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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 20Mbps and Slew Rate Limited 15kV RS485/RS422 Transceivers DESCRIPTION
The LTC(R)2856-1, LTC2856-2, LTC2857-1, LTC2857-2, LTC2858-1 and LTC2858-2 are low power, RS485/RS422 transceivers operating on 5V supplies at maximum data rates of 20Mbps or 250kbps for low EMI. The receiver has a one-eighth unit load, supporting up to 256 nodes per bus (C, I-Grade), and a failsafe feature that guarantees a high output state under conditions of floating or shorted inputs. The driver maintains a high output impedance over the entire common mode range when disabled or when the supply is removed. Excessive power dissipation caused by bus contention or a fault is prevented by current limiting all outputs and by thermal shutdown. Enhanced ESD protection allows these parts to withstand 15kV (human body model) on the transceiver interface pins without latchup or damage.
PART NUMBER LTC2856-1 LTC2856-2 LTC2857-1 LTC2857-2 LTC2858-1 LTC2858-2 MAX DATA RATE (Mbps) 20 0.25 20 0.25 20 0.25 DUPLEX Half Half Full Full Full Full PACKAGE MSOP-8, DFN-8 MSOP-8, DFN-8 MSOP-8, DFN-8 MSOP-8, DFN-8 MSOP-10, DFN-10 MSOP-10, DFN-10
FEATURES

20Mbps or Low EMI 250kbps Maximum Data Rate No Damage or Latchup to ESD: 15kV HBM High Input Impedance Supports 256 Nodes (C, I-Grade) Operation Up to 125C (H-Grade) Guaranteed Failsafe Receiver Operation Over the Entire Common Mode Range Current Limited Drivers and Thermal Shutdown Delayed Micropower Shutdown: 5A Maximum (C, I-Grade) Power Up/Down Glitch-Free Driver Outputs Low Operating Current: 900A Maximum in Receive Mode Compatible with TIA/EIA-485-A Specifications Available in 8-Lead and 10-Lead 3mm x 3mm DFN and 8-Lead and 10-Lead MSOP Packages
APPLICATIONS

Low Power RS485/RS422 Transceiver Level Translator Backplane Transceiver
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
LTC2856-1 RO1 RE1 DE1 DI1 D GND1 2V/DIV R VCC1 RT
LTC2858-1 at 20Mbps
DI Y Z
LTC2856-1 RO2 RE2 DE2 DI2 D GND2
285678 TA01a
R
VCC2
RT
Y-Z
20ns/DIV
2856 TA01b
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 ABSOLUTE MAXIMUM RATINGS
(Note 1)
Supply Voltage (VCC) ................................... -0.3V to 7V Logic Input Voltages (RE, DE, DI) ................ -0.3V to 7V Interface I/O (A, B, Y, Z) ....................(VCC - 15V) to 15V Receiver Output Voltage (RO) .......-0.3V to (VCC + 0.3V) Operating Temperature Range (Note 4) LTC285xC ................................................ 0C to 70C LTC285xI.............................................. -40C to 85C LTC285xH .......................................... -40C to 125C
Storage Temperature Range MSOP ................................................ -65C to 150C DFN.................................................... -65C to 150C Lead Temperature (Soldering, 10 sec) MSOP ............................................................... 300C
PACKAGE INFORMATION
LTC2856
TOP VIEW RO 1 RE 2 DE 3 DI 4 9 8 7 6 5 VCC B A GND TOP VIEW RO RE DE DI 1 2 3 4 8 7 6 5 VCC B A GND
