otherwise noted. YMOL PRMETER CONDITION MIN TYP MX NIT V OD Differential Driver Output Voltage (nloaded) I O = 5 V V OD Differential Driver Output Voltage (ith Load) R = 5Ω, (R) V R = 7Ω, (R85) (Figure ).5 5 V V OD Change in Magnitude of Driver Differential R = 7Ω or R = 5Ω (Figure ). V Output Voltage for Complementary Output tates V OC Driver Common-Mode Output Voltage R = 7Ω or R = 5Ω (Figure ) V V OC Change in Magnitude of Driver Common-Mode R = 7Ω or R = 5Ω (Figure ). V Output Voltage for Complementary Output tates V INH Input High Voltage DI, DE, RE. V V INL Input Low Voltage DI, DE, RE.8 V I IN Input Current DI, DE, RE ± µ I IN Input Current (, ) V CC = V or 5., V IN = V. m V CC = V or 5., V IN = 7V.8 m V TH Differential Input Threshold Voltage for Receiver 7V V CM V.. V V TH Receiver Input Hysteresis V CM = V 6 mv V OH Receiver Output High Voltage I O = m, V ID =.V.5 V V OL Receiver Output Low Voltage I O = m, V ID =.V. V I OZR Three-tate Output Current at Receiver V CC = Max.V V O.V ± µ I CC upply Current No Load; DI = GND or V CC Outputs Enabled.8. m Outputs Disabled.7. m R IN Receiver Input Resistance 7V V CM V kω I OD Driver hort-circuit Current, V OT = High V O = 7V 5 m I OD Driver hort-circuit Current, V OT = Low V O = V 5 m I OR Receiver hort-circuit Current V V O V CC 7 85 m

3 LTC85 ITCHI GCHRCTERITIC V CC = (Notes, ), unless otherwise noted. YMOL PRMETER CONDITION MIN TYP MX NIT t PLH Driver Input to Output R DIFF = 5Ω, C L = C L = pf 5 ns (Figures, 5) t PHL Driver Input to Output R DIFF = 5Ω, C L = C L = pf 5 ns (Figures, 5) t KE Driver Output to Output R DIFF = 5Ω, C L = C L = pf ns (Figures, 5) t r, t f Driver Rise or Fall Time R DIFF = 5Ω, C L = C L = pf ns (Figures, 5) t ZH Driver Enable to Output High C L = pf (Figures, 6) Closed 7 ns t ZL Driver Enable to Output Low C L = pf (Figures, 6) Closed 7 ns t LZ Driver Disable Time from Low C L = 5pF (Figures, 6) Closed 7 ns t HZ Driver Disable Time from High C L = 5pF (Figures, 6) Closed 7 ns t PLH Receiver Input to Output R DIFF = 5Ω, C L = C L = pf (Figures, 7) ns t PHL Receiver Input to Output R DIFF = 5Ω, C L = C L = pf (Figures, 7) 55 ns t KE t PLH t PHL R DIFF = 5Ω, C L = C L = pf (Figures, 7) 5 5 ns Differential Receiver kew t ZL Receiver Enable to Output Low C L = 5pF (Figures, 8) Closed 5 ns t ZH Receiver Enable to Output High C L = 5pF (Figures, 8) Closed 5 ns t LZ Receiver Disable from Low C L = 5pF (Figures, 8) Closed 5 ns t HZ Receiver Disable from High C L = 5pF (Figures, 8) Closed 5 ns The denotes specifications which apply over the operating temperature range. Note : bsolute Maximum Ratings are those values beyond which the safety of the device cannot be guaranteed. Note : ll currents into device pins are positive. ll currents out of device pins are negative. ll voltages are referenced to device ground unless otherwise specified. Note : ll typicals are given for V CC = and T = 5 C. TYPICL PERFOR CE CHRCTERITIC 6 Receiver Output Low Voltage vs Output Current T = 5 C 8 6 Receiver Output High Voltage vs Output Current T = 5 C.8.6 Receiver Output High Voltage vs Temperature I = 8m OTPT CRRENT (m) OTPT CRRENT (m) 8 6 OTPT VOLTGE (V) OTPT VOLTGE (V). 5 OTPT VOLTGE (V) TEMPERTRE ( C) 85 G 85 G 85 G

