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ADM3232E

±15 kV ESD Protected, 3.3 V, RS-232 Line Driver/Receiver

Analog Devices 

ADM3232E

THEORY OF OPERATION

The ADM3232E is a single-channel RS-232 line driver/receiver.

Step-up voltage converters, coupled with level-shifting transmitters

and receivers, allow RS-232 levels to be developed while operating

from a single 3.3 V supply.

CMOS technology is used to keep the power dissipation to an

absolute minimum, allowing maximum battery life in portable

applications.

CIRCUIT DESCRIPTION

The internal circuitry consists of the following main sections:

• A charge pump voltage converter

• A 3.3 V logic to RS-232 transmitter

• An RS-232 to 3.3 V logic receiver

Charge Pump Voltage Converter

The charge pump voltage converter consists of a 200 kHz

oscillator and a switching matrix. The converter generates a

±6.6 V supply from the input 3.3 V level. This is accomplished

in two stages by using a switched capacitor technique as shown

in Figure 10. First, the 3.3 V input supply is doubled to 6.6 V by

using Capacitor C1 as the charge storage element. The +6.6 V

level is then inverted to generate −6.6 V, using C2 as the storage

element.

Capacitor C3 and Capacitor C4 are used to reduce the output

ripple. Their values are not critical and can be increased, if desired.

Capacitor C3 is shown connected between V+ and VCC. It is also

acceptable to connect this capacitor between V+ and GND.

If desired, larger capacitors (up to 10 μF) can be used for

Capacitor C1 to Capacitor C4.

+3.3V INPUT

C1 +

0.1µF

10V

C2 +

0.1µF

10V

TTL/CMOS

INPUTS

T1IN

T2IN

ADM3232E

C1+

+3.3V TO +6.6V

VOLTAGE

VCC

C1– DOUBLER V+

C2+ +6.6V TO –6.6V V–

VOLTAGE

C2– INVERTER

T1

T2

C3

0.1µF

6.3V

C4

0.1µF

10V

C5

0.1µF

T1OUT

T2OUT

RS-232

OUTPUTS

TTL/CMOS

OUTPUTS

R1OUT

R2OUT

R1

R2

GND

R1IN

5kΩ

RS-232

INPUTS*

R2IN

5kΩ

*INTERNAL 5kΩ PULL-DOWN RESISTOR

ON EACH RS-232 INPUT.

Figure 10. Typical Operating Circuit

VCC

GND

S1

S3

+

C1

+

C3

V+ = 2VCC

S2

S4

VCC

INTERNAL

OSCILLATOR

Figure 11. Charge Pump Voltage Doubler

V+

FROM

VOLTAGE

DOUBLER

GND

S1

S3

GND

+

C2

+

C4

S2

S4

V– = –(V+)

INTERNAL

OSCILLATOR

Figure 12. Charge Pump Voltage Inverter

3.3 V Logic to RS-232 Transmitter

The drivers convert 3.3 V logic input levels into RS-232 output

levels. With VCC = 3.3 V and driving an RS-232 load, the output

voltage swing is typically ±6 V.

RS-232 to 3.3 V Logic Receiver

The receivers are inverting level shifters that accept RS-232 input

levels and translate them into 3 V logic output levels. The inputs

have internal 5 kΩ pull-down resistors to ground and are pro-

tected against overvoltages up to ±30 V. Unconnected inputs

are pulled to 0 V by the internal 5 kΩ pull-down resistor. This

results in a Logic 1 output level for unconnected inputs or for

inputs connected to GND.

The receivers have Schmitt trigger inputs with a hysteresis level

of 0.4 V. This ensures error-free reception for both noisy inputs

and for inputs with slow transition times.

ESD Protection on RS-232 Pins

All RS-232 (TxOUT and RxIN) inputs and outputs are protected

against electrostatic discharges (up to ±15 kV). This ensures

compliance with IEC 1000-4-2 requirements.

HIGH BAUD RATE

The ADM3232E features high slew rates, permitting data trans-

mission at rates well in excess of the EIA-232E specifications.

RS-232 voltage levels are maintained at data rates up to 460 kbps,

even under worst-case loading conditions. The slew rate is

internally controlled to less than 30 V/μs to minimize EMI

interference.

Rev. B | Page 8 of 12

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