MS8 PACKAGE 8-LEAD PLASTIC MSOP DD PACKAGE 8-LEAD (3mm 3mm) PLASTIC DFN
TJMAX = 150C, JA = 200C/W, JC = 40C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB TJMAX = 150C, JA = 43C/W, JC = 3C/W LTC2857
TOP VIEW VCC 1 RO 2 DI 3 GND 4 9 8 7 6 5 A B Z Y TOP VIEW VCC RO DI GND 1 2 3 4 8 7 6 5 A B Z Y
MS8 PACKAGE 8-LEAD PLASTIC MSOP DD PACKAGE 8-LEAD (3mm 3mm) PLASTIC DFN
TJMAX = 150C, JA = 200C/W, JC = 40C/W
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB TJMAX = 150C, JA = 43C/W, JC = 3C/W LTC2858
TOP VIEW RO RE DE DI GND 1 2 3 4 5 11 10 VCC 9A 8B 7Z 6Y TOP VIEW RO RE DE DI GND 1 2 3 4 5 10 9 8 7 6 VCC A B Z Y
DD PACKAGE 10-LEAD (3mm 3mm) PLASTIC DFN
MS PACKAGE 10-LEAD PLASTIC MSOP
TJMAX = 150C, JA = 120C/W, JC = 45C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB TJMAX = 150C, JA = 43C/W, JC = 3C/W
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 ORDER INFORMATION
LTC2856 C DD -1 #TR PBF LEAD FREE DESIGNATOR TAPE AND REEL TR = Tape and Reel SPEED -1 = 20Mbps Max. Data Rate -2 = 250kbps Max. Data Rate PACKAGE TYPE DD = 8-Lead Plastic DFN (LTC2856, LTC2858) MS8 = 8-Lead Plastic MSOP (LTC2856, LTC2857) MS = 10-Lead Plastic MSOP (LTC2858) TEMPERATURE GRADE C = Commercial Temperature Range (0C to 70C) I = Industrial Temperature Range (-40C to 85C) H = Automotive Temperature Range (-40C to 125C) PRODUCT PART NUMBER LTC2856 = Half Duplex, with Enables LTC2857 = Full Duplex, no Enables LTC2858 = Full Duplex, with Enables Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
PRODUCT SELECTION GUIDE
PART NUMBER LTC2856-1 LTC2856-2 LTC2857-1 LTC2857-2 LTC2858-1 LTC2858-2 PART MARKING LTCMF, LCMG LTCMH, LCMJ LTCMC, LCMD LTCMK, LCMM LTCGQ, LCGR LTCMQ, LCMR MAX DATA RATE (Mbps) 20 0.25 20 0.25 20 0.25 DUPLEX Half Half Full Full Full Full LOW POWER SHUTDOWN MODE Yes Yes No No Yes Yes PACKAGE MSOP-8, DFN-8 MSOP-8, DFN-8 MSOP-8, DFN-8 MSOP-8, DFN-8 MSOP-10, DFN-10 MSOP-10, DFN-10
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 ELECTRICAL CHARACTERISTICS
SYMBOL Driver |VOD| Differential Driver Output Voltage R = , VCC = 4.5V (Figure 1) R = 27 (RS485), VCC = 4.5V (Figure 1) R = 50 (RS422), VCC = 4.5V (Figure 1) |VOD| 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 Driver Three-State (High Impedance) Output Current on Y and Z Maximum Driver Short-Circuit Current Receiver Input Current (A, B) R = 27 or R = 50 (Figure 1)

The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V unless otherwise noted. (Note 2)
PARAMETER CONDITIONS MIN TYP MAX VCC 1.5 2 VCC VCC 0.2 UNITS V V V V
VOC |VOC|
R = 27 or R = 50 (Figure 1) R = 27 or R = 50 (Figure 1)

3 0.2
V V
IOZD IOSD Receiver IIN
DE = 0V, (Y or Z) = -7V, 12V, LTC2858-1, LTC2858-2 -7V (Y or Z) 12V (Figure 2) DE = TE = 0V, VCC = 0V or 5V, VIN = 12V (Figure 3) (C, I-Grade) DE = TE = 0V, VCC = 0V or 5V, VIN = -7V, (Figure 3) (C, I-Grade) DE = TE = 0V, VCC = 0V or 5V, VIN = 12V (Figure 3) (H-Grade) DE = TE = 0V, VCC = 0V or 5V, VIN = -7V, (Figure 3) (H-Grade) RE = VCC or 0V, DE = TE = 0V, VIN = -7V, -3V, 3V, 7V, 12V (Figure 3) (C, I-Grade) RE = VCC or 0V, DE = TE = 0V, VIN = -7V, -3V, 3V, 7V, 12V (Figure 3) (H-Grade) -7V B 12V B = 0V I(RO) = -4mA, A-B = 200mV, VCC = 4.5V I(RO) = 4mA, A-B = -200mV, VCC = 4.5V RE = 5V, 0V RO VCC, LTC2856-1, LTC2856-2, LTC2858-1, LTC2858-2 DE, DI, RE, VCC = 5.5V DE, DI, RE, VCC = 4.5V DE, DI, RE DE = 0V, RE = VCC, LTC2856, LTC2858 (C and I-Grade) LTC2856, LTC2858 (H-Grade) No Load, DE = 0V, RE = 0V, LTC2856-1, LTC2856-2, LTC2858-1, LTC2858-2

