ADM223/ADM230L–ADM241L
S1
V+
FROM
VOLTAGE
DOUBLER
GND
C2
S2
S4
V– = – (V+)
C4
S3
GND
INTERNAL
OSCILLATOR
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 imped-
ance on the V+ and V– supplies.
The V+ and V– supplies may also be used to power external cir-
cuitry if the current requirements are small.
Transmitter (Driver) Section
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
EN*
0V
T
EN
3.5V
R
OUT
0.8V
T
DIS
V
OH
– 0.1V
V
VOL
+ 0.1V
The drivers convert TTL/CMOS input levels into EIA-232-E
output levels. With V
CC
= +5 V and driving a typical 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-
ible with the switching threshold set at V
CC
/4. With a nominal
V
CC
= 5 V the switching threshold is 1.25 V typical. Unused in-
puts may be left unconnected, as an internal 400 kΩ pull-up re-
sistor 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
*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 pos-
sible to trade off baud rate vs. cable length. Large load capaci-
tances cause a reduction in slew rate, and hence the maximum
transmission 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 mul-
tiple output transitions as the signal passes slowly through the
transition region. The ADM230L-ADM241L have 0.5 V of hys-
teresis to guard against this. This ensures that, even in noisy en-
vironments, error-free reception can be achieved.
High Baud Rate Operation
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 kΩ 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 inter-
preted 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 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.
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 shut-
down the charge pump is turned off, the transmitters are dis-
abled 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 moni-
toring 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.
REV. 0
–13–
AD [ ANALOG DEVICES ]
