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LTC1487 Ultra-Low Power RS485 with Low EMI, Shutdown and High Input Impedance
FEATURES
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DESCRIPTIO
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High Input Impedance: Up to 256 Transceivers on the Bus Low Power: ICC = 120A Max with Driver Disabled ICC = 200A Max with Driver Enabled, No Load 1A Quiescent Current in Shutdown Mode Controlled Slew Rate Driver for Reduced EMI Single 5V Supply ESD Protection to 10kV On Receiver Inputs and Driver outputs - 7V to 12V Common-Mode Range Permits 7V Ground Difference Between Devices on the Data Line Thermal Shutdown Protection Power Up/Down Glitch-Free Driver Outputs Permit Live Insertion or Removal of Transceiver Driver Maintains High Impedance in Three-State or with the Power Off Pin Compatible with the LTC485
The LTC(R)1487 is an ultra-low power differential line transceiver designed with high impedance inputs allowing up to 256 transceivers to share a single bus. It meets the requirements of RS485 and RS422. The LTC1487 features output drivers with controlled slew rate, decreasing the EMI radiated from the RS485 lines, and improving signal fidelity with misterminated lines. The CMOS design offers significant power savings without sacrificing ruggedness against overload or ESD damage. Typical quiescent current is only 80A while operating and 1A in shutdown. 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. The receiver has a fail-safe feature which guarantees a high output state when the inputs are left open. I/O pins are protected against multiple ESD strikes of over 10kV using the Human Body Model. The LTC1487 is fully specified over the commercial temperature range and is available in 8-pin DIP and SO packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATI
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Battery-Powered RS485/RS422 Applications Low Power RS485/RS422 Transceiver Level Translator
TYPICAL APPLICATI
LTC1487 1 RO 2 RE 3 DE 4 DI R 7 D 6 120
LTC1487 2000 FEET OF TWISTED-PAIR WIRE 7 120 6 D R 1 RO 2 RE 3 DE 4 DI
DI RECEIVER INPUT
A B
330 4.7nF
RO EQUIVALENT LOAD OF 256 LTC1487 TRANSCEIVERS
LTC1487 * TA01
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LTC1487 * TA02
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LTC1487 ABSOLUTE
(Note 1)
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RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW RO 1 RE 2 DE 3 DI 4 N8 PACKAGE 8-LEAD PDIP D R 8 7 6 5 VCC B A GND
Supply Voltage (VCC) .............................................. 12V Control Input Voltage ..................... - 0.5V to VCC + 0.5V Driver Input Voltage ....................... - 0.5V to VCC + 0.5V Driver Output Voltage ........................................... 14V Receiver Input Voltage .......................................... 14V Receiver Output Voltage ................ - 0.5V to VCC + 0.5V Operating Temperature Range ............. 0C TA 70C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LTC1487CN8 LTC1487CS8 S8 PART MARKING 1487
S8 PACKAGE 8-LEAD PLASTIC SO
TJMAX = 125C, JA = 130C/ W (N8) TJMAX = 125C, JA = 150C/ W (S8)
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL VOD1 VOD2 VOD VOC VOC VIH VIL IIN1 IIN2 VTH VTH VOH VOL IOZR RIN ICC ISHDN IOSD1 IOSD2 IOSR PARAMETER Differential Driver Output Voltage (Unloaded) Differential Driver Output Voltage (with Load) Change in Magnitude of Driver Differential Output Voltage for Complementary Output States Driver Common-Mode Output Voltage Change in Magnitude of Driver Common-Mode Output Voltage for Complementary Output States Input High Voltage Input Low Voltage Input Current Input Current (A, B) Differential Input Threshold Voltage for Receiver Receiver Input Hysteresis Receiver Output High Voltage Receiver Output Low Voltage Three-State (High Impedance) Output Current at Receiver Receiver Input Resistance Supply Current Supply Current in Shutdown Mode Driver Short-Circuit Current, VOUT = HIGH Driver Short-Circuit Current, VOUT = LOW Receiver Short-Circuit Current
0C TA 70C, VCC = 5V (Notes 2, 3) unless otherwise noted.
