Datasheet LTC1487 Datasheet (Linear Technology)

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Page 1

LTC1487

Ultra-Low Power RS485

with Low EMI, Shutdown

and High Input Impedance

EATU

■

High Input Impedance: Up to 256 Transceivers

on the Bus

■

Low Power: I

■

ICC = 200µA Max with Driver Enabled, No Load

■

1µA Quiescent Current in Shutdown Mode

■

Controlled Slew Rate Driver for Reduced EMI

■

Single 5V Supply

■

ESD Protection to ±10kV On Receiver Inputs and

= 120µA Max with Driver Disabled

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

PPLICATI

■

Battery-Powered RS485/RS422 Applications

■

Low Power RS485/RS422 Transceiver

■

Level Translator

DUESCRIPTIO

The LTC®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 80µA while operating and 1µA 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.

PPLICATITYPICAL

LTC1487 LTC1487

2000 FEET OF TWISTED-PAIR WIRE

120Ω

330Ω

4.7nF

EQUIVALENT LOAD OF 256

LTC1487 TRANSCEIVERS

120Ω

LTC1487 • TA01

RECEIVER INPUT

LTC1487 • TA02

Page 2

LTC1487

1 2 3 4

8 7 6 5



TOP VIEW

 B A GND

N8 PACKAGE 8-LEAD PDIP

S8 PACKAGE

8-LEAD PLASTIC SO

RO

RE DE

LUTEXI T

WUW

ARB

U G

PACKAGE

RDER I FOR ATIO

(Note 1)

Supply Voltage (VCC) .............................................. 12V

Control Input Voltage..................... –0.5V to VCC + 0.5V

ORDER PART

NUMBER

Driver Input Voltage....................... –0.5V to VCC + 0.5V

Driver Output Voltage ........................................... ±14V

Receiver Input Voltage.......................................... ±14V

LTC1487CN8 LTC1487CS8

Receiver Output Voltage ................ –0.5V to VCC + 0.5V

Operating Temperature Range ............. 0°C ≤ TA ≤ 70°C

Lead Temperature (Soldering, 10 sec)................. 300°C

LECTRICAL C CHARA TERIST

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

OD1

OD2

∆V

∆V

IN1

IN2

∆V

OZR

SHDN

OSD1

OSD2

OSR

Differential Driver Output Voltage (Unloaded) IO = 0 ● 5V Differential Driver Output Voltage (with Load) R = 50Ω (RS422) ● 2.0 V

Change in Magnitude of Driver Differential Output R = 27Ω or R = 50Ω, Figure 1 ● 0.2 V Voltage for Complementary Output States

Driver Common-Mode Output Voltage R = 27Ω or R = 50Ω, Figure 1 ● 3V

 Change in Magnitude of Driver Common-Mode R = 27Ω or R = 50Ω, Figure 1 ● 0.2 V

Output Voltage for Complementary Output States Input High Voltage DE, DI, RE ● 2V Input Low Voltage DE, DI, RE ● 0.8 V Input Current DE, DI, RE ● ±2 µA Input Current (A, B) DE = 0, VCC = 0V or 5.25V, VIN = 12V ● 0.30 mA

Differential Input Threshold Voltage for Receiver –7V ≤ VCM ≤ 12V ● – 0.2 0.2 V Receiver Input Hysteresis VCM = 0V ● 45 mV Receiver Output High Voltage IO = –4mA, VID = 200mV ● 3.5 V Receiver Output Low Voltage IO = 4mA, VID = –200mV ● 0.4 V Three-State (High Impedance) Output VCC = Max, 0.4V ≤ VO ≤ 2.4V ● ±1 µA

Current at Receiver Receiver Input Resistance –7V ≤ VCM ≤ 12V ● 70 96 kΩ Supply Current No Load, Output Enabled ● 120 200 µA

Supply Current in Shutdown Mode DE = 0V, RE = V Driver Short-Circuit Current, V Driver Short-Circuit Current, V Receiver Short-Circuit Current 0V ≤ VO ≤ V

= HIGH –7V ≤ VO ≤ 12V ● 35 250 mA

OUT

= LOW –7V ≤ VO ≤ 12V ● 35 250 mA

OUT

ICS

0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted.

R = 27Ω (RS485), Figure 1

DE = 0, V

No Load, Output Disabled ● 80 120 µA

= 0V or 5.25V, VIN = –7V ● –0.15 mA

= 125°C, θ

JMAX

= 125°C, θ

JMAX

Consult factory for Industrial and Military grade parts.

