ST ST485AB User Manual

low power RS-485/RS-422 transceiver

Features

■ Low supply current: 5 mA max

■ High data rate > 30 Mbps

■ Designed for RS 485 interface applications

■ -7 to 12 common mode input voltage range

■ Driver maintains high impedance in 3-state or

with the power off

■ 70mV typical input hysteresis

■ Operate from single 5 supply

■ ±4 kV ESD protection

■ Current limiting and thermal shutdown for

driver overload protection

■ Latch-up free up to 500 mA

ST485AB

Very high speed

DIP-8 SO-8

The ST485A is designed for bi-directional data
communications or multipoint bus transmission
lines.

Description

The ST485A is a low power, high speed
transceiver for RS-485 and RS-422
communications. The device contains one driver
and one receiver in half duplex configuration. The
ST485A draws 3 mA (typ.) of supply current when
unloaded or fully loaded with disabled drivers. It
operates from a single 5 V supply.

Driver is short-circuit current limited and is
protected against excessive power dissipation by
thermal shutdown circuitry that place the driver
outputs into a high-impedance state. The receiver
input has a fail-safe feature that guarantees a
logic high output if both inputs are open circuit.

Table 1. Device summary

Order code

Temperature

range

Package Packaging

ST485ABN -40 to 85 °C DIP-8 50parts per tube / 40tube per box

ST485ABDR -40 to 85 °C SO-8 (Tape & reel) 2500 parts per reel

August 2007 Rev. 3 1/19

www.st.com

19

Contents ST485AB

Contents

1 Pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Truth tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 Test circuit and typical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2/19

ST485AB Pin configuration

1 Pin configuration

Figure 1. Pin configuration

Table 2. Pin description

Pin n° Symbol Name and function

1 RO Receiver output. If A>B by 200 mV, RO will be high; if A<B by 200 mV, RO will be low

2RE

3DE

4DI

5 GND Ground

6 A Non-inverting receiver input and non-inverting driver output

7 B Inverting receiver input and inverting driver output

8V

Receiver output enable. RO is enabled when RE is low; RO is high impedance when RE is
high. If RE is high and DE is low, the device will enter a low power shutdown mode.

Driver output enable. The driver outputs are enabled by bringing DE high. They are high
impedance when DE is low. If RE is high DE is low, the device will enter a low-power shutdown
mode. If the driver outputs are enabled, the part functions as line driver, while they are high
impedance, it functions as line receivers if RE is low.

Driver input. A low on DI forces output A low and output B high. Similarly, a high on DI forces
output A high and output B low

Supply voltage: VCC= 4.75 V to 5.25 V

CC

3/19

Truth tables ST485AB

2 Truth tables

Table 3. Truth table (driver)

Inputs Outputs Mode

RE

XHHLHNormal

XHLHLNormal

LLXZZNormal

DE DI B A A

Note: X = Don’t care; Z = High impedance

Table 4. Truth table (receiver)

Inputs Outputs

RE

LL

LL

L L Inputs open H Normal

DE A-B RO

≥ -0.2 V H Normal

≤ -0.2 V L Normal

Note: X = Don’t care; Z = High impedance

Mode

4/19

ST485AB Maximum ratings

3 Maximum ratings

Table 5. Absolute maximum ratings

Symbol Parameter Value Unit

V

V

V

V

V

V

CC

DI

DO

RI

RO

Supply voltage 7 V

Control input voltage (RE, DE) -0.3 to 7 V

I

Driver input voltage (DI) -0.3 to 7 V

Driver output voltage (A, B) -7.5 to 12.5 V

Receiver input voltage (A, B) -7.5 to 12.5 V

Receiver output voltage (RO) -0.3 to (VCC + 0.3) V

Note: Absolute maximum ratings are those values beyond which damage to the device may occur.

Functional operation under these condition is not implied.

5/19

Electrical characteristics ST485AB

4 Electrical characteristics

Table 6. DC electrical characteristics (V

specified. Typical values are referred to T

= 4.5 V to 5.5 V, TA = -40 to 85°C, unless otherwise

CC

= 25°C, VCC = 5 V) (Note: 1)

A

Symbol Parameter Test conditions Min. Typ. Max. Unit

I

SUPPLYVCC

C

C

Input/output capacitance 10 pF

I/O

Driver output capacity 10 pF

AB

Power supply current No load, VRE=0 V or V

CC

= V

DE

CC

= 0 V 2.6 5 mA

V

DE

2.3 5 mA

V

Table 7. Logic input electrical characteristics (VCC = 4.5 V to 5.5 V, TA = -40 to 85°C, unless

otherwise specified. Typical values are referred to T

Symbol Parameter Test conditions Min. Typ. Max. Unit

V

Input logic threshold low DE, DI, RE 0.8 V

IL

V

I

I

Input logic threshold high DE, DI, RE 2 V

IH

Logic input current DE, DI, RE ± 2.0 µA

IN1

=0 V,

V

Input current (A, B)

IN2

DE

VCC= 0 or 5.25 V

= 25°C, VCC = 5 V) (Note: 1)

A

=12 V 1 mA

V

IN

=-7 V -0.8 mA

V

IN

Table 8. Transmitter electrical characteristics (VCC = 4.5 V to 5.5 V, TA = -40 to 85°C, unless

otherwise specified. Typical values are referred to T

Symbol Parameter Test conditions Min. Typ. Max. Unit

= 25°C, VCC = 5 V) (Note: 1)

A

V

V

Differential driver output (No load) 5 V

OD1

= 54 Ω (RS-422)

R

Differential driver output (with load)

OD2

L

VCM= -7 V to 12 V

Change in magnitude of driver

ΔV

differential output voltage for

OD

complementary output states

= 27 Ω or 50 Ω 00.2V

R

L

(Note: 1)

V

Driver common mode output

OC

voltage

= 27 Ω or 50 Ω 13V

R

L

Change in magnitude of driver

ΔV

common mode output voltage

OC

RL= 54 Ω or 100 Ω 00.2V

(Note: 1)

I

OSD

Driver short circuit output current VO= -7 V to 12 V ± 250 mA

6/19

1.5 2.7 5 V