4 LTC85 TYPICL PERFOR.9.8 Receiver Output Low Voltage vs Temperature I = 8m CE CHRCTERITIC 6 Driver Differential Output Voltage vs Output Current T = 5 C. Driver Differential Output Voltage vs Temperature R L =5Ω OTPT VOLTGE (V) OTPT CRRENT (m) 8 6 DIFFERENTIL VOLTGE (V) TEMPERTRE ( C) OTPT VOLTGE (V) TEMPERTRE ( C) 85 G 85 G5 85 G6 Driver Output Low Voltage vs Output Current Driver Output High Voltage vs Output Current TTL Input Threshold vs Temperature 8 T = 5 C 96 T = 5 C.6 OTPT CRRENT (m) 6 OTPT CRRENT (m) 7 8 INPT THREHOLD VOLTGE (V) OTPT VOLTGE (V) OTPT VOLTGE (V) TEMPERTRE ( C) 85 G7 85 G8 85 G9 Receiver t PLH t PHL vs Temperature Driver kew vs Temperature upply Current vs Temperature ENLED TIME (ns) TIME (ns) PPLY CRRENT (m) DILED TEMPERTRE ( C) TEMPERTRE ( C) TEMPERTRE ( C) 85 G 85 G 85 G

5 PI F CTIO RO (Pin ): Receiver Output. If the receiver output is enabled (RE low), then if > by mv, RO will be high. If < by mv, then RO will be low. RE (Pin ): Receiver Output Enable. low enables the receiver output, RO. high input forces the receiver output into a high impedance state. DE (Pin ): Driver Output Enable. high on DE enables the driver outputs, and. low input will force the driver outputs into a high impedance state. LTC85 DI (Pin ): Driver Input. If the driver outputs are enabled (DE high), then a low on DI forces the driver outputs low and high. high on DI will force high and low. GND (Pin 5): Ground Connection. (Pin 6): Driver Output/Receiver Input. (Pin 7): Driver Output/Receiver Input. V CC (Pin 8): Positive upply..7 V CC 5.. TET CIRCIT R V OD R V OC DI R DIFF C L C L RO 5pF 85 F 85 F Figure. Driver DC Test Load Figure. Driver/Receiver Timing Test Circuit OTPT C L k k V CC OTPT NDER TET 5Ω C L V CC 85 F 85 F Figure. Receiver Timing Test Load Figure. Driver Timing Test Load 5

6 LTC85 ITCHI G TI E VEFOR V DI. f = MHz; t r ns; t f ns. V t PLH t PHL V O V V V % O 5% 9% 9% 5% % t r t f V O / V O / V O t KE t KE 85 F5 Figure 5. Driver Propagation Delays V DE. f = MHz; t r ns; t f ns. V t ZL t LZ, V OL.V OTPT NORMLLY LO. V OH, V t ZH.V OTPT NORMLLY HIGH t HZ. 85 F6 Figure 6. Driver Enable and Disable Times V V V OD V INPT f = MHz; t r ns; t f ns V V OD tplh t PHL OTPT V OH.. RO V OL 85 F7 Figure 7. Receiver Propagation Delays 6

7 LTC85 ITCHI G TI E VEFOR V RE V. t ZL f = MHz; t r ns; t f ns. t LZ RO V OL. OTPT NORMLLY LO. V OH RO V t ZH. OTPT NORMLLY HIGH t HZ. 85 F8 Figure 8. Receiver Enable and Disable Times PPLICTI O I FOR TIO Typical pplication typical connection of the LTC85 is shown in Figure 9. Two twisted pair wires connect up to driver/receiver pairs for half duplex data transmission. There are no restrictions on where the chips are connected to the wires and it isn t necessary to have the chips connected at the ends. However, the wires must be terminated only at the ends with a resistor equal to their characteristic impedance, typically Ω. The input impedance of a receiver is typically k to GND, or.6 unit R85 load, so in practice 5 to 6 transceivers can be connected to the same wires. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GND at one end. LTC85 LTC85 RX RX DX 7 Ω Ω DX 6 LTC85 85 F9 RX 7 6 DX Figure 9. Typical Connection 7

