DESCRIPTIO. LTC490 Differential Driver and Receiver Pair

Transcription

1 FEATE Low Power: I CC = µa Typical esigned for or Applications ingle V upply V to 1V Bus Common-Mode ange Permits ±V Ground ifference Between evices on the Bus Thermal hutdown Protection Power-p/own Glitch-Free river utputs Permit Live Insertion or emoval of Package river Maintains High Impedance with the Power ff Combined Impedance of a river utput and eceiver Allows up to Transceivers on the Bus mv Typical Input Hysteresis ns Typical river Propagation elays with ns kew Pin Compatible with the N19 ECIPTI LTC9 ifferential river and eceiver Pair The LTC9 is a low power differential bus/line transceiver designed for multipoint data transmission standard applications with extended common-mode range (1V to V). It also meets the requirements of. The CM design offers significant power savings over its bipolar counterpart without sacrificing ruggedness against overload or E damage. 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. The receiver has a fail safe feature which guarantees a high output state when the inputs are left open. Both AC and C specifications are guaranteed from C to C and.v to.v supply voltage range. Low Power / Transceiver Level Translator TPICAL LTC9 LTC9 ECEIVE FT BELEN 91 ECEIVE FT BELEN 91 LTC9 TA1 1

2 LTC9 ABLTE AXI ATI G (Note 1) upply Voltage (V CC )... 1V river Input Currents... ma to ma river Input Voltages....V to V CC +.V river utput Voltages... ±1V eceiver Input Voltages... ±1V eceiver utput Voltages....V to V CC +.V perating Temperature ange LTC9C... C to C LTC9I... C to C torage Temperature ange... C to 1 C Lead Temperature (oldering, 1 sec)... C PACKAGE/E I F V CC 1 GN TP VIE A B N PACKAGE -LEA PLATIC IP PACKAGE -LEA PLATIC IC T JMAX = 1 C, θ JA = 1 C/ (N) T JMAX = 1 C, θ JA = 1 C/ () Consult factory for Military grade parts. ATI E PAT NMBE LTC9CN LTC9C LTC9IN LTC9I PAT MAKING 9 9I C ELECTICAL CHAA CTEITIC V CC = V ±% MBL PAAMETE CNITIN MIN TP MAX NIT V 1 ifferential river utput Voltage (nloaded) I = V V ifferential river utput Voltage (with Load) = Ω () V = Ω () (Figure 1) 1. V V Change in Magnitude of river ifferential utput = Ω or = Ω (Figure 1). V Voltage for Complementary utput tates V C river Common-Mode utput Voltage = Ω or = Ω (Figure 1) V V C Change in Magnitude of river Common Mode = Ω or = Ω (Figure 1). V utput Voltage for Complementary utput tates V IH Input High Voltage (). V V IL Input Low Voltage (). V l IN1 Input Current () ± µa l IN Input Current (A, B) V CC = V or.v V IN = 1V 1 ma V IN = V. ma V TH ifferential Input Threshold Voltage for eceiver V V CM 1V.. V V TH eceiver Input Hysteresis V CM = V mv V H eceiver utput High Voltage I = ma, V I =.V. V V L eceiver utput Low Voltage I = ma, V I =.V. V I Three-tate utput Current at eceiver V CC = Max.V V.V ±1 µa I CC upply Current No Load; = GN or V CC µa IN eceiver Input esistance V V 1V 1 kω I 1 river hort-circuit Current, V T = High V = V 1 ma I river hort-circuit Current, V T = Low V = 1V 1 ma I eceiver hort-circuit Current V V V CC ma I river Three-tate utput Current V = V to 1V ± ± µa