10 120 250 125 -100 250 -145 96 125
A mA A A A A k

RIN
Receiver Input Resistance
48
125
k
VTH VTH VOH VOL IOZR Logic VIH VIL IINL Supplies ICCS
Receiver Differential Input Threshold Voltage Receiver Input Hysteresis Receiver Output High Voltage Receiver Output Low Voltage Receiver Three-State (High Impedance) Output Current on RO Logic Input High Voltage Logic Input Low Voltage Logic Input Current Supply Current in Shutdown Mode
0.2 25 2.4 0.4 1
V mV V V A

2 0.8 0 0 0 540 10 5 15 900
V V A A A A

ICCR
Supply Current in Receive Mode
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 SWITCHING CHARACTERISTICS
SYMBOL ICCT PARAMETER Supply Current in Transmit Mode
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 5V unless otherwise noted. (Note 2)
CONDITIONS No Load, DE = VCC, RE = VCC LTC2856-1 LTC2858-1 LTC2856-2 LTC2858-2 LTC2856-1 LTC2857-1 LTC2858-1 LTC2856-2 LTC2857-2 LTC2858-2

MIN
TYP 630 670 660
MAX 1000 1100 1100
UNITS A A A
ICCTR
Supply Current with Both Driver and Receiver Enabled
No Load, DE = VCC, RE = 0V
700
1200
A
Driver in LTC2856-1, LTC2857-1, LTC2858-1 fMAX tPLHD, tPHLD tPD tSKEWD tRD, tFD tZLD, tZHD, tLZD, tHZD tZHSD, tZLSD tSHDN Maximum Data Rate Driver Input to Output Driver Output Y to Output Z Driver Rise or Fall Time Driver Enable or Disable Time Driver Enable from Shutdown Time to Shutdown (Note 3) RDIFF = 54, CL = 100pF (Figure 4) RDIFF = 54, CL = 100pF (Figure 4) RDIFF = 54, CL = 100pF (Figure 4) RL = 500, CL = 50pF, RE = 0V, LTC2856-1, LTC2858-1 (Figure 5) RL = 500, CL = 50pF, RE = VCC, LTC2856-1, LTC2858-1 (Figure 5) (DE = , RE = VCC) or (DE = 0V, RE = ), LTC2856-1, LTC2858-1 (Figure 5) (Note 3) RDIFF = 54, CL = 100pF (Figure 4) RDIFF = 54, CL = 100pF (Figure 4) RDIFF = 54, CL = 100pF (Figure 4) RL = 500, CL = 50pF, RE = 0V, LTC2856-2, LTC2858-2 (Figure 5) RL = 500, CL = 50pF, RE = 0V, LTC2856-2, LTC2858-2 (Figure 5) RL = 500, CL 50pF, RE = VCC, LTC2856-2, LTC2858-2 (Figure 5) (DE = 0V, RE) or (DE = , RE = VCC), LTC2856-2, LTC2858-2 (Figure 5) CL = 15pF, VCM = 1.5V, |VAB| = 1.5V, tR and tF < 4ns (Figure 6) CL = 15pF (Figure 6) CL = 15pF (Figure 6) RL = 1k, CL = 15pF, DE = VCC, LTC2856-1, LTC2856-2, LTC2858-1, LTC2858-2 (Figure 7) RL = 1k, CL = 15pF, DE = 0V, LTC2856-1, LTC2856-2, LTC2858-1, LTC2858-2 (Figure 7)

20 10 1 1 4 50 6 6 12.5 70 8 100
Mbps ns ns ns ns ns s ns
Driver Input to Output Difference |tPLHD - tPHLD| RDIFF = 54, CL = 100pF (Figure 4)
Driver in LTC2856-2, LTC2857-2, LTC2858-2 fMAXS tPDS tSKEWDS tRDS, tFDS tZHDS, tZLDS tLZDS, tHZDS Maximum Data Rate

250 0.95 50 200 0.90 1.5 500 500 1.5 300 70 8 500
kbps s ns ns s ns ns s ns
