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Texas Instruments Fault Handling Using TPS2660 eFuse Application notes
Application Report
SLVA934 – November 2017
Fault Handling Using TPS2660 eFuse
Karikalan Selvaraj
ABSTRACT
The TPS2660 eFuse provides integrated protection to various system faults such as overcurrent,
overvoltage, undervoltage, short-circuit, and reverse input polarity protection. Integrated reverse input
polarity protection helps to protect electronic systems from reverse input supply due to miswiring. This
application note describes methods to handle the fault of the TPS2660 by downstream circuits under a
reverse input polarity condition.
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Contents
Introduction ...................................................................................................................
Fault Status Monitoring Using TPS2660 eFuse .........................................................................
Fault Status Monitoring During Reverse Input Supply Connection ...................................................
Suggested Fault Handling Methods .......................................................................................
Conclusion ....................................................................................................................
1
2
3
4
5
List of Figures
1
TPS2660 Power Good Indication Using FLTb ........................................................................... 2
2
TPS2660 Fault Status Monitoring Using MCU ........................................................................... 2
3
Fault Status During Reverse Input Supply Connection ................................................................. 3
4
Power Supply Sequence Where FLTb Sinks More Current
5
Proposed System Fix With Series Current Limiting Resistor .......................................................... 4
6
Proposed System Fix With Blocking Diode
...........................................................
..............................................................................
4
5
List of Tables
1
Maximum DC Current versus Reverse Supply Voltage
................................................................
3
Trademarks
All trademarks are the property of their respective owners.
1
Introduction
A PLC system is usually powered by an external 24-V DC power supply to provide power to the controller
unit, backplane, and I/O modules within the PLC system. Input protection circuits are required to protect
the PLC system from system faults such as overcurrent, overvoltage, and overload. Because the input
supply connectors are screw type, there can always be a possibility of reverse connections at the input
supply.
The TPS2660 eFuse protects downstream circuits from various systems faults including integrated
protection to reverse input supply conditions (see the TPS2660 reference design). This device provides
status monitor functions like fault indication and load current monitor, which is used by a downstream
microcontroller unit (MCU) to take control decisions or a DC/DC converter to do power sequencing.
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Fault Status Monitoring Using TPS2660 eFuse
2
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Fault Status Monitoring Using TPS2660 eFuse
The FLTb signal of the TPS2660 combines power good indication along with system faults such as
overload, overvoltage, undervoltage, and shutdown. This combination enables downstream loads like
DC/DC converters to turn on heavy loads after power good indication. An application example where FLTb
is directly connected to enable pin of a DC/DC converter is shown in Figure 1. During startup, FLTb is
pulled low by the TPS2660 eFuse initially and is released after eFuse output is fully ON. Pullup resistor R4
and pulldown resistor R5 are used to scale down the pullup voltage and are chosen based on DC/DC
converters' enable threshold voltage and its operating maximum rating.
+
Supply wire
-
GND wire
Power Supply
R1
VIN
COUT
150 m
R4
UVLO
OVP
FLT
TPS2660x
R2
18-V to 36-V DC
OUT
OUT
IN
CIN *
EN
SHDN
R5
IMON
dVdT
R3
RTN
CdVdT
DC/DC
Converter
GND
ILIM
GND
RILIM
*Optional and only for noise suppression
Copyright © 2017, Texas Instruments Incorporated
Figure 1. TPS2660 Power Good Indication Using FLTb
Alternatively, the FLTb signal can be used by an external MCU to take control decisions under various
systems fault conditions. An application example where FLTb is directly connected to the IO pin of the
MCU and pulled up to the 5-V IO supply is shown in Figure 2.
