Data Sheet ADM3251E
THEORY OF OPERATION
The ADM3251E is a high speed, 2.5 kV fully isolated, single-
channel RS-232 transceiver device that operates from a single
power supply.
The internal circuitry consists of the following main sections:
• Isolation of power and data
• A charge pump voltage converter
• A 5.0 V logic to EIA/TIA-232E transmitter
• A EIA/TIA-232E to 5.0 V logic receiver
07388-013
DECODE
RECT
REG
V–
C4
0.1µF
16V
VOLTAGE
DOUBLER
C1+ C1–
V+
V
ISO
C2+
C2–
R
T
VOLTAGE
INVERTER
V
CC
R
OUT
T
IN
GND GND
ISO
R
IN
*
T
OUT
ADM3251E
OSC
ENCODE
ENCODE
DECODE
*INTERNAL 5kΩ PULL-DOWN RESISTOR ON THE RS-232 INPUT.
0.1µF
C3
0.1µF
10V
C2
0.1µF
16V
0.1µF
C1
0.1µF
16V
Figure 13. Functional Block Diagram
ISOLATION OF POWER AND DATA
The ADM3251E incorporates a dc-to-dc converter section,
which works on principles that are common to most modern
power supply designs. V
CC
power is supplied to an oscillating
circuit that switches current into a chip-scale air core transformer.
Power is transferred to the secondary side, where it is rectified
to a high dc voltage. The power is then linearly regulated to
about 5.0 V and supplied to the secondary side data section and
to the V
ISO
pin. The V
ISO
pin should not be used to power
external circuitry.
Because the oscillator runs at a constant high frequency
independent of the load, excess power is internally dissipated
in the output voltage regulation process. Limited space for
transformer coils and components also adds to internal power
dissipation. This results in low power conversion efficiency.
The ADM3251E can be operated with the dc-to-dc converter
enabled or disabled. The internal dc-to-dc converter state of the
ADM3251E is controlled by the input V
CC
voltage. In normal
operating mode, V
CC
is set between 4.5 V and 5.5 V and the
internal dc-to-dc converter is enabled. To disable the dc-to-dc
converter, lower V
CC
to a value between 3.0 V and 3.7 V. In this
mode, the user must externally supply isolated power to the
V
ISO
pin. An isolated secondary side voltage of between 3.0 V
and 5.5 V and a secondary side input current, I
ISO
, of 12 mA
(maximum) is required on the V
ISO
pin. The signal channels of
the ADM3251E then continue to operate normally.
The T
IN
pin accepts CMOS input levels (and TTL levels at
V
CC
= 3.3 V). The driver input signal that is applied to the T
IN
pin is referenced to logic ground (GND). It is coupled across the
isolation barrier, inverted, and then appears at the transceiver
section, referenced to isolated ground (GND
ISO
). Similarly, the
receiver input (R
IN
) accepts RS-232 signal levels that are
referenced to isolated ground. The R
IN
input is inverted and
coupled across the isolation barrier to appear at the R
OUT
pin,
referenced to logic ground.
The digital signals are transmitted across the isolation barrier
using iCoupler technology. Chip-scale transformer windings
couple the digital signals magnetically from one side of the
barrier to the other. Digital inputs are encoded into waveforms
that are capable of exciting the primary transformer of the
winding. At the secondary winding, the induced waveforms are
decoded into the binary value that was originally transmitted.
There is hysteresis in the V
CC
input voltage detect circuit. Once the
dc-to-dc converter is active, the input voltage must be decreased
below the turn-on threshold to disable the converter. This feature
ensures that the converter does not go into oscillation due to
noisy input power.
+
+
C3
0.1µF
10V
+
C1
0.1µF
16V
+
C2
0.1µF
16V
0.1µF
+
C4
0.1µF
16V
EIA/TIA-232E OUTPUT
EIA/TIA-232E INPUT
V
ISO
V+
C1+
C1–
T
OUT
R
IN
C2+
C2–
V–
GND
ISO
07388-014
ISOLATION
BARRIER
CMOS OUTPUT
CMOS INPUT
4.5V TO 5.5V
V
CC
R
OUT
T
IN
GND
0.1µF
ADM3251E
Figure 14. Typical Operating Circuit with the DC-to-DC Converter Enabled
(V
CC
= 4.5 V to 5.5 V)
+
+
C3
0.1µF
10V
+
C1
0.1µF
16V
+
C2
0.1µF
16V
0.1µF
+
C4
0.1µF
16V
EIA/TIA-232E OUTPUT
EIA/TIA-232E INPUT
V
ISO
V+
C1+
C1–
T
OUT
R
IN
C2+
C2–
V–
GND
ISO
07388-015
ISOLATION
BARRIER
CMOS OUTPUT
CMOS INPUT
3.0V TO 3.7V
3.0V TO 5.5V
ISOLATED SUPPLY
V
CC
R
OUT
T
IN
GND
0.1µF
ADM3251E
Figure 15. Typical Operating Circuit with the DC-to-DC Converter Disabled
(V
CC
= 3.0 V to 3.7 V)
Rev. G | Page 11 of 16