tPLHDS, tPHLDS Driver Input to Output Driver Output A to Output B Driver Rise or Fall Time Driver Enable Time Driver Disable Time
Driver Input to Output Difference |tPLHD - tPHLD| RDIFF = 54, CL = 100pF (Figure 4)
tZHSDS, tZLSDS Driver Enable from Shutdown tSHDNS Receiver tPLHR, tPHLR tSKEWR tRR, tFR tZLR, tZHR, tLZR, tHZR tZHSR, tZLSR Receiver Input to Output Differential Receiver Skew, |tPLHR - tPHLR| Receiver Output Rise or Fall Time Receiver Enable/Disable Time to Shutdown

50 1 3
70 6 12.5 50
ns ns ns ns
Receiver Enable from Shutdown
8
s
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. High temperatures degrade operating lifetimes. Operating lifetime is derated at temperatures greater than 105C. Note 2: 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 3: Maximum data rate is guaranteed by other measured parameters and is not tested directly. Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Overtemperature protection actives at a junction temperature exceeding 150C. Continuous operation above the specified maximum operating junction temperature may result in device degradation or failure.
TEST CIRCUITS
Y GND OR VCC DI Y
DRIVER
+
VOD
R
-
R
Z
VOC
+ -
GND OR VCC
DI
IOSD DRIVER
Z
+ -
285678 F02
-7V TO +12V
285678 F01
Figure 1. Driver DC Characteristics
Figure 2. Driver Output Short-Circuit Current
IIN
A OR B RECEIVER B OR A
285678 F03
VIN
+ -
V RIN = IN IIN
Figure 3. Receiver Input Current and Input Resistance
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 TEST CIRCUITS
VCC Y DI CL DRIVER RDIFF CL Z
285678 F04a
DI 0V
tPLHD, tPLHDS
tPHLD, tPHLDS tSKEWD,tSKEWDS
Y, Z
VO
1/2 VO
90% Y-Z 10% 0 tRD, tRDS 0
90% 10% tFD, tFDS
285678 F04b
Figure 4. Driver Timing Measurement
VCC Y VCC OR GND DI CL DRIVER RL DE Z CL VCC OR GND
285678 F05a
RL
GND OR VCC
DE 0V VCC Y OR Z VOL VOH Z OR Y 0V VO
1/2 VCC
tZLD, tZLDS, tZLSD, tZLSDS 1/2 VCC
tLZD, tLZDS 0.5V 0.5V tHZD, tHZDS, tSHDN, tSHDNS
285678 F05b
1/2 VCC tZHD, tZHDS, tZHSD, tZHSDS
Figure 5. Driver Enable and Disable Timing Measurement
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 TEST CIRCUITS
VAB/2 VCM VAB/2 B A RECEIVER RO CL
285678 F06
VAB A-B -VAB VCC RO 0 VO
0 tPLHR 90% 1/2 VCC 10% tRR tSKEWR = tPLHR - tPHLR 1/2 VCC tPHLR 90% 10% tFR
285678 F06b
Figure 6. Receiver Propagation Delay Measurements
VCC RE 0V OR VCC A RECEIVER VCC OR 0V B RE RO CL RL VCC OR GND 0V VCC RO VOL VOH DI = 0V OR VCC
285678 F07a
1/2 VCC tZLR, tZLSR VO 1/2 VCC tLZR 0.5V 0.5V
285678 F05b
RO 0V tZHR, tZHSR
1/2 VCC tHZR
Figure 7. Receiver Enable/Disable Time Measurements
FUNCTION TABLES
LTC2856-1, LTC2856-2
LOGIC INPUTS DE 0 0 1 1 RE 0 1 0 1 MODE Receive Shutdown Transceive Transmit A, B RIN RIN Driven Driven RO Driven High-Z Driven High-Z
LTC2858-1, LTC2858-2
LOGIC INPUTS DE 0 0 1 1 RE 0 1 0 1 MODE Receive Shutdown Transceive Transmit A, B RIN RIN RIN RIN Y, Z High-Z High-Z Driven Driven RO Driven High-Z Driven High-Z
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 TYPICAL PERFORMANCE CHARACTERISTICS
Receiver Skew vs Temperature
2 VAB = 1.5V CL = 15pF RECEIVER SKEW (ns) 2 RDIFF = 54 CL = 100pF DRIVER PROP DELAY (ns)