CIN *
Supply wire
Power Supply
-
UVLO
OVP
GND wire
R2
18-V to 36-V DC
5V
COUT
150 m
R1
+
To Downstream Load
OUT
OUT
IN
VIO
R4
TPS2660x
FLT
IO
MCU
GND
dVdT
RTN
R3
CdVdT
ILIM
GND
RILIM
*Optional and only for noise suppression
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Figure 2. TPS2660 Fault Status Monitoring Using MCU
2
Fault Handling Using TPS2660 eFuse
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Fault Status Monitoring During Reverse Input Supply Connection
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3
Fault Status Monitoring During Reverse Input Supply Connection
During a faulty reverse input supply connection, the FLTb pin of the TPS2660 can sink current more than
its absolute maximum rating under certain power supply sequence. In particular, after a normal power-up
and power-down sequence, FLTb pin remains pulled to RTN to indicate brownout faults. Now, if a reverse
input supply is applied immediately, the FLTb pin sinks current back into the supply through an external
DC/DC converter enable pin or MCU IO pin as it remains pulled to RTN. The current flow path during a
reverse input supply connection is indicated in Figure 3.
IN
Supply wire
R1
+
CIN *
Power Supply
GND wire
R2
18-V to 36-V DC
To Downstream Load
COUT
150 m
UVLO
OVP
-
OUT
OUT
5V
VIO
R4
FLT
TPS2660x
IO
MCU
GND
dVdT
RTN
R3
ILIM
GND
CdVdT
RILIM
*Optional and only for noise suppression
Copyright © 2017, Texas Instruments Incorporated
Figure 3. Fault Status During Reverse Input Supply Connection
A specific power supply sequence where FLTb sinks more current is shown in Figure 4 and described
below.
• A normal power up is followed by a normal power down.
• The FLTb switch is pulled low to RTN due to a reverse current blocking fault or undervoltage fault.
• A reverse input supply is applied with a slew rate of –50 μV/μs > dVIN/dt > –500 mV/μs within 15
minutes.
• The FLTb switch remains pulled low to RTN and results in a current conduction path shown in
Figure 3.
During this reverse input supply sequence, a 60-mA ±20% DC current can flow through the ESD structure
of the IO pin of the MCU or the enable pin of the DC/DC converter for a 24-V reverse supply. Table 1
shows the maximum DC current that can flow through the FLTb pin for various power supply voltages.
Also note that this DC current does not flow for subsequent reverse supply connections because the FLTb
switch is opened after the first reverse supply is removed.
Table 1. Maximum DC Current versus Reverse Supply Voltage
PARAMETER
Maximum DC current through FLTb
REVERSE SUPPLY VOLTAGE
–18 V
–24 V
–36 V
45 mA ±20%
60 mA ±20%
90 mA ±20%
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Suggested Fault Handling Methods
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24-V DC
VIN
VOUT
Unpowered State
Normal Power Up
Power Supply OFF
0V
< 15 minutes
Reverse Power Supply
Connection
-50 V/ S > dVIN/dt > -500 mV/ s
±24-V DC
FLTb switch is ON due
to RCB event or UVLO
FLTb switch is OFF
VFLTb
0V
FLTb switch remains ON
IFLTb
VIO/R4
60 mA (± 20%) through MCU or DC/DC
(10 mA Max)
0 mA
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Figure 4. Power Supply Sequence Where FLTb Sinks More Current
4
Suggested Fault Handling Methods
Generally, ESD structures of MCUs or DC/DC converters can take 60 mA of current for a short period
during their operating life without degradation in performance. However, if the ESD structures are not able
to handle 60 mA of current during a faulty reverse input supply, the current through FLTb needs to be
limited by external means.
Adding a current limit resistance of R5 > VINMAX/2 mA between FLTb and IO of MCU as shown in Figure 5
limits current through ESD structure to be less than 2 mA. R5 can be chosen so that current through the
ESD structure is less than the absolute maximum ratings of the IO pin of the MCU or the enable pin of the
DC/DC converter, and current through the FLTb pin of the TPS2660 is always less than its absolute
maximum rating of 10 mA.
VIN
R1