ADM223/ADM230L–ADM241L

–13–

REV. B

S1

S3

S2

S4

INTERNAL

OSCILLATOR

C2

C4

GND

V– = –(V+)

+

+

GND

FROM

VOLTAGE

DOUBLER

Figure 34. Charge-Pump Voltage Inverter

Capacitors C3 and C4 are used to reduce the output ripple.

Their values are not critical and can be reduced if higher

levels of ripple are acceptable. The charge pump capacitors

C1 and C2 may also be reduced at the expense of higher

output impedance on the V+ and V– supplies.

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

circuitry if the current requirements are small.

Transmitter (Driver) Section

The drivers convert TTL/CMOS input levels into EIA-232-E

load, the output voltage swing is

±9 V. Even under worst-case

conditions the drivers are guaranteed to meet the

±5 V EIA-232-E

minimum requirement.

The input threshold levels are both TTL and CMOS compat-

inputs may be left unconnected, as an internal 400 k

Ω pull-up

resistor pulls them high forcing the outputs into a low state.

As required by the EIA-232-E standard, the slew rate is limited

to less than 30 V/

µs without the need for an external slew limit-

ing capacitor and the output impedance in the power-off state is

greater than 300

Ω.

Receiver Section

The receivers are inverting level shifters which accept EIA-232-E

input levels (

±5 V to ±15 V) and translate them into 5 V TTL/

CMOS levels. The inputs have internal 5 k

Ω pull-down resistors

to ground and are also protected against overvoltages of up to

±30 V. The guaranteed switching thresholds are 0.8 V minimum

and 2.4 V maximum which are well within the

±3 V EIA-232-E

requirement. The low level threshold is deliberately positive

as it ensures that an unconnected input will be interpreted as a

low level.

The receivers have Schmitt trigger inputs with a hysteresis level

of 0.5 V. This ensures error-free reception for both noisy inputs

and for inputs with slow transition times.

Shutdown (SD)

The ADM223, ADM230L, ADM235L, ADM236L and

ADM241L feature a control input that may be used to disable

the part and reduce the power consumption to less than 5

µW.

This is very useful in battery operated systems. During shutdown

the charge pump is turned off, the transmitters are disabled

and all receivers except R4 and R5 on the ADM223 are put into

a high-impedance disabled state. Receivers R4 and R5 on the

ADM223 remain enabled during shutdown. This feature allows

monitoring external activity such as ring indicator monitoring

while the device is in a low power shutdown mode.

The shutdown control input is active high on all parts except the

ADM223 where it is active low. Refer to Tables I and II.

Enable Input

The ADM235, ADM239, ADM241L and ADM223 feature

an enable input used to enable or disable the receiver outputs.

The enable input is active low on the ADM235L, ADM239L,

ADM241L and active high on the ADM223. Refer to Tables I

and II. When disabled, all receiver outputs are placed in a high

impedance state. This function allows the outputs to be con-

nected directly to a microprocessor data bus. It can also be used

to allow receivers from different devices to share a common data

line. The timing diagram for the enable function is shown in

Figure 35.

3V

0V

3.5V

0.8V

EN*

*POLARITY OF

EN IS REVERSED FOR ADM223.

Figure 35. Enable Timing

APPLICATION HINTS

Driving Long Cables

In accordance with the EIA-232-E standard, long cables are per-

missible provided that the total load capacitance does not exceed

2500 pF. For longer cables which do exceed this, then it is possible

to trade off baud rate vs. cable length. Large load capacitances

cause a reduction in slew rate, and hence the maximum transmis-

sion baud rate is decreased. The ADM230L-ADM241L are

designed so that the slew rate reduction with increasing load

capacitance is minimized.

For the receivers, it is important that a high level of noise immu-

nity be inbuilt so that slow rise and fall times do not cause

multiple output transitions as the signal passes slowly through

the transition region. The ADM230L-ADM241L have 0.5 V of

hysteresis to guard against this. This ensures that, even in noisy

environments, error-free reception can be achieved.

High Baud Rate Operation

The ADM230L-ADM241L feature high slew rates permitting

data transmission at rates well in excess of the EIA-232-E speci-

fication. The drivers maintain

±5 V signal levels at data rates up

to 100-kB/s under worst-case loading conditions.

OBSOLETE