Texas Instruments | An Isolated 24 V Power Supply Window Monitor | Application notes | Texas Instruments An Isolated 24 V Power Supply Window Monitor Application notes

Texas Instruments An Isolated 24 V Power Supply Window Monitor Application notes
Isolated 24 V Power Supply Window Monitor Options
Lars Lotzenburger, Ahmed Noeman
The 24 V field voltage is commonly used on the
factory floor. It powers field transmitters, sensors,
actuators, and more. According to IEC61131, the 24 V
has a tolerance of -15% to +20%, which translates to a
voltage range of 20.4 V to 28.8 V. In critical
applications, the PLC side may need to know whether
the voltage is in the valid range for correct operation.
There may be no inbound isolation path to get the
information to the PLC side, though. Just think about
an output-oriented application like an analog output
module. The processor feeds the data to the DAC
where all serial interface signals are outbound-directed
(for example, SCLK, CS, and DOUT of an SPI). The
power valid signal you want to transfer may also sneak
in here by spending an additional channel in an
anyway-used digital isolator (see Figure 1). In
applications where this is not possible or size/power
consumption is not a big constraint, Option 2 or Option
3 may be of interest.
The TLV6700 has 1% detection accuracy on the
overvoltage (OV) threshold, and 1.5% for the
undervoltage (UV) detection. The resistor ladderproduced voltage has 1.9% accuracy when 1%
resistors are used. Using the sum of squares gives
2.3% overall accuracy for OV detection, and 2.4% for
UV detection.
Option 1: Digital Isolator with Back Channel and
Window Comparator
For the DAC example described above, a digital
isolator with three channels (ISO7730) works, but the
four channel derivative (ISO7741) can be used to
transfer the power supply information. The TLV6700, a
window comparator with internal reference, and opendrain outputs monitors the power supply voltage. It
outputs high if the voltage is in the expected range or
outputs low otherwise. The resistor ladder for the
window comparator is calculated using the procedure
presented in Window Comparator with Integrated
Reference Circuit Application Note (SNVA832). For the
calculation, the lower threshold (VL) is set to 20.4 V
and the higher threshold (VH) is set to 28.8 V. See
Figure 1 for the schematics.
The power supply for the TLV6700 comes "for free" as
a 3.3-V to 5-V voltage rail. It is required for the isolator
anyway (pin VCC2). The supply rail VCC_ISO is
generated from VIN, meaning if the power is lost at the
PLC side, the power of the isolator on the isolated side
(pin VCC2) also goes down. As a consequence, the
ISO7741F sets the output of channel D (pin OUTD)
low. This presents a reliable fault to the processor
down to zero voltage (power loss). The TLV6700 starts
operating at 1.8 V while the ISO77xx family starts at
2.25 V. This always ensures a valid level at the
isolator input when powered up.
SLLA457 – August 2019
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Figure 1. Schematics Option 1
Option 2: Dedicated Isolator with Shunt Reference
The second option consists of two components: The
ISO1211, an isolated 24-V to 60-V digital input
receiver, and the shunt reference TL431LIAQ. If the
field voltage is in valid range, the ISO1211 outputs
high, otherwise it is low. The ISO1211 takes care of
the isolation and the lower threshold detection. See
Figure 2 for the schematics of this circuit.
Figure 2. Schematics option 2
Isolated 24 V Power Supply Window Monitor Options Lars Lotzenburger, Ahmed Noeman
Copyright © 2019, Texas Instruments Incorporated
1
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The lower, positive-edge, threshold is set by R7 and
R8. R8 sets the required input current, IL, which is set
to the lowest possible current of 2.25 mA. R7 and R8
define the lower voltage threshold of the window
comparator.
(1)
With a field supply threshold of 20.4 V, the threshold
resistor R7 is 5.4 kΩ (5.49 kΩ of E96 series). The
upper, negative-edge, threshold is triggered by the
TL431LIAQ. The resistor divider R9/R10 sets the
threshold. The minimum current through the divider is
set to 40 µA (100x the reference input current of
TL431LIAQ). The resistance of the divider for the
upper threshold voltage of 28.8 V should therefore not
exceed 720 kΩ.
ISO1211. The window comparator ensures that the
open-drain output is low in case the supply is out of
the valid voltage range (OV/UV condition). The
ISO1211 gets the required 2.25 mA as long as the
window comparator output is high to generate output
high on the processor side. The TLV6700 can
withstand voltage up to 40 V, and has a level detection
accuracy of ±1.4%. The 1% resistor ladder has an
accuracy of ±1.9%. Using sum of squares, the
expected accuracy of this OV/UV detector is better
than ±2.4%.
The reference voltage of the TL431LIAQ is 2.495 V
nominal. The resistor divider splits up to 28.8 V/ 2.495
V = 11.54:1. R10 must not exceed 720 kΩ / 11.54 =
62.39 kΩ (61.9 kΩ). R9 results in 62.39 kΩ × (11.54 1) = 658 kΩ (649 kΩ). With real resistor values, the
upper threshold excluding errors is calculated by:
Figure 3. Schematics Option 3
(2)
As soon as this threshold is reached, the TL431LIAQ
conducts and provides a low impedance path to
ground. As a result, the input voltage at the ISO1211
goes below the lower threshold, forcing a low at the
ISO1211 output.
The circuit works with an input voltage up to 75 V. The
current thought the TL431LIAQ for a SELV/PELV
power supply with 60 V input voltage (max) is about 12
mA. According to the simulation, the voltage drop
across R7 at 60 V input voltage is 59.08 V. This
results in a maximum power dissipation of PMAX_R7 =
0.636 W in this overvoltage condition. A reverse
polarity input connection is also tolerated down to a
input voltage of -55 V. The ISO1212 can be used for a
dual voltage monitor.
The TL4311LIAQ is quite accurate for OV detection –
it has 0.6% accuracy. If a 1.7% voltage accuracy out
of the 1% resistor ladder is considered, the OV
detection accuracy is about 1.8%. The case for the UV
is worse, as the ISO1211 has about 7% inaccuracy for
threshold detection. This leaves UV detection at about
7% accuracy.
Option 3: Dedicated Isolator with Window
Comparator
The third option consists of window comparator
TLV6710. The TLV6710 is similar to the TLV6700, but
with a supply voltage up to +36 V and a slightly
different threshold. The resistor ladder is adapted
accordingly. The isolation is performed by the
2
Table 1. Comparison of the Three Options
COMPARISON
PARAMETER
OPTION 1
OPTION 2
OPTION 3
Devices
ISO7741F,
TLV6700
ISO1211,
TL431LIAQ
ISO1211,
TLV6710
Input Voltage
0...>100 V
0..75 V
0..40 V
Reverse polarity
protected
No
Yes
No
Accuracy
~±2.4%
~±1.8% OV, ±7%
UV
~±2.4%
Power
consumption
(nominal)
~7 mW
(ISO7741F: one
channel)
~55 mW
~55 mW
Isolation
Reinforced/basic
Basic
Basic
Temperature
range
-40ºC to +125ºC
-40ºC to +125ºC
-40ºC to +125ºC
Size
Small
Medium
Medium
Cost
$$
$
$$$
Application
Needs to fit in
application
concept
Independent of
other application
circuit
Independent of
other application
circuit
References
• TLV6700 Product Page
• TLV6710 Product Page
• ISO7741 Product Page
• ISO1211 Product Page
• TL431LI-Q1 Product Page
Isolated 24 V Power Supply Window Monitor Options Lars Lotzenburger, Ahmed Noeman
Copyright © 2019, Texas Instruments Incorporated
SLLA457 – August 2019
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