Detailed Description
The MAX22191 features an integrated current source, voltage
comparator, and current steering network to create an input
load compliant with IEC 61131-2 Type 1 and Type 3 24V
DC
inputs, while generating a drive current for opto-isolators
that turn-on/-off in compliance with the voltage thresholds
of the standard. The addition of external voltage-dropping
resistors also allows the MAX22191 to operate with 48V
DC
inputs (see the Typical Operating Circuits).
Power-Up/Power-Down
As the input voltage (V
IN
) rises, the MAX22191 transitions
through three phases of operation:
Phase 1: V
IN
is rising but is inadequate to fully power the
current source or voltage comparator. Any current that
does flow into the MAX22191 is diverted to GND through
the internal current steering switches, bypassing the optical
isolator.
Phase 2: V
IN
continues to increase to a level that is
adequate to power the comparator and the current
source, but the input voltage threshold has not been
reached. The output of the internal current source
continues to be diverted to GND.
Phase 3: V
IN
exceeds the comparator threshold (V
INTHU
),
and the current is switched to the OUT pin. If connected to
an external optical isolator, the current passes through the
LED and returns to the negative field input.
As V
IN
drops, the phases are reversed. The internal
current source is switched from OUT to GND when V
IN
falls below the lower voltage threshold (V
INTHL
).
Boost Current
To allow for a faster response time, the MAX22191
includes a boost current, I
INB
, during IN power up. The
boost current is used to set and stabilize the output
current while the voltage on IN is rising (V
IN
< V
INTHU
).
When V
IN
> V
INTHU
, and the output current is enabled,
the input current is the sum of both the output current and
boost current (I
INB
+ I
INH
) for a short period before the
output current is steady at 2.3mA (typ).
Integrated Diagnostic (TEST) Input
The MAX22191 features an integrated TEST input for
easy diagnostic checks. When IN is high, toggle TEST
from low-to-high to verify that OUT toggles high-to-low.
See Table 1. The current on IN is not affected during this
diagnostic test.
When IN is low, TEST has no effect on OUT, it remains low.
Applications Information
Powering the MAX22191 With the V
CC
Pin
The MAX22191 can be powered parasitically from a
digital input or from an external power supply.
To power the device parastically, connect V
CC
to GND.
In this configuration, power is derived from the signal on
the IN pin.
To power the device from a local power supply, connect
V
CC
to a source between 3.0V and 5.5V. When V
CC
is powered, the output (OUT) changes from a current
source to a CMOS output and the propagation delay from
IN to OUT is reduced.
Layout Considerations
Place the 40.2kΩ (±1%) REXT resistor as close to the
pin as possible. Too much distance between the resistor
and the IC can create unwanted input current overshoots/
undershoots.
Table 1. TEST Mode Functionality
IN TEST OUT
< V
INTHL
Low Low
< V
INTHL
High Low
≥ V
INTHU
Low High
≥ V
INTHU
High Low
MAX22191 Parasitically Powered Digital Input
www.maximintegrated.com
Maxim Integrated
│
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