3 LTC9 ITCHI V CC = V ±% GCHAACTEITIC MBL PAAMETE CNITIN MIN TP MAX NIT t PLH river Input to utput IFF = Ω, C L1 = C L = 1pF (Figures, ) 1 ns t PHL river Input to utput IFF = Ω, C L1 = C L = 1pF (Figures, ) 1 ns t KE river utput to utput IFF = Ω, C L1 = C L = 1pF (Figures, ) ns t r, t f river ise or Fall Time IFF = Ω, C L1 = C L = 1pF (Figures, ) ns t PLH eceiver Input to utput IFF = Ω, C L1 = C L = 1pF (Figures, ) 1 ns t PHL eceiver Input to utput IFF = Ω, C L1 = C L = 1pF (Figures, ) 1 ns t K t PLH t PHL ifferential eceiver kew IFF = Ω, C L1 = C L = 1pF (Figures, ) 1 ns The denotes specifications which apply over the full operating temperature range. Note 1: Absolute maximum ratings are those beyond which the safety of the device cannot be guaranteed. Note : All currents into device pins are positive; all currents out of device pins are negative. All voltages are referenced to device ground unless otherwise specified. Note : All typicals are given for V CC = V and Temperature = C. TPICAL PEF A CECHAA CTEITIC river utput High Voltage vs river ifferential utput Voltage river utput Low Voltage vs utput Current vs utput Current utput Current 9 T A = C T A = C T A = C TPT CENT (ma) TPT CENT (ma) 1 TPT CENT (ma) 1 TPT VLTAGE (V) LTC9 TPC1 1 TPT VLTAGE (V) LTC9 TPC 1 TPT VLTAGE (V) LTC9 TPC TTL Input Threshold vs Temperature river kew vs Temperature upply Current vs Temperature 1. INPT THEHL VLTAGE (V) TIME (ns) PPL CENT (µa) 1. 1 TEMPEATE ( C ) 1 1 TEMPEATE ( C ) 1 1 TEMPEATE ( C ) LTC9 TPC LTC9 TPC LTC9 TPC

4 LTC9 TPICAL PEF A CE CHAA CTEITIC river ifferential utput Voltage eceiver t PLH -t PHL vs eceiver utput Low Voltage vs vs Temperature Temperature Temperature. = Ω. I = ma IFFEENTIAL VLTAGE (V) TIME (ns) TPT VLTAGE (V) TEMPEATE ( C ) 1 TEMPEATE ( C ) 1 TEMPEATE ( C ) LTC9 TPC LTC9 TPC LTC9 TPC9 PI F CTI V CC (Pin 1): Positive upply;.v V CC.V. (Pin ): eceiver utput. If A > B by mv, will be high. If A < B by mv, then will be low. (Pin ): river Input. A low on forces the driver outputs A low and B high. A high on will force A high and B low. GN (Pin ): Ground Connection. (Pin ): river utput. (Pin ): river utput. B (Pin ): eceiver Input. A (Pin ): eceiver Input. TET CICIT V V C IFF C L1 C L A B ECEIVE 1pF LTC9 TA LTC9 TA Figure 1. river C Test Load Figure. river/eceiver Timing Test Circuit

5 LTC9 ITCHI G TI E AVEF V V 1.V f = 1MHz : t r 1ns : t f 1ns 1.V t PLH t PHL V % % V 1% V IFF = V() V() 9% % % t r t f V 1/ V t KE 1/ V t KE Figure. river Propagation elays LTC9 TA V A-B V V INPT f = 1MHz ; t r 1ns : t f 1ns V t PLH t PHL V H 1.V TPT 1.V V L Figure. eceiver Propagation elays LTC9 TA I F ATI Typical Application A typical connection of the LTC9 is shown in Figure. Two twisted-pair wires connect two driver/receiver pairs for full duplex data transmission. Note that the driver and receiver outputs are always enabled. If the outputs must be disabled, use the LTC91. There are no restrictions on where the chips are connected, and it isn t necessary to have the chips connected at the ends of the wire. However, the wires must be terminated only at the ends with a resistor equal to their characteristic impedance, typically. Because only V V X 1 LTC9 ECEIVE HIEL LTC9 1 X +.1µF X HIEL ECEIVE X +.1µF Figure. Typical Connection LTC9 TA

