ADM202E/ADM1181A
Rev. C | Page 6 of 16
GENERAL DESCRIPTION
The ADM202E/ADM1181E are rugged RS-232 line
drivers/receivers. Step-up voltage converters coupled with level-
shifting transmitters and receivers allow RS-232 levels to be
developed while operating from a single 5 V supply.
Features include low power consumption, high transmission
rates, and compliance with the EU directive on electromagnetic
compatibility. EM compatibility includes protection against
radiated and conducted interference, including high levels of
electrostatic discharge.
All inputs and outputs contain protection against electrostatic
discharges of up to ±15 kV and electrical fast transients of up
to ±2 kV. This ensures compliance to IEC1000-4-2 and
IEC1000-4-4 requirements.
The devices are ideally suited for operation in electrically harsh
environments or where RS-232 cables are frequently being
plugged/unplugged. They are also immune to high RF field
strengths without special shielding precautions.
CMOS technology is used to minimize the power dissipation,
allowing maximum battery life in portable applications.
The ADM202E/ADM1181A serve as a modification,
enhancement, and improvement to the ADM230–ADM241
family and its derivatives. It is essentially plug-in compatible
and do not have materially different applications.
CIRCUIT DESCRIPTION
The internal circuitry consists of four main sections:
• A charge-pump voltage converter
• 5 V logic to EIA-232 transmitters
• EIA-232 to 5 V logic receivers.
• Transient protection circuit on all I/O lines
Charge-Pump DC-to-DC Voltage Converter
The charge-pump voltage converter consists of a 200 kHz
oscillator and a switching matrix. The converter generates a
±10 V supply from the input 5 V level. This is done in two stages,
using a switched capacitor technique, as illustrated in Figure 6
and Figure 7. First, the 5 V input supply is doubled to 10 V, using
Capacitor C1 as the charge storage element. The 10 V level is
then inverted to generate −10 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. On the ADM202E, Capacitor C3 is shown connected
between V+ and V
CC
, whereas it is connected between V+ and
GND on the ADM1181A. It is acceptable to use either
configuration with both the ADM202E and ADM1181A. If
desired, larger capacitors (up to 47 µF) can be used for
Capacitor C1 to Capacitor C4. This facilitates direct substitution
with older generation charge-pump RS-232 transceivers.
S1
S2
C1
S4
S3
C3
V+ = 2V
CC
V
CC
V
CC
GND
INTERNAL
OSCILLATOR
NOTE: C3 CONNECTS BETWEEN V+ AND GND ON THE ADM1181A
00066-006
Figure 6. Charge-Pump Voltage Doubler
S1
S2
C2
S4
S3
C4
V– = –(V+)
V+ GND
INTERNAL
OSCILLATOR
GND
FROM
VOLTAGE
DOUBLER
00066-007
Figure 7. Charge-Pump Voltage Inverter
Transmitter (Driver) Section
The drivers convert 5 V logic input levels into RS-232 output
levels. When driving an RS-232 load with V
CC
= 5 V, the output
voltage swing is typically ±9 V.
Receiver Section
The receivers are inverting level shifters that accept RS-232
input levels and translate them into 5 V logic output levels. The
inputs have internal 5 kΩ pull-down resistors to ground and are
also protected against overvoltages of up to ±30 V. Unconnected
inputs are pulled to 0 V by the internal 5 kΩ pull-down resistor.
Therefore, unconnected inputs and those connected to GND
have a Logic 1 output level.
The receivers have Schmitt-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.
HIGH BAUD RATE
The ADM202E/ADM1181A feature high slew rates, permitting
data transmission at rates well in excess of the EIA/RS-232-E
specifications. RS-232 voltage levels are maintained at data rates
of up to 230 kbps, even under worst case loading conditions.
This allows for high speed data links between two terminals and
is also suitable for the new generation ISDN modem standards,
which require data rates of 230 kbps. The slew rate is internally
controlled to less than 30 V/µs to minimize EMI interference.