8 LTC85 PPLICTI O I FOR TIO Thermal hutdown The LTC85 has a thermal shutdown feature which protects the part from excessive power dissipation. If the outputs of the driver are accidentally shorted to a power supply or low impedance source, up to 5m can flow through the part. The thermal shutdown circuit disables the driver outputs when the internal temperature reaches 5 C and turns them back on when the temperature cools to C. If the outputs of two or more LTC85 drivers are shorted directly, the driver outputs can not supply enough current to activate the thermal shutdown. Thus, the thermal shutdown circuit will not prevent contention faults when two drivers are active on the bus at the same time. LO PER FT (d).. FREQENCY (MHz) 85 F Figure. ttenuation vs Frequency for elden 98 k Cables and Data Rate The transmission line of choice for R85 applications is a twisted pair. There are coaxial cables (twinaxial) made for this purpose that contain straight pairs, but these are less flexible, more bulky, and more costly than twisted pairs. Many cable manufacturers offer a broad range of Ω cables designed for R85 applications. Losses in a transmission line are a complex combination of DC conductor loss, C losses (skin effect), leakage, and C losses in the dielectric. In good polyethylene cables such as the elden 98, the conductor losses and dielectric losses are of the same order of magnitude, leading to relatively low overall loss (Figure ). hen using low loss cables, Figure can be used as a guideline for choosing the maximum line length for a given data rate. ith lower quality PVC cables the dielectric loss factor can be times worse. PVC twisted pairs have terrible losses at high data rates (>kbs), and greatly reduce the maximum cable length. t low data rates however, they are acceptable and much more economical. Cable Termination The proper termination of the cable is very important. If the cable is not terminated with its characteristic impedance, distorted waveforms will result. In severe cases, distorted (false) data and nulls will occur. quick look at the output of the driver will tell how well the cable is terminated. It is best to look at a driver connected to the 8 CLE LENGTH (FT) k k k M.5M M DT RTE (bps) 85 F Figure. Cable Length vs Data Rate end of the cable, since this eliminates the possibility of getting reflections from two directions. imply look at the driver output while transmitting square wave data. If the cable is terminated properly, the waveform will look like a square wave (Figure). If the cable is loaded excessively (7Ω) the signal initially sees the surge impedance of the cable and jumps to an initial amplitude. The signal travels down the cable and is reflected back out of phase because of the mistermination. hen the reflected signal returns to the driver, the amplitude will be lowered. The width of the pedestal is equal to twice the electrical length of the cable (about.5ns/foot). If the cable is lightly loaded (7Ω) the signal reflects in phase and increases the amplitude at the driver output. n input frequency of khz is adequate for tests out to feet of cable.

