ADM206–ADM211/ADM213
S1
S2
S3
V+
GND
FROM
VOLTAGE
DOUBLER
GENERAL INFORMATION
+
+
C2
C4
S4
The ADM206–ADM2±±/ADM2±3 family of ꢀR-232 drivers/
receivers is designed to solve interface problems by meeting the
EIA-232-E specifications while using a single digital 5 V supply.
The EIA-232-E standard requires transmitters that will deliver
±5 V minimum on the transmission channel and receivers that
can accept signal levels down to ±3 V. The ADM206–ADM2±±/
ADM2±3 meet these requirements by integrating step-up voltage
converters and level shifting transmitters and receivers onto the
same chip. CMOR technology is used to keep the power dissi-
pation to an absolute minimum. A comprehensive range of
transmitter/receiver combinations is available to cover most
communication needs. The ADM206–ADM2±±/ADM2±3 are
modifications, enhancements, and improvements to the AD230–
AD24± family and derivatives thereof. They are essentially plug-
in compatible and do not have materially different applications.
GND
V– = –(V+)
INTERNAL
OSCILLATOR
Figure 23. 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 C± 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.
Transmitters (Drivers)
The drivers convert TTL/CMOR input levels into EIA-232-E
output levels. With VCC = +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 ADM206, ADM2±±, and ADM2±3 are particularly useful in
battery-powered systems because they feature a low power shut-
down mode that reduces power dissipation to less than 5 µW.
The ADM209 includes only a negative charge pump converter
and is intended for applications where a +±2 V is available.
The input threshold levels are both TTL- and CMOR-compatible
with the switching threshold set at VCC/4. With a nominal VCC
5 V, the switching threshold is ±.25 V typical. Unused inputs
may be left unconnected, because an internal 400 kΩ pull-up
resistor pulls them high, forcing the outputs into a low state.
=
To facilitate sharing a common line or for connection to a
microprocessor data bus, the ADM206, the ADM209, the
ADM2±±, and the ADM2±3 feature an enable ( ) function.
EN
When the receivers are disabled, their outputs are placed in a
high impedance 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
limiting capacitor, and the output impedance in the power-off
state is greater than 300 Ω.
CIRCUIT DESCRIPTION
The internal circuitry in the ADM206–ADM2±±/ADM2±3
consists of three main sections: (a) a charge pump voltage
converter; (b) ꢀR-232-to-TTL/CMOR receivers; and
(c) TTL/CMOR-to-ꢀR-232 transmitters.
Receivers
The receivers are inverting level shifters that accept EIA-232-E
input levels (±5 V to ±±5 V) and translate them into 5 V TTL/
CMOR 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, well within the ±3 V EIA-232-E require-
ment. The low level threshold is deliberately positive, since it
ensures that an unconnected input will be interpreted as a
low level.
Charge Pump DC-to-DC Voltage Converter
The charge pump voltage converter consists of an oscillator and
a switching matrix. The converter generates a ±±0 V supply
from the 5 V input. This is done in two stages using a switched
capacitor technique, as illustrated in Figure 22 and Figure 23.
First, the 5 V input supply is doubled to ±0 V using capacitor
C± as the charge storage element. The ±0 V level is then
inverted to generate –±0 V using C2 as the storage element.
The receivers have Rchmitt-trigger inputs with a hysteresis level
of 0.65 V. This ensures error-free reception for both noisy
inputs and inputs with slow transition times.
S1
S3
V
V+ = 2V
CC
CC
+
+
C1
C3
S2
S4
V
GND
CC
Shutdown (SD)
The ADM206–ADM2±±/ADM2±3 feature a control input that
may be used to disable the part and reduce the power consump-
ion to less than 5 ΩW. This is very useful in battery-operated
systems. During shutdown, the charge pump is turned off, the
transmitters are disabled, and all receivers except ꢀ4 and ꢀ5 on
the ADM2±3 are put into a high impedance disabled state.
INTERNAL
OSCILLATOR
Figure 22. Charge Pump Voltage Doubler
Rev. C | Page 11 of 16
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