6 LTC9 I F ATI one driver can be connected on the bus, the cable can be terminated only at the receiving end. The optional shields around the twisted pair help reduce unwanted noise, and are connected to GN at one end. The LTC9 can also be used as a line repeater as shown in Figure. If the cable length is longer than feet, the LTC9 is inserted in the middle of the cable with the receiver output connected back to the driver input. Losses in a transmission line are a complex combination of C conductor loss, AC losses (skin effect), leakage and AC losses in the dielectric. In good polyethylene cables such as the Belden 91, the conductor losses and dielectric losses are of the same order of magnitude, leading to relatively low overall loss (Figure ). 1 X LTC9 ECEIVE ATA IN L PE 1 FT (db) 1. X ATA T FEQENC (MHz) LTC9 TA Figure. Line epeater LTC9 TA Thermal hutdown The LTC9 has a thermal shutdown feature which protects the part from excessive power dissipation. If the outputs of the driver are accidently shorted to a power supply or low impedance, source, up to ma can flow through the part. The thermal shutdown circuit disables the driver outputs when the internal temperature reaches 1 C and turns them back on when the temperature cools to 1 C. If the outputs of two or more LTC9 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. Cables and ata ate The transmission line of choice for 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 cables designed for applications. Figure. Attenuation vs Frequency for Belden 91 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 1 times worse. PVC twisted pairs have terrible losses at high data rates (>1kbs), and greatly reduce the maximum cable length. At low data rates however, they are acceptable and much more economical. CABLE LENGTH (FT) 1k 1k 1 1 1k 1k 1M.M 1M ATA ATE (bps) LTC9 TA9 Figure. Cable Length pecification. Applies for Gauge, Polyethylene ielectric Twisted Pair.

7 LTC9 I F ATI 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. A 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 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 9). If the cable is loaded excessively (Ω), 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 1.ns/foot). If the cable is lightly loaded (Ω), the signal reflects in phase and increases the amplitude at the driver output. An input frequency of khz is adequate for tests out to feet of cable. t X ECEIVE X t = t = Ω PBE HEE AC 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 resistors, causing ma of C current to flow in the cable when no data is being sent. This C current is about times greater than the supply current of the LTC9. ne way to eliminate the unwanted current is by AC coupling the termination resistors as shown in Figure 1. C ECEIVE C = LINE LENGTH (FT) 1.pF Figure 1. AC Coupled Termination X LTC9 TA11 The coupling capacitor must allow high frequency energy to flow to the termination, but block C 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 of the coupling capacitor should therefore be set at 1.pF per foot of cable length for 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 1 or state. A 1nF capacitor is adequate for lines up to feet in length. Be aware that the power savings start to decrease once the data rate surpasses 1/( C). Fault Protection All of LTC s products are protected against E transients up to kv using the human body model (1pF, 1.kΩ). However, some applications need more protection. The best protection method is to connect a bidirectional Transorb from each line side pin to ground (Figure 11). A Transorb is a silicon transient voltage t = Ω Figure 9. Termination Effects LTC9 TA1 Transorb is a registered trademark of General Instruments, GI 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 its circuits as described herein will not infringe on existing patent rights.

8 LTC9 I F ATI suppressor that has exceptional surge handling capabilities, fast response time, and low series resistance. They are available from General Instruments, GI and come in a variety of breakdown voltages and prices. Be sure to pick a breakdown voltage higher than the common- mode voltage required for your application (typically 1V). Also, don t forget to check how much the added parasitic capacitance will load down the bus. LTC9 TA1 Figure 11. E Protection with Transorbs TPICAL eceiver to Level Transistor with Hysteresis = k IN.k ECEIVE X IN 1k 1/ LTC9 LTC9 TA1.k 1/ LTC9 V - V 19k HTEEI = 1k LTC9 TA1 PACKAGE ECIPTI imensions in inches (millimeters) unless otherwise noted. N Package -Lead Plastic IP.. (..1).. ( ).1 ±. (. ±.1). (1.1) MAX.9.1 (.9.1) ( ). (1.1) TP. ±.1 (1.1 ±.1).1 ±.1 (. ±.).1 (.1) MIN.1 ±. (. ±.). (.) MIN 1. ±.1 (. ±.)..1 (..).1. (..) TP..9 (1. 1.) Package -Lead Plastic IC..1 (.11.) (.1.) (..). (1.) BC.. (.91.19).1.1 (.1.9) Linear Technology Corporation 1 McCarthy Blvd., Milpitas, CA 9-1 BA/LT/GP 9 K EV A PINTE IN A () -19 FAX: () - TELEX: 99-9 LINEA TECHNLG CPATIN 199