TA = 25C, VCC = 5V unless otherwise noted. Driver Propagation Delay vs Temperature
18 16 14 12 10 8 6 RDIFF = 54 CL = 100pF
Driver Skew vs Temperature
1
DRIVER SKEW (ns)
1
0
0
-1 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
385678 G01
-1 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
385678 G02
4 -40 -20
0
20
40
60
80
100 120
285678 G03
TEMPERATURE (C)
Driver Output Short-Circuit Current vs Temperature
160 OUTPUT SHORT-CIRCUIT CURRENT (mA) 5
Driver Output Low/High Voltage vs Output Current
5 VOH OUTPUT VOLTAGE (V)
Driver Differential Output Voltage vs Temperature
RDIFF =
OUTPUT VOLTAGE (V)
140
SOURCE (VOUT = 0V)
4
4 3 RDIFF = 100
3
120 SINK (VOUT = 5V) 100
2 VOL 1
2
RDIFF = 54
1
80 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
285678 G04
0 0 10
40 30 20 50 OUTPUT CURRENT (mA)
60
70
0 -40 -20
0
20 40 60 80 TEMPERATURE (C)
100 120
285678 G06
285678 G05
Receiver Output Voltage vs Output Current (Source and Sink)
5 SOURCE 4 70 65 60 PROP DELAY (ns) 3 55 50 45 40 SINK 0 0 1 3 4 2 OUTPUT CURRENT (mA) 5
285678 G07
Receiver Propagation Delay vs Temperature
60 VAB = 1.5V CL = 15pF SUPPLY CURRENT (mA) 50 40
Supply Current vs Data Rate
CL = 100pF RDIFF = 54
OUTPUT VOLTAGE (V)
RDIFF = 100 30 20 10 RDIFF = 0
2
1
35 30 -40 -20 0 20 40 60 80 TEMPERATURE (C) 100 120
285678 G08
0.1
1 10 DATA RATE (Mbps)
100
285678 G09
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 PIN FUNCTIONS
PIN NUMBER PIN NAME RO LTC2856-1 LTC2856-2 1 LTC2857-1 LTC2857-2 2 LTC2858-1 LTC2858-2 1 DESCRIPTION Receiver Output. If the receiver output is enabled (RE low) and A > B by 200mV, then RO will be high. If A < B by 200mV, then RO will be low. If the receiver inputs are open, shorted or terminated without a valid signal, RO will be high. Receiver Enable. A low enables the receiver. A high input forces the receiver output into a high impedance state. Driver Enable. A high on DE enables the driver. A low input will force the driver outputs into a high impedance. If RE is high with DE low, the part will enter a low power shutdown state. Driver Input. If the driver outputs are enabled (DE high), then a low on DI forces the driver positive output low and negative output high. A high on DI, with the driver outputs enabled, forces the driver positive output high and negative output low. Ground. *The Exposed Pad on the DFN packages should be connected to ground. Noninverting Driver Output for the LTC2857-1, LTC2857-2, LTC2858-1 and LTC2858-2. High impedance when the driver is disabled (LTC2858-1) or unpowered. Inverting Driver Output for the LTC2857-1, LTC2857-2, LTC2858-1 and LTC2858-2. High impedance when the driver is disabled (LTC2858-1) or unpowered. Inverting Receiver Input (and Inverting Driver Output for the LTC2856-1 and LTC2856-2). Impedance is > 96k in receive mode or unpowered. Noninverting Receiver Input (and Noninverting Driver Output for the LTC2856-1 and LTC2856-2). Impedance is > 96k in receive mode or unpowered. Positive Supply. 4.5V < VCC < 5.5V. Bypass with a 0.1F ceramic capacitor.
