ISO255 Datasheet, PDF (7/9 Pages) Burr-Brown (TI) – Precision, Powered, Three-Port Isolated INSTRUMENTATION AMPLIFIER

INPUT PROTECTION
The inputs of the amplifier are individually protected for
voltages up to ±40V. Internal circuitry on each input pro-
vides low series impedance under normal signal conditions.
If the input is overloaded, the protection circuitry limits the
input current to a safe value (approximately 1.5mA). The
inputs are protected even if no power supply is present.
DC/DC CONVERTER
ISO255 provides a reliable solution to the need for integral
power. The high isolation rating being achieved by careful
design and attention to the physical construction of the
transformer. In addition to the high dielectric strength a low
leakage coating increases the isolation voltage range. The
soft start oscillator/driver design eliminates high inrush
currents during turn-on. Input current sensing protects both
the converter and the load from possible thermal damage
during a fault condition. The DC/DC converter is synchro-
nized to the amplifier and when multiple ISO255’s are used,
each channel can be synchronized via the SYNC pin.
The DC/DC converter consists of a free-running oscillator,
control and switch driver circuitry, MOSFET switches, a
transformer, rectifier diodes and filter capacitors all con-
tained within the ISO255 package. The control circuitry
consists of current limiting, soft start and synchronization
features. In instances where several ISO255’s are used in a
system, beat frequencies developed between the ISO255’s
are a potential source of low frequency noise in the supply
and ground paths. This noise may couple into the signal path
and can be avoided by synchronizing the individual ISO255’s
together by tying the SYNC pins together or using the circuit
in Figure 2 to drive the SYNC pins from an external source.
MC1472
or Equivalent
Peripheral
Driver
+5V
330Ω
+15V
TTL
SYNC
620Ω
100Ω
2N3904
2N3904
16
17
16
17
ISO255
15
ISO255
15
To Other
ISO255’s
FIGURE 2. External SYNC drive.
When connecting up to eight ISO255’s without a driver the
unit with the highest natural frequency will determine the
synchronized running frequency. The SYNC pin is sensitive
to capacitive loading: 150pF or less is recommended. If
unused, the SYNC pin should be left open. Avoid shorting
the SYNC pin directly to ground or supply potentials;
otherwise damage may result.
Soft start circuitry protects the MOSFET switches during
startup. This is accomplished by holding the gate-to-source
voltage of both MOSFET switches low until the free-run-
ning oscillator is fully operational. In addition, soft start
circuitry and input current sensing also protects the switches.
This current limiting keeps the MOSFET switches operating
in their safe operating area under fault conditions or exces-
sive loads. When either of these conditions occur, the peak
input current exceeds a safe limit. The result is an approxi-
mate 5% duty cycle, 300µs drive period to the MOSFET
switches. This protects the internal MOSFET switches as
well as the external load from any thermal damage. When
the fault or excessive load is removed, the converter resumes
normal operation. A delay period of approximately 50µs
incorporated in the current sensing circuitry allows the
output filter capacitors to fully charge after a fault is re-
moved. This delay period corresponds to a filter capacitance
of no more than 1µF at either of the output pins. This
provides full protection of the MOSFET switches and also
sufficiently filters the output ripple voltage. The current
sensing circuitry is designed to provide thermal protection
for the MOSFET switches over the operating temperature
range as well. When these conditions are exceeded, the unit
will go into its shutdown mode.
OUTPUT CURRENT RATINGS
The total current which can be drawn from each output
supply on the ISO255 is a function of the total power drawn
from all outputs. For example if three outputs are not used
then maximum current can be drawn from one output. In all
cases, the total maximum current that can be drawn from any
combination of outputs is:
I₁₊ + I_1– + I₂₊ + I_2– ≤ 50mΑ
+V_S3, GND3
The waveform of the ground return current is an 800kHz
sawtooth. A capacitor between +V_S3 and GND3 provides a
bypass for the AC portion of this current. The power should
never be instantaneously interrupted to the ISO255 (i.e., a
break in the line to +V_S3 either by accidental or switch
means.) Normal power down of the +V_S3 supply is not
considered instantaneous. Should a rapid break in input
power occur the internal transformers voltage will rapidly
rise to maintain current flow and may cause internal damage
to the ISO255.
SYNCHRONIZED OPERATION
ISO255 can be synchronized to an external signal source.
This capability is useful in eliminating troublesome beat
®
7
ISO255