RS232 Interface Products - Linear Technology

10/93/75

RS232 Interface Circuits for 3.3V Systems – Design Note 75

Gary Maulding

The rapid, widespread use of 3.3V logic circuits compli-

cates the selection of RS232 interface circuits. The opti-

mum choice of an interface circuit should be based upon

several application dependent factors:

1) Logic circuitry connected to interface chip

2) Power supply voltages available

3) Power consumption constraints

4) Serial interface environment

5) Mouse driving requirements

As Figure 1 illustrates, 5V interface circuits cannot be used

to directly connect to 3.3V CMOS logic circuits. The re-

ceiver output level will forward bias the logic circuit’s input

protection diode, causing large current flow. In the worst

case the CMOS logic circuit may latch up. Resistor voltage

dividers or level shift buffers may be used to prevent

forward biasing the CMOS input diode, but an RS232

transceiver designed for 3V logic application prevents this

problem without extra components or power dissipation.

Many of today’s systems have both 5V and 3V power

supplies. In these systems, an RS232 interface chip which

uses the 5V supply for charge pump and driver operation

and the 3V supply for receiver output levels, provides the

best performance. The 5V operation of the charge pump

and drivers gives full RS232 output levels and sufficient

current drive for operating a serial port mouse. The LT1342,

LT1330, and LT1331 are all good RS232 transceiver choices

for systems with both 5V and 3V power. Typical perfor-

mance waveforms for the LT1342 operating with V

= 5V

and V

= 3.3V are shown in Figure 2.

Systems with only a 3V power supply are unable to use 5V

powered RS232 interface circuits. Charge pump triplers (or

quadruplers) have losses too great for generating RS232

voltage and current levels from a 3.3V supply. The LT1331

and LTC1327 provide solutions for 3V only systems. The

LT1331 circuit is usable in both 5V/3V mixed or 3V only

systems. When the charge pump is operated from 3V sup-

plies, it powers the driver circuitry to provide RS562 output

levels (see Figure 3). RS562 is a newer serial data interface

standard than RS232 with lower (±3.7V) driver output levels

and extended (64k baud vs 20k baud) data rates. RS562

systems and RS232 systems are universally interoperable.

The LTC1327 also provides RS562 output levels from a 3V

supply. This circuit features ultra-low 300µA supply current

to maximize battery life. An advanced CMOS process makes

this low current operation possible without compromising

the rugged overvoltage and ESD protection available on

Linear Technology’s bipolar interface circuits.

VPP Switcher Drives 3V RS232

When fully RS232 compliant operation or mouse driving is

required in a 3V only system, the LT1332 provides the

solution. The

LT1332 is specifically designed to be used

DRIVER OUTPUT

= 3k

= 2500pF

RECEIVER

OUTPUT

INPUTS

DN75 • F02

Figure 2. LT1342 Outputs for VCC = 5V and VL = 3.3V

= 5V V

LOGIC

= 3.3V

RS232 RECEIVER OUTPUT LOGIC INPUT

DN75 • F01

Figure 1. 5V Receiver Forward Biases Logic Input Diode

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487

(408) 432-1900

●

FAX

: (408) 434-0507

●

TELEX

: 499-3977

LT/GP 1093 190K

 LINEAR TECHNOLOGY CORPORATION 1993

For literature on our RS232 Interface Products,

call (800) 637-5545. For applications help,

call (408) 432-1900, Ext. 453

Figure 3. LT1331 Outputs for VCC = VL = 3.3V

DN75 • F03

DRIVER OUTPUT

= 3k

= 2500pF

INPUTS

RECEIVER

OUTPUT

in conjunction with a micropower switching regulator

like the LT1109A. The switcher provides 12V needed for

flash memory VPP and the RS232 V+. A capacitor from

the switcher’s drive pin (VSW) to on-chip diodes in the

LT1332 form a charge pump to generate the V – needed

for the RS232 drivers. This two chip solution for VPP

generation and RS232 interface is a very economical

solution in 3V systems where both these needs coexist.

The output driver levels of the LT1332 are fully RS232

compliant and capable of driving serial mice, a capability

which cannot be met by other 3V operating circuits.

Battery-powered 3V systems can use the RS232

transceiver’s SHUTDOWN and Driver Disable controls to

maximize battery charge life. These operating mode con-

trols reduce power consumption when communications

needs allow the transceiver to be partially or fully turned off.

Keep-alive receivers, available on some transceivers, con-

sume little power (60µA) while monitoring a data line. When

data is detected, the system can be fully powered up to

accept and process the incoming data.

ESD Protection

ESD transient protection of data lines is essential for

equipment reliability. Traditional protection measures us-

ing TransZorbs

and diodes are a large percentage of total

interface port component costs. Linear Technology’s RS232

and RS562 interface circuits reduce this cost by providing

10kV “Human Body Model” ESD protection on the RS232

data lines without external components. This level of pro-

tection is adequate in most applications, but when even

higher levels of protection are needed, a simple RC network

(see Figure 4) may be used. The RC network raises the ESD

protection level to 10kV “Machine Model” discharges at a

lower cost than TransZorb

based protection networks.

Table 1. RS232/RS562 Transceivers for 5V/3V

and 3V Systems

5V/3V 3V 10kV

Part No. RS232 RS562 ESD Comments

LT1342 ✓✓LT1137A Pin Compatible

LT1330 ✓✓Low Power Burst ModeTM

LT1331 ✓✓✓V

CC Not Used in SHUTDOWN

LT1327 ✓✓300µA Supply Current

LT1332 3V RS232 Used with LT1109A VPP Generator

Burst ModeTM is a trademark of Linear Technology Corporation

Figure 4. LT1109A-12 and LT1332 Provide VPP Supply and RS232 Interface

TransZorb® is a registered trademark of General Instruments, GSI

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14

–

10µF

–

12V

VPP OUTPUT

22µF*

3

8

1

7

MBRS130T3

VIN

ON/OFF



GND

SW

SENSE

PGND

22µF*

SWITCHER V



2 AA BATTERIES

UP TO 6V

ON/OFF

L1**

33µH

RS232 ON/OFF

LOGIC SIDE

RS232 SIDE

0.1µF

3V RS232 V

LT1109A -12

LT1332

STANDARD FLASH MEMORY VPP GENERATOR

1µF

DN75 • F04

AVX TAJE226K035

SUMIDA CD54-330N (708-956-0666)

*

200pF

300Ω300Ω

10kV “MACHINE

MODEL” ESD

PROTECTION