RE DE
2 3
2 3
DI
4
3
4
GND Y
5/9*
4/9* 5
5/11* 6
Z
6
7
B A VCC
7 6 8
7 8 1
8 9 10
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 BLOCK DIAGRAMS
LTC2856-1, LTC2856-2 LTC2857-1, LTC2857-2
A (15kV) RO RECEIVER RO RECEIVER B (15kV) RE DE A (15kV) SLEEP/SHUTDOWN LOGIC AND DELAY B (15kV) Z (15kV) DI DRIVER DI DRIVER Y (15kV)
2856 BD 2857 BD
LTC2858-1, LTC2858-2
A (15kV) RO RECEIVER B (15kV) RE DE
SLEEP/SHUTDOWN LOGIC AND DELAY
Z (15kV) DI DRIVER Y (15kV)
2858 BD
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 APPLICATIONS INFORMATION
Driver The driver provides full RS485 and RS422 compatibility. When enabled, if DI is high, Y-Z is positive for the full-duplex devices and A-B is positive for the half-duplex device. When the driver is disabled, both outputs are high impedance. For the full-duplex devices, the leakage on the driver output pins is guaranteed to be less than 10A over the entire common mode range of -7V to 12V. On the halfduplex device, the impedance is dominated by the receiver input resistance, RIN. Driver Overvoltage and Overcurrent Protection The driver outputs are protected from short circuits to any voltage within the Absolute Maximum range of (VCC - 15V) to 15V. The maximum current in this condition is 250mA. If the pin voltage exceeds about 10V, current limit folds back to about half of the peak value to reduce overall power dissipation and avoid damaging the part. All devices also feature thermal shutdown protection that disables the driver and receiver output in case of excessive power dissipation (see Note 4). Slew Limiting for EMI Emissions Control The LTC2856-2, LTC2857-2 and the LTC2858-2 feature reduced slew rate driver outputs to control the high frequency EMI emissions from equipment and data cables. These devices are limited to data rates of 250kbaud or less. Slew limiting also mitigates the adverse affects of imperfect transmission line termination caused by stubs or mismatched cable. Figures 10 and 11 show the output waveforms from the LTC2858-1 and its slew rate limited counterpart, the LTC2858-2, operating at 250kbps. The corresponding frequency spectrums show significant reduction in the high frequency harmonics for the slew rate limited device. Receiver and Failsafe With the receiver enabled, when the absolute value of the differential voltage between the A and B pins is greater than 200mV, the state of RO will reflect the polarity of (A-B). These parts have a failsafe feature that guarantees the receiver output to be in a logic-high state when the inputs are either shorted, left open or terminated, but not driven for more than about 3s. The delay prevents signal zero crossings from being interpreted as shorted inputs and causing RO to go high inadvertently. This failsafe feature is guaranteed to work for inputs spanning the entire common mode range of -7V to 12V. The receiver output is internally driven high (to VCC) or low (to ground) with no external pull-up needed. When the receiver is disabled the RO pin becomes high-Z with leakage of less than 1A for voltages within the supply range. Receiver Input Resistance The receiver input resistance from A or B to ground is guaranteed to be greater than 96k (C, I-Grade). This is 8x higher than the requirements for the RS485 standard and thus this receiver represents a one-eighth unit load. This, in turn, means that 8x the standard number of receivers, or 256 total, can be connected to a line without loading it beyond what is called out in the RS485 standard. The receiver input resistance from A or B to ground on high temperature H-Grade parts is greater than 48k providing a one-quarter unit load. The input resistance of the receivers is unaffected by enabling/disabling the receiver and by powering/unpowering the part. Supply Current The unloaded static supply currents in these devices are very low--typically under 700A for all modes of operation. In applications with resistively terminated cables, the supply current is dominated by the driver load. For example, when using two 120 terminators with a differential driver output voltage of 2V, the DC load current is 33mA, which is sourced by the positive voltage supply. Power supply current increases with toggling data due to capacitive loading and this term can increase significantly at high data rates. Figure 8 shows supply current vs data rate for two different capacitive loads for the circuit configuration of Figure 4.
