ADM2209E
GENERAL DESCRIPTION
The ADM2209E is a rugged dual-port RS-232 line driver/re-
ceiver that operates from a single, +12 V supply, thus removing
the need for a –12 V power supply. It contains ten receivers and
six drivers, and provides a one-chip solution for both serial ports
in desktop or portable personal computers.
Features include low power consumption, 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
±
2 kV. This ensures compliance to IE1000-4-2 and
IEC1000-4-4 requirements.
This device is 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. Emissions are
also controlled to within very strict limits.
A novel feature of this device is that one receiver (R5) in each
port can be kept active by a low-current, +3 V/+5 V power
supply, while the rest of the system is powered down. This al-
lows the system to be awakened when peripheral devices begin
to communicate with it.
CIRCUIT DESCRIPTION
The inverted output V– goes to GND. All transmitter outputs
are disabled and receivers R1 through R4 on each port are three-
stated. The remaining receiver on each port (R5) remains fully
active.
The standby current I
STBY
remains less than 200
µA
in this
mode.
V
DD
(12V)
V
STBY
(5V)
INTERNAL
SHUTDOWN SIGNAL
Figure 21. Standby Detection Circuit
Charge Pump Capacitors And Supply Decoupling
For proper operation of the charge pump, the capacitors should
have an equivalent series resistance (ESR) less than 1
Ω.
As the
charge pump draws current pulses from V
DD
, the V
DD
decou-
pling capacitor should also have low ESR. The V– decoupling
capacitor and reservoir capacitor should also have low ESR
because they determine how effectively ESD pulses are clamped
to V
DD
or V– by the on-chip clamp diodes. Tantalum or mono-
lithic ceramic capacitors are suitable for these components. If
using tantalum capacitors, do not forget to observe polarity.
Transmitter (Driver) Section
The internal circuitry consists of four main sections. These are:
1. A charge pump dc-to-dc converter
2. Logic (3 V/5 V)-to-EIA-232 transmitters
3. EIA-232-to-logic receivers
4. Transient protection circuit on all I-O lines
Charge Pump DC-DC Converter
The drivers convert 5 V logic input levels into EIA-232 output
levels. With V
DD
= +12 V and driving an EIA-232 load, the
output voltage swing is typically
±
9 V.
Unused inputs may be left unconnected, as an internal 400 kΩ
pull-up resistor pulls them high, forcing the outputs into a low
state. The input pull-up resistors typically source 10
µA
when
grounded, so unused inputs should either be connected to V
STBY
or left unconnected in order to minimize power consumption.
Receiver Section
The dc-dc converter generates a negative supply voltage from
the +12 V supply, thus removing the need for a separate –12 V
rail. It consists of an on-chip 200 kHz oscillator, switching ma-
trix and two external capacitors, as shown in Figure 20.
V
DD
+12V
GND
INTERNAL
OSCILLATOR
S1
S2
C1
+
S4
S3
+
C2
C2–
GND
The receivers are inverting level shifters that accept EIA-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. The guaran-
teed switching thresholds are 0.4 V minimum and 2.4 V maxi-
mum. Unconnected inputs are pulled to 0 V by the internal
5 kΩ pull-down resistor. This, therefore, results in a Logic 1
output level for unconnected inputs or for inputs connected to
GND.
The receivers have Schmitt trigger input with a hysteresis level
of 0.25 V. This ensures error-free reception for both noisy in-
puts and for inputs with slow transition times.
HIGH BAUD RATE
Figure 20. Charge Pump DC-DC Converter
When S1 and S2 are closed, S3 and S4 are open, and C1 charges
to +12 V. S1 and S2 are then opened, while S3 and S4 are
closed to connect C1 across C2, dumping charge into C2. Since
the positive terminal of C2 is at ground, a negative voltage will
be built up on its negative terminal with each cycle of the oscil-
lator. This voltage depends on the current drawn from C2. If
the current is small, the voltage will be close to –12 V, but will
fall as the current drawn increases.
Standby Operation
The ADM2209E features high slew rates permitting data trans-
mission at rates well in excess of the EIA-232-E specifications.
RS-232 levels are maintained at data rates up to 920 kb/s. This
allows for high speed data links between two terminals and,
indeed, is suitable for the new generation modem standards.
ESD/EFT TRANSIENT PROTECTION SCHEME
The ADM2209E automatically enters a standby or shutdown
mode when the V
DD
power supply is removed. An on-chip com-
parator circuit generates an internal shutdown signal. This sig-
nal disables the internal oscillator and hence the charge pump.
–8–
The ADM2209E uses protective clamping structures on all in-
puts and outputs, which clamps the voltage to a safe level and
dissipates the energy present in ESD (Electrostatic) and EFT
REV. 0
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