= 130°C/ W (N8)

= 150°C/ W (S8)

● 1.5 5 V

● 785mA

S8 PART MARKING

1487

110µA

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LTC1487

LECTRICAL C CHARA TERIST

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

OD1

OD2

VTHDifferential Input Threshold Voltage for Receiver –7V ≤ VCM ≤ 12V ● – 0.2 0.2 V ∆V

SHDN

PLH

PHL

SKEW

tr, t t

PLH

PHL

SKD

MAX

Differential Driver Output Voltage (Unloaded) IO = 0 ● 5V Differential Driver Output Voltage (with Load) R = 50Ω (RS422) ● 2.0 V

Driver Common-Mode Output Voltage R = 27Ω or R = 50Ω, Figure 1 ● 3V

Receiver Input Hysteresis VCM = 0V ● 45 mV Supply Current No Load, Output Enabled ● 120 200 µA

Supply Current in Shutdown Mode DE = 0V, RE = V Driver Input to Output R Driver Input to Output ● 150 1200 ns Driver Output to Output ● 100 600 ns Driver Rise or Fall Time ● 150 2000 ns

Receiver Input to Output R Receiver Input to Output ● 30 140 250 ns

t

– t

PLH

Maximum Data Rate ● 250 kbps

 Differential Receiver Skew ● 13 ns

PHL

ICS

–40°C ≤ TA ≤ 85°C, VCC = 5V (Note 4) unless otherwise noted.

R = 27Ω (RS485), Figure 1

No Load, Output Disabled

= 54Ω, CL1 = CL2 = 100pF, ● 150 1200 ns

DIFF

(Figures 3, 5)

= 54Ω, CL1 = CL2 = 100pF, ● 30 140 250 ns

DIFF

(Figures 3, 7)

● 1.5 5 V

● 80 120 µA

110µA

SWITCHI G CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

PLH

PHL

SKEW

tr, t t

PLH

PHL

SKD

MAX

SHDN

Driver Input to Output R Driver Input to Output ● 150 1200 ns Driver Output to Output ● 250 600 ns Driver Rise or Fall Time ● 150 1200 ns Driver Enable to Output High CL = 100pF (Figures 4, 6), S2 Closed ● 100 1500 ns Driver Enable to Output Low CL = 100pF (Figures 4, 6), S1 Closed ● 100 1500 ns Driver Disable Time from Low CL = 15pF (Figures 4, 6), S1 Closed ● 150 1500 ns Driver Disable Time from High CL = 15pF (Figures 4, 6), S2 Closed ● 150 1500 ns Receiver Input to Output R Receiver Input to Output ● 30 140 250 ns

t

– t

PLH

Receiver Enable to Output Low CRL = 15pF (Figures 2, 8), S1 Closed ● 20 50 ns Receiver Enable to Output High CRL = 15pF (Figures 2, 8), S2 Closed ● 20 50 ns Receiver Disable from Low CRL = 15pF (Figures 2, 8), S1 Closed ● 20 50 ns Receiver Disable from High CRL = 15pF (Figures 2, 8), S2 Closed ● 20 50 ns Maximum Data Rate ● 250 kbps Time to Shutdown DE = 0, RE = ● 50 200 600 ns

 Differential Receiver Skew ● 13 ns

PHL

0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted.

= 54Ω, CL1 = CL2 = 100pF, ● 150 1200 ns

DIFF

(Figures 3, 5)

= 54Ω, CL1 = CL2 = 100pF, ● 30 140 250 ns

DIFF

(Figures 3, 7)

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LTC1487

TEMPERATURE (°C)

–50



350

400

500

25 75

LTC1487 • G06

300

250

–25 0

50 100 125

200

150

450

TIME (ns)

TEMPERATURE (°C)

–50

DIFFERENTIAL VOLTAGE (V)

2.08

2.20

2.22

2.24

LTC1487 • TPC03

2.04

2.02

2.16

2.12

2.06

2.18

2.00

2.14

2.10

–25

100

125

RL = 54Ω

SWITCHI G CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

ZH(SHDN)

ZL(SHDN)

ZH(SHDN)

ZL(SHDN)

Driver Enable from Shutdown to Output High CL = 100pF (Figures 4, 6), S2 Closed ● 2000 ns Driver Enable from Shutdown to Output Low CL = 100pF (Figures 4, 6), S1 Closed ● 2000 ns Receiver Enable from Shutdown to Output High CL = 15pF (Figures 2, 8), S2 Closed ● 2000 ns Receiver Enable from Shutdown to Output Low CL = 15pF (Figures 2, 8), S1 Closed ● 2000 ns

0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted.

● denotes specifications which apply over the full operating

The 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

350

300

250

200

150

SUPPLY CURRENT (µA)

100

DRIVER DISABLED WITH NO LOAD

–25 125

–50

Driver Output Low Voltage vs Output Current

120

100

OUTPUT CURRENT (mA)

THERMAL SHUTDOWN

WITH DRIVER ENABLED

AND NOMINAL LOAD

DRIVER ENABLED

WITH NO LOAD

25 175

0 150

100

LTC1487 • TPC01

LTC1487 • TPC04

TEMPERATURE (°C)

= 25°C

1234

OUTPUT VOLTAGE (V)

OUTPUT CURRENT (mA)

–10 –20 –30 –40 –50 –60 –70

OUTPUT CURRENT (mA)

–80 –90

–100

Note 3: All typicals are given for VCC = 5V and TA = 25°C. Note 4: The LTC1487 is not tested and is not quality-assurance sampled at

–40°C and at 85°C. These specifications are guaranteed by design, correlation, and/or inference from 0°C, 25°C and/or 70°C tests.