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 APPLICATIONS INFORMATION
75 70 65 CL = 1000pF 60 55 CL = 100pF 50 45 RDIFF = 54
Cable Length vs Data Rate For a given data rate, the maximum transmission distance is bounded by the cable properties. A typical curve of cable length vs data rate compliant with the RS485 standard is shown in Figure 9. Three regions of this curve reflect different performance limiting factors in data transmission. In the flat region of the curve, maximum distance is determined by resistive losses in the cable. The downward sloping region represents limits in distance and data rate due to AC losses in the cable. The solid vertical line represents the specified maximum data rate in the RS485 standard. The dashed line at 250kbps shows the maximum data rate of the low-EMI LTC2856-2, LTC2857-2, and LTC2858-2. The dashed line at 20Mbps shows the maximum data rates of the LTC2856-1, LTC2857-1 and LTC2858-1.
10k
CURRENT (mA)
102
104 103 DATA RATE (kbps)
105
285678 F08
Figure 8. Supply Current vs Data Rate
High Speed Considerations A ground plane layout is recommended. A 0.1F bypass capacitor less than one-quarter inch away from the VCC pin is also recommended. The PC board traces connected to signals A/B and Z/Y should be symmetrical and as short as possible to maintain good differential signal integrity. To minimize capacitive effects, the differential signals should be separated by more than the width of a trace and should not be routed on top of each other if they are on different signal planes. Care should be taken to route outputs away from any sensitive inputs to reduce feedback effects that might cause noise, jitter or even oscillations. For example, in the full-duplex LTC2857-1, DI and A/B should not be routed near the driver or receiver outputs. The logic inputs have 100mV of hysteresis to provide noise immunity. Fast edges on the outputs can cause glitches in the ground and power supplies which are exacerbated by capacitive loading. If a logic input is held near its threshold (typically 1.5V), a noise glitch from a driver transition may exceed the hysteresis levels on the logic and data input pins causing an unintended state change. This can be avoided by maintaining normal logic levels on the pins and by slewing inputs through their thresholds by faster than 1V/s when transitioning. Good supply decoupling and proper line termination also reduce glitches caused by driver transitions.
CABLE LENGTH (FT)
1k
LOW-EMI MODE MAX DATA RATE
100 RS485 MAX DATA RATE 10 10k 100k
NORMAL MODE MAX DATA RATE
1M 10M DATA RATE (bps)
100M
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Figure 9. Cable Length vs Data Rate (RS485 Standard Shown in Solid Vertical Lines)
Cable Termination Proper cable termination is very important for good signal fidelity. If the cable is not terminated with its characteristic impedance, reflections will result in distorted waveforms. RS485 transceivers typically communicate over twistedpair cables with characteristic impedance ranging from 100 to 120. Each end of the network should be terminated with a discrete resistor matching the characteristic impedance or with an LTC2859/LTC2861 transceiver with integrated termination capability.
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13
LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 APPLICATIONS INFORMATION
Time-Based Traces Frequency Spectrum
Y, Z 1V/DIV Y-Z 10dB/DIV Y-Z 2V/DIV
2s/DIV
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1.25MHz/DIV
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Figure 10. LTC2858-1 Driver Output at 100kHz Into 100 Resistor
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14
LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 APPLICATIONS INFORMATION
Time-Based Traces Frequency Spectrum
Y, Z 1V/DIV Y-Z 10dB/DIV
Y-Z 2V/DIV
2s/DIV
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1.2MHz/DIV
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Figure 11. LTC2858-2 Driver Output at 100kHz Into 100 Resistor
TYPICAL APPLICATIONS
Failsafe "0" Application (Idle State = Logic "0")
VCC 100k RO I1 R B "A"
