ADM3311E

–6–

REV. A

GENERAL DESCRIPTION

The ADM3311E is a ruggedized RS-232 line driver/receiver

that operates from a single supply of +2.7 V to +3.6 V. Step-up

voltage converters, coupled with level-shifting transmitters and

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

from a single supply. Features include low power consumption,

Green Idle operation, high transmission rates and compatibility

with the EU directive on electromagnetic compatibility. EM

compatibility includes protection against radiated and conducted

interference including high levels of electrostatic discharge.

All RS-232 inputs and outputs contain protection against

electrostatic discharges up to

± 15 kV and electrical fast tran-

sients up to

± 4 kV.

The device is ideally suited for operation in electrically harsh

environments or where RS-232 cables are frequently being

plugged/unplugged, and is immune to high RF field strengths

without special shielding precautions.

Emissions are also controlled to within very strict limits. 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 three main sections. These are:

1. A charge pump voltage converter.

2. 3.3 V logic to EIA-232 transmitters.

3. EIA-232 to 3 V logic receivers.

4. Transient protection circuit on all I-O lines.

Charge Pump DC-DC Voltage Converter

The charge pump voltage converter consists of a 180 kHz oscil-

lator and a switching matrix. The converter generates a

±9 V

supply from the input +3.0 V level. This is done in two stages

using a switched capacitor technique as illustrated below. First,

the +3.0 V input supply is tripled to +9.0 V using capacitor C4

as the charge storage element. The +9.0 V level is then inverted

to generate –9.0 V using C5 as the storage element.

However, it should be noted that, unlike other charge-pump dc-

dc converters, the charge pump on the ADM3311E does not

run open-loop. The output voltage is regulated to

±7.25 V by

the Green Idle circuit (as described later) and will never reach

±9 V in practice. This saves power as well as maintaining a

more constant output voltage.

The tripler operates in two phases. During the oscillator low

S4 and S5 are open. S6 and S7 are closed.

During the oscillator high phase, S1 and S2 are open. S3 and

voltage is used to charge C2. In the absence of any discharge

ing the oscillator high phase, as previously mentioned, S6 and

This voltage is used to charge C3.

+

+

INTERNAL

OSCILLATOR

GND

C2

C1

S1

S2

S3

S4

+

C4

S5

S6

S7

Figure 15. Charge Pump Voltage Tripler

The voltage inverter is illustrated in Figure 14. During the oscil-

lator high phase S10 and S11 are open, S8 and S9 are closed

put of the voltage tripler. During the oscillator low phase, S8

and S9 are open, while S10 and S11 are closed. C3 is connected

across C5, whose positive terminal is grounded and whose nega-

tive terminal is the V– output. Over several cycles C5 charges to

+

INTERNAL

OSCILLATOR

GND

C3

S8

S9

S11

V+

+

C5

V– = –(V+)

FROM

VOLTAGE

TRIPLER

S10

GND

Figure 16. Charge Pump Voltage Inverter

The V+ and V– supplies may also be used to power external

circuitry if the current requirements are small. Please refer to

Figures 13 and 14 in the Typical Performance section.

1

T

[

]

Tek Stop 500kS/s

101 Acqs

T

T

SD

2

Ch1 5.00V

Ch2

5.00V

M 100 s

Ch1

0V

V–

Figure 13. Charge Pump V– Exiting Shutdown

1

T

[

]

Tek Stop 500kS/s

244 Acqs

T

SD

2

V+

Ch1 5.00V

Ch2

5.00V

M 100 s

Ch1

0V

T

Figure 14. Charge Pump V+ Exiting Shutdown