Driver Differential Output Voltage vs Output Current

0.5 4.5

2.0

1.5

2.5

1.0 OUTPUT VOLTAGE (V)

3.0

TA = 25°C

3.5

LTC1487 • TPC02

Driver Output High Voltage vs Output Current

TA = 25°C

OUTPUT VOLTAGE (V)

LTC1487 • TPC05

Driver Differential Output Voltage vs Temperature

4.0

Driver Skew vs Temperature

Page 5

UUU

PIN FUNCTIONS

LTC1487

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 (1µA) shutdown state.

FU CTIO TABLES

LTC1487 Transmitting

INPUTS OUTPUTS

RE DE DI B A

X1101 X1010 00XZZ 1 0 X Z* Z*

*Shutdown mode

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.

LTC1487 Receiving

INPUTS OUTPUTS

RE DE A – B RO

00≥0.2V 1 00≤–0.2V 0 0 0 Inputs Open 1 10 X Z*

*Shutdown mode

TEST CIRCUITS

Figure 1. Driver DC Test Load Figure 2. Receiver Timing Test Load

DIFF

Figure 3. Driver/Receiver Timing Test Circuit Figure 4. Driver Timing Test Load

LTC1487 • F04

LTC1487 • F02

OUTPUT

TEST POINT



500Ω

LTC1487 • F01

LTC1487 • F03

15pF

RECEIVER

OUTPUT

UNDER TEST

Page 6

LTC1487

UW W

SWITCHI G TI E WAVEFOR S

–V

A, B

1/2 V

1.5V

10% t

f = 1MHz, tr ≤ 10ns, tf ≤ 10ns

PLH

SKEW

90%

DIFF

= V(A) – V(B)

1.5V

1/2 V

PHL

SKEW

90%

10%

LTC1487 • F05

Figure 5. Driver Propagation Delays

1.5V

f = 1MHz, tr ≤ 10ns, tf ≤ 10ns

ZL(SHDN), tZL

2.3V



2.3V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

ZH(SHDN), tZH

1.5V

0.5V

LTC1487 • F06

A – B

–V

Figure 6. Driver Enable and Disable Times

OD2

PHL

1.5V

f = 1MHz, tr ≤ 10ns, tf ≤ 10ns

0V 0V

OUTPUT

INPUT

PLH

1.5V

LTC1487 • F07

Figure 7. Receiver Propagation Delays

1.5V

f = 1MHz, tr ≤ 10ns, tf ≤ 10ns

, t

ZL(SHDN)

1.5V

OUTPUT NORMALLY LOW

OUTPUT NORMALLY HIGH

, t

ZH(SHDN)

1.5V

0.5V

LTC1487 • F08

Figure 8. Receiver Enable and Disable Times

Page 7

UU W U

APPLICATIO S I FOR ATIO

High Input Impedance

The LTC1487 is designed with a 96kΩ (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 96kΩ input impedance of the LTC1487 will draw only 125µA 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.

LTC1487

SD3

LOGIC

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 150°C and turns them back on when the temperature cools to 130°C. This cycle will repeat as necessary until the fault condition is removed.

OUTPUT

SD4

LTC1487 • F10

LOGIC

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.

OUTPUT

LTC1487 • F09

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

Page 8

LTC1487

UU W U

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 10µA 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 1µA. With the receiver active and the driver outputs disabled, the LTC1487 will typically draw 80µ A 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 72µA. Typical quiescent current rise with the driver enabled is about 40µA.

The quiescent current rises significantly if the driver is enabled when it is externally terminated. With 1/2 termination load (120Ω 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 60Ω line attached, the current will rise to greater than 25mA with the driver enabled, completely overshadowing the extra 40µA 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.

In shutdown the LTC1487 typically draws only 1µA 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

3.5µs. If the driver is active immediately prior to shutdown, the

supply current will not drop to 1µA until the driver outputs have reached a steady state; this can take as long as 2.6µs under worst case conditions. If the driver is disabled prior to shutdown the supply current will drop to 1µA 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

, tf < 6ns

Figure 11. Receiver Propagation Delay Test Circuit

R 100Ω

100pF

LTC1487 • F11

RECEIVER

OUT

The receiver delay times are:

t

– t

PLH

 = 13ns Typ, V

PHL

= 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 = –40°C to 85°C

PACKAGE DESCRIPTION

For package descriptions consult the

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

(408) 432-1900

●

FAX

: (408) 434-0507

1994 Linear Databook Volume III

●

TELEX

: 499-3977

LT/GP 0395 10K • PRINTED IN THE USA

 LINEAR TECHNOLOGY CORPORATION 1995