A DI I2 D LTC2856-1
"B"
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15
LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 PACKAGE DESCRIPTION
DD Package 8-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1698)
0.675
0.05
3.5
0.05 2.15
1.65 0.05 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.38 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP 5
0.38 8
0.10
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
1.65 0.10 (2 SIDES)
(DD) DFN 1203
0.200 REF
0.75
0.05
0.25
4 0.05 2.38 0.10 (2 SIDES)
1 0.50 BSC
0.00 - 0.05
BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON TOP AND BOTTOM OF PACKAGE
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16
LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 PACKAGE DESCRIPTION
DD Package 10-Lead Plastic DFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1699)
R = 0.115 TYP 6 0.675 0.05 10 0.38 0.10
3.50
0.05 1.65 0.05 2.15 0.05 (2 SIDES) PACKAGE OUTLINE 0.25 0.05 0.50 BSC 2.38 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE PIN 1 TOP MARK (SEE NOTE 6)
3.00 0.10 (4 SIDES)
1.65 0.10 (2 SIDES)
(DD) DFN 1103
5 0.200 REF 0.75 0.05 2.38 0.10 (2 SIDES)
1 0.25 0.05 0.50 BSC
0.00 - 0.05
BOTTOM VIEW--EXPOSED PAD
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 PACKAGE DESCRIPTION
MS8 Package 8-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1660)
0.889 (.035
0.127 .005)
5.23 (.206) MIN
3.20 - 3.45 (.126 - .136)
0.42 0.038 (.0165 .0015) TYP
0.65 (.0256) BSC
3.00 0.102 (.118 .004) (NOTE 3)
8
7 65
0.52 (.0205) REF
RECOMMENDED SOLDER PAD LAYOUT
DETAIL "A" 0 - 6 TYP 4.90 0.152 (.193 .006) 3.00 0.102 (.118 .004) (NOTE 4)
0.254 (.010) GAUGE PLANE
1 0.53 0.152 (.021 .006) DETAIL "A" 0.18 (.007) SEATING PLANE 0.22 - 0.38 (.009 - .015) TYP 1.10 (.043) MAX
23
4 0.86 (.034) REF
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.65 (.0256) BSC
0.1016 (.004
0.0508 .002)
MSOP (MS8) 0307 REV F
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LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 PACKAGE DESCRIPTION
MS Package 10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661)
0.889 (.035
0.127 .005)
5.23 (.206) MIN
3.20 - 3.45 (.126 - .136) 3.00 0.102 (.118 .004) (NOTE 3)
0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT
10 9 8 7 6
0.497 0.076 (.0196 .003) REF
0.254 (.010) GAUGE PLANE
DETAIL "A" 0 - 6 TYP
4.90 0.152 (.193 .006)
3.00 0.102 (.118 .004) (NOTE 4)
12345 0.53 0.152 (.021 .006) DETAIL "A" 0.18 (.007) SEATING PLANE 0.17 -0.27 (.007 - .011) TYP 0.1016 (.004 0.0508 .002) 1.10 (.043) MAX 0.86 (.034) REF
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.50 (.0197) BSC
MSOP (MS) 0307 REV E
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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.
19
LTC2856-1/LTC2856-2 LTC2857-1/LTC2857-2 LTC2858-1/LTC2858-2 TYPICAL APPLICATION
Multi-Node Network and End Termination Using the LTC2856-1
R
D
R
D
LTC2856-1
LTC2856-1
R TE = 5V
R TE = 5V
D LTC2859 LTC2859
D
285678 TA03
RELATED PARTS
PART NUMBER LTC485 LTC491 LTC1480 LTC1483 LTC1485 LTC1487 LTC1520 LTC1535 LTC1685 LT1785 LTC2859/ LTC2861 DESCRIPTION Low Power RS485 Interface Transceiver Differential RS485 Driver and Receiver Pair 3.3V Ultralow Power RS485 Transceiver Ultralow Power RS485 Low EMI Transceiver Differential Bus Transceiver Ultralow Power RS485 with Low EMI, Shutdown and High Input Impedance 50Mbps Precision Quad Line Receiver Isolated RS485 Full-Duplex Transceiver 52Mbps RS485 Transceiver with Precision Delay 60V Fault Protected RS485 Transceiver 20Mbps RS485 Transceivers with Integrated Switchable Termination COMMENTS ICC = 300A (Typ) ICC = 300A 3.3V Operation Controlled Driver Slew Rate 10Mbaud Operation Up to 256 Transceiver on the Bus Channel-to-Channel Skew 400ps (Typ) 2500VRMS Isolation in Surface Mount Package Propagation Delay Skew 500ps (Typ) 60V Tolerant, 15kV ESD Integrated, Switchable, 120 Termination Resistor, 15kV ESD
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20 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0308 REV E * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2006

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