9 LTC85 PPLICTI O PROE HERE I FOR TIO DX R t RX Rt = Ω of the coupling capacitor should therefore be set at 6.pF per foot of cable length for Ω cables. ith the coupling capacitors in place, power is consumed only on the signal edges and not when the driver output is idling at a or state. nf capacitor is adequate for lines up to feet in length. e aware that the power savings start to decrease once the data rate surpasses /(Ω C). Rt = 7Ω Rt = 7Ω Figure. Termination Effects 85 F C Cable Termination Cable termination resistors are necessary to prevent unwanted reflections, but they consume power. The typical differential output voltage of the driver is V when the cable is terminated with two Ω resistors, causing m of DC current to flow in the cable when no data is being sent. This DC current is about times greater than the supply current of the LTC85. One way to eliminate the unwanted current is by C-coupling the termination resistors as shown in Figure. Receiver Open-Circuit Fail-afe ome data encoding schemes require that the output of the receiver maintains a known state (usually a logic ) when the data is finished transmitting and all drivers on the line are forced into three-state. The receiver of the LTC85 has a fail-safe feature which guarantees the output to be in a logic state when the receiver inputs are left floating (open-circuit). If the receiver output must be forced to a known state, the circuits of Figure can be used. Ω Ω Ω Ω.5k RX Ω RX Ω C RX.5k 85 F C = LINE LENGTH (FT) 6.pF Figure. C-Coupled Termination k The coupling capacitor must allow high frequency energy to flow to the termination, but block DC and low frequencies. The dividing line between high and low frequency depends on the length of the cable. The coupling capacitor must pass frequencies above the point where the line represents an electrical one-tenth wavelength. The value C Ω RX Figure. Forcing hen ll Drivers re Off 85 F 9

10 LTC85 PPLICTI O I FOR TIO The termination resistors are used to generate a DC bias which forces the receiver output to a known state, in this case a logic. The first method consumes about 8m and the second about 8m. The lowest power solution is to use an C termination with a pull-up resistor. imply swap the receiver inputs for data protocols ending in logic. Ω 85 F5 Fault Protection ll of LTC s R85 products are protected against ED transients up to kv using the human body model (pf,.5kω). However, some applications need more protection. The best protection method is to connect a bidirectional TransZorb from each line side pin to ground (Figure 5). TransZorb is a silicon transient voltage suppressor that has exceptional surge handling capabilities: fast response Figure 5. ED Protection with TransZorbs time and low series resistance. They are available from General emiconductor Industries and come in a variety of breakdown voltages and prices. e sure to pick a breakdown voltage higher than the common-mode voltage required for your application (typically V). lso, don t forget to check how much the added parasitic capacitance will load down the bus. TransZorb is a registered trademark of General Instruments, GI TYPICL PPLICTI O R Receiver R IN 5.6k RX 85 T R to R85 Level Translator with Hysteresis k R IN k Ω 5.6k 85 T HYTEREI = k V V /R 9 (kω VOLT)/R

11 LTC85 PCKGE DECRIPTIO Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead Plastic DIP.* (.6) MX ±.5* (6.77 ±.8)..5 ( ).5.65 (..65). ±.5 (. ±.7).9.5 (.9.8) ( ).65 (.65) TYP.5 ±.5 (. ±.8). ±. (.5 ±.5) *THEE DIMENION DO NOT INCLDE MOLD FLH OR PROTRION. MOLD FLH OR PROTRION HLL NOT EXCEED. INCH (.5mm)..5 (.75) MIN.8 ±. (.57 ±.76).5 (.8) MIN N8 69 Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of circuits as described herein will not infringe on existing patent rights.

12 LTC85 PCKGE DECRIPTIO Dimensions in inches (millimeters) unless otherwise noted. 8 Package 8-Lead Plastic OIC.89.97* (.8 5.) ( ).5.57* (.8.988).8. (..5).. (.5.58) 5 8 TYP.5.69 (.6.75).. (..5) (.55.8) *THEE DIMENION DO NOT INCLDE MOLD FLH OR PROTRION. MOLD FLH OR PROTRION HLL NOT EXCEED.6 INCH (.5mm)..5 (.7) C O8 9 RELTED PRT PRT NMER DECRIPTION COMMENT LTC86 Quad R85 Driver Fits 757 Pinout, Only µ I Q LTC88 Quad R85 Receiver Fits 757 Pinout, Only 7m I Q LTC9 Full Duplex R85 Transceiver Fits 7579 Pinout, Only µ I Q LTC8 ltra-low Power Half Duplex R85 Transceiver Fits 7576 Pinout, 8µ I Q Linear Technology Corporation 6 McCarthy lvd., Milpitas, C (8) -9 FX: (8) -57 TELEX: sn85 85fs LT/GP 795 K REV PRINTED IN THE LINER TECHNOLOGY CORPORTION 995