CONDITIONS IO = 0 R = 50 (RS422) R = 27 (RS485), Figure 1 R = 27 or R = 50, Figure 1 R = 27 or R = 50, Figure 1 R = 27 or R = 50, Figure 1 DE, DI, RE DE, DI, RE DE, DI, RE DE = 0, VCC = 0V or 5.25V, VIN = 12V DE = 0, VCC = 0V or 5.25V, VIN = - 7V - 7V VCM 12V VCM = 0V IO = - 4mA, VID = 200mV IO = 4mA, VID = - 200mV VCC = Max, 0.4V VO 2.4V - 7V VCM 12V No Load, Output Enabled No Load, Output Disabled DE = 0V, RE = VCC - 7V VO 12V - 7V VO 12V 0V VO VCC
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MIN 2.0 1.5
TYP
MAX 5 5 0.2 3 0.2
UNITS V V V V V V V
2 0.8 2 0.30 - 0.15 - 0.2 45 3.5 0.4 1 70 96 120 80 1 35 35 7 200 120 10 250 250 85 0.2
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V A mA mA V mV V V A k A A A mA mA mA
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LTC1487
ELECTRICAL CHARACTERISTICS
SYMBOL VOD1 VOD2 VOC VTH VTH ICC ISHDN tPLH tPHL tSKEW t r , tf tPLH tPHL tSKD fMAX PARAMETER Differential Driver Output Voltage (Unloaded) Differential Driver Output Voltage (with Load) Driver Common-Mode Output Voltage Differential Input Threshold Voltage for Receiver Receiver Input Hysteresis Supply Current Supply Current in Shutdown Mode Driver Input to Output Driver Input to Output Driver Output to Output Driver Rise or Fall Time Receiver Input to Output Receiver Input to Output
tPLH - tPHL Differential Receiver Skew
- 40C TA 85C, VCC = 5V (Note 4) unless otherwise noted.
CONDITIONS IO = 0 R = 50 (RS422) R = 27 (RS485), Figure 1 R = 27 or R = 50, Figure 1 - 7V VCM 12V VCM = 0V No Load, Output Enabled No Load, Output Disabled DE = 0V, RE = VCC RDIFF = 54, CL1 = CL2 = 100pF, (Figures 3, 5)
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MIN 2.0 1.5 - 0.2
TYP
MAX 5 5 3 0.2
UNITS V V V V V mV A A A ns ns ns ns ns ns ns kbps
45 120 80 1 150 150 100 150 30 30 250 140 140 13 200 120 10 1200 1200 600 2000 250 250
RDIFF = 54, CL1 = CL2 = 100pF, (Figures 3, 7)
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Maximum Data Rate
SWITCHI G CHARACTERISTICS
SYMBOL tPLH tPHL tSKEW tr, tf tZH tZL tLZ tHZ tPLH tPHL tSKD tZL tZH tLZ tHZ fMAX tSHDN PARAMETER Driver Input to Output Driver Input to Output Driver Output to Output Driver Rise or Fall Time Driver Enable to Output High Driver Enable to Output Low Driver Disable Time from Low Driver Disable Time from High Receiver Input to Output Receiver Input to Output
tPLH - tPHL Differential Receiver Skew
Receiver Enable to Output Low Receiver Enable to Output High Receiver Disable from Low Receiver Disable from High Maximum Data Rate Time to Shutdown
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0C TA 70C, VCC = 5V (Notes 2, 3) unless otherwise noted.
MIN
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CONDITIONS RDIFF = 54, CL1 = CL2 = 100pF, (Figures 3, 5)
TYP
MAX 1200 1200
UNITS ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns kbps ns
150 150 250 150 100 100 150 150 30 30 140 140 13 20 20 20 20 250 50 200
600 1200 1500 1500 1500 1500 250 250 50 50 50 50 600
CL = 100pF (Figures 4, 6), S2 Closed CL = 100pF (Figures 4, 6), S1 Closed CL = 15pF (Figures 4, 6), S1 Closed CL = 15pF (Figures 4, 6), S2 Closed RDIFF = 54, CL1 = CL2 = 100pF, (Figures 3, 7)
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CRL = 15pF (Figures 2, 8), S1 Closed CRL = 15pF (Figures 2, 8), S2 Closed CRL = 15pF (Figures 2, 8), S1 Closed CRL = 15pF (Figures 2, 8), S2 Closed DE = 0, RE =
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LTC1487
SWITCHI G CHARACTERISTICS
SYMBOL tZH(SHDN) tZL(SHDN) tZH(SHDN) tZL(SHDN) PARAMETER Driver Enable from Shutdown to Output High Driver Enable from Shutdown to Output Low Receiver Enable from Shutdown to Output High Receiver Enable from Shutdown to Output Low
The q 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 2: All currents into device pins are positive; all currents out ot device pins are negative. All voltages are referenced to device ground unless otherwise specified.
TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current vs Temperature
450 400
OUTPUT CURRENT (mA)
SUPPLY CURRENT (A)
350 300 250 200 150 100 50 DRIVER ENABLED WITH NO LOAD DRIVER DISABLED WITH NO LOAD 0 25 50 75 100 125 150 175 TEMPERATURE (C)
LTC1487 * TPC01
DIFFERENTIAL VOLTAGE (V)
THERMAL SHUTDOWN WITH DRIVER ENABLED AND NOMINAL LOAD
0 - 50 -25
Driver Output Low Voltage vs Output Current
120 TA = 25C 100
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
80 60 40 20 0 0 1 2 3 OUTPUT VOLTAGE (V) 4
TIME (ns)
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0C TA 70C, VCC = 5V (Notes 2, 3) unless otherwise noted.
MIN
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CONDITIONS CL = 100pF (Figures 4, 6), S2 Closed CL = 100pF (Figures 4, 6), S1 Closed CL = 15pF (Figures 2, 8), S2 Closed CL = 15pF (Figures 2, 8), S1 Closed
TYP
MAX 2000 2000 2000 2000
UNITS ns ns ns ns
Note 3: All typicals are given for VCC = 5V and TA = 25C. Note 4: The LTC1487 is not tested and is not quality-assurance sampled at - 40C and at 85C. These specifications are guaranteed by design, correlation, and/or inference from 0C, 25C and/or 70C tests.
Driver Differential Output Voltage vs Output Current
80 TA = 25C 70 60 50 40 30 20 10 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 OUTPUT VOLTAGE (V)
LTC1487 * TPC02
Driver Differential Output Voltage vs Temperature
2.24 2.22 2.20 2.18 2.16 2.14 2.12 2.10 2.08 2.06 2.04 2.02 2.00 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125 RL = 54
LTC1487 * TPC03
Driver Output High Voltage vs Output Current
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 0 1 3 4 2 OUTPUT VOLTAGE (V) 5
400 350 300 250 200 500
Driver Skew vs Temperature
450
TA = 25C
150 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
LTC1487 * TPC04
LTC1487 * TPC05
LTC1487 * G06
LTC1487
PIN FUNCTIONS
RO (Pin 1): Receiver Output. If the receiver output is enabled (RE LOW), and A > B by 200mV, RO will be HIGH. If A < B by 200mV, then RO will be LOW. RE (Pin 2): Receiver Output Enable. A LOW enables the receiver output, RO. A HIGH input forces the receiver output into a high impedance state. DE (Pin 3): Driver Outputs Enable. A HIGH on DE enables the driver output. A and B and the chip will function as a line driver. A LOW input will force the driver outputs into a high impedance state and the chip will function as a line receiver. If RE is HIGH and DE is LOW, the part will enter a low power (1A) shutdown state. DI (Pin 4): Driver Input. If the driver outputs are enabled (DE HIGH) then a LOW on DI forces the outputs A LOW and B HIGH. A HIGH on DI with the driver outputs enabled will force A HIGH and B LOW. GND (Pin 5): Ground. A (Pin 6): Driver Output/Receiver Input. B (Pin 7): Driver Output/Receiver Input. VCC (Pin 8): Positive Supply. 4.75V < VCC < 5.25V.
FU CTIO TABLES
LTC1487 Transmitting
INPUTS RE X X 0 1 DE 1 1 0 0 DI 1 0 X X 0 1 Z Z* OUTPUTS B A 1 0 Z Z* RE 0 0 0 1
*Shutdown mode
TEST CIRCUITS
A R VOD R B
LTC1487 * F01
Figure 1. Driver DC Test Load
3V DE A DI B RDIFF CL2 CL1 A
Figure 3. Driver/Receiver Timing Test Circuit
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LTC1487 Receiving
INPUTS DE 0 0 0 0 A-B 0.2V - 0.2V Inputs Open X OUTPUTS RO 1 0 1 Z*
*Shutdown mode
RECEIVER OUTPUT
VOC
TEST POINT
S1
1k VCC
CRL
1k
S2
LTC1487 * F02
Figure 2. Receiver Timing Test Load
S1
RO B RE
LTC1487 * F03
15pF
OUTPUT UNDER TEST
500 S2 CL
LTC1487 * F04
VCC
Figure 4. Driver Timing Test Load
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LTC1487
SWITCHI G TI E WAVEFOR S
3V DI 0V t PLH B VO A VO 0V -VO 1/2 VO 10% tr tSKEW 90% VDIFF = V(A) - V(B) tf t SKEW 90% 10%
LTC1487 * F05
1.5V
Figure 5. Driver Propagation Delays
3V DE 0V 5V A, B VOL VOH A, B 0V 2.3V 1.5V
Figure 6. Driver Enable and Disable Times
VOH RO VOL t PHL VOD2 A-B -VOD2 0V 1.5V OUTPUT f = 1MHz, tr 10ns, tf 10ns INPUT t PLH 0V
LTC1487 * F07
Figure 7. Receiver Propagation Delays
3V RE 0V 5V RO 1.5V f = 1MHz, tr 10ns, tf 10ns t ZL(SHDN), tZL 1.5V OUTPUT NORMALLY LOW t LZ 0.5V 1.5V
RO 0V
Figure 8. Receiver Enable and Disable Times
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f = 1MHz, tr 10ns, tf 10ns t PHL
1.5V 1/2 VO
f = 1MHz, tr 10ns, tf 10ns t ZL(SHDN), t ZL t LZ
1.5V
2.3V
OUTPUT NORMALLY LOW
0.5V
OUTPUT NORMALLY HIGH t HZ
0.5V
LTC1487 * F06
t ZH(SHDN), t ZH
1.5V
1.5V
OUTPUT NORMALLY HIGH t HZ
0.5V
LTC1487 * F08
t ZH(SHDN), tZH
LTC1487
APPLICATIO S I FOR ATIO
High Input Impedance
The LTC1487 is designed with a 96k (typ) input impedance to allow up to 256 transceivers to share a single RS485 differential data bus. The RS485 specification requires that a transceiver be able to drive as many as 32 "unit loads." One unit load (UL) is defined as an impedance that draws a maximum of 1mA with up to 12V across it. Typical RS485 transceivers present between 0.5 and 1 unit load at their inputs. The 96k input impedance of the LTC1487 will draw only 125A under the same 12V condition, presenting only 0.125UL to the bus. As a result, 256 LTC1487 transceivers (32UL/0.125UL = 256) can be connected to a single RS485 data bus without exceeding the RS485 driver load specification. The LTC1487 meets all other RS485 specifications, allowing it to operate equally well with standard RS485 transceiver devices or high impedance transceivers. CMOS Output Driver The RS485 specification requires that a transceiver withstand common-mode voltages of up to 12V or -7V at the RS485 line connections. Additionally, the transceiver must be immune to both ESD and latch-up. This rules out traditional CMOS drivers, which include parasitic diodes from their driver outputs to each supply rail (Figure 9). The LTC1487 uses a proprietary process enhancement which adds a pair of Schottky diodes to the output stage (Figure 10), preventing current from flowing when the commonmode voltage exceeds the supply rails. Latch-up at the output drivers is virtually eliminated and the driver is prevented from loading the line under RS485 specified fault conditions. A proprietary output protection structure protects the transceiver line terminals against ESD strikes (Human Body Model) of up to 10kV.
VCC P1 D1 LOGIC N1 OUTPUT D2
LTC1487 * F09
Figure 9. Conventional CMOS Output Stage
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.
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VCC SD3 P1 D1 OUTPUT LOGIC SD4 D2 N1
LTC1487 * F10
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Figure 10. LTC1487 Output Stage
When two or more drivers are connected to the same transmission line, a potential condition exists whereby more than two drivers are simultaneously active. If one or more drivers is sourcing current while another driver is sinking current, excessive power dissipation may occur within either the sourcing or sinking element. This condition is defined as driver contention, since multiple drivers are competing for one transmission line. The LTC1487 provides a current limiting scheme to prevent driver contention failure. When driver contention occurs, the current drawn is limited to about 70mA, preventing excessive power dissipation within the drivers. The LTC1487 has a thermal shutdown feature which protects the part from excessive power dissipation. Under extreme fault conditions, up to 250mA can flow through the part, causing rapid internal temperature rise. The thermal shutdown circuit will disable the driver outputs when the internal temperature reaches 150C and turns them back on when the temperature cools to 130C. This cycle will repeat as necessary until the fault condition is removed. Receiver Inputs The LTC1487 receiver features an input common-mode range covering the entire RS485 specified range of -7V to 12V. Internal 96k input resistors from each line terminal to ground provide the 0.125UL load to the RS485 bus. Differential signals of greater than 200mV within the specified input common-mode range will be converted to a TTL-compatible signal at the receiver output. A small amount of input hysteresis is included to minimize the
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LTC1487
APPLICATIO S I FOR ATIO
effects of noise on the line signals. If the line is terminated or the receiver inputs are shorted together, the receiver output will retain the last valid line signal due to the 45mV of hysteresis incorporated in the receiver circuit. If the LTC1487 transceiver inputs are left floating (unterminated), an internal pull-up of 10A at the A input will force the receiver output to a known high state. Low Power Operation The LTC1487 draws very little supply current whenever the driver outputs are disabled. In shutdown mode, the quiescent current is typically less than 1A. With the receiver active and the driver outputs disabled, the LTC1487 will typically draw 80A quiescent current. With the driver outputs enabled but unterminated, quiescent current will rise slightly as one of the two outputs sources current into the internal receiver input resistance. With the minimum receiver input resistance of 70k and the maximum output swing of 5V, the quiescent current will rise by a maximum of 72A. Typical quiescent current rise with the driver enabled is about 40A. The quiescent current rises significantly if the driver is enabled when it is externally terminated. With 1/2 termination load (120 between the driver outputs), the quiescent current will jump to at least 13mA as the drivers force a minimum of 1.5V across the termination resistance. With a fully terminated 60 line attached, the current will rise to greater than 25mA with the driver enabled, completely overshadowing the extra 40A drawn by the internal receiver inputs. Shutdown Mode Both the receiver output (RO) and the driver outputs (A, B) can be placed in three-state mode by bringing RE HIGH and DE LOW respectively. In addition, the LTC1487 will enter shutdown mode when RE is HIGH and DE is LOW.
PACKAGE DESCRIPTION
For package descriptions consult the 1994 Linear Databook Volume III.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
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In shutdown the LTC1487 typically draws only 1A of supply current. In order to guarantee that the part goes into shutdown, RE must be HIGH and DE must be LOW for at least 600ns simultaneously. If this time duration is less than 50ns the part will not enter shutdown mode. Toggling either RE or DE will wake the LTC1487 back up within 3.5s. If the driver is active immediately prior to shutdown, the supply current will not drop to 1A until the driver outputs have reached a steady state; this can take as long as 2.6s under worst case conditions. If the driver is disabled prior to shutdown the supply current will drop to 1A immediately. Slew Rate and Propagation Delay Many digital encoding schemes are dependent upon the difference in the propagation delay times of the driver and receiver. Figure 11 shows the test circuit for the LTC1487 propagation delay.
100pF TTL IN t r, t f < 6ns D R 100
LTC1487 * F11
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BR R
RECEIVER OUT
100pF
Figure 11. Receiver Propagation Delay Test Circuit
The receiver delay times are: tPLH - tPHL = 13ns Typ, VCC = 5V The LTC1487 drivers feature controlled slew rate to reduce system EMI and improve signal fidelity by reducing reflections due to misterminated cables. The driver's skew times are: Skew = 250ns Typ, VCC = 5V 600ns Max, VCC = 5V, TA = - 40C to 85C
LT/GP 0395 10K * PRINTED IN THE USA
(c) LINEAR TECHNOLOGY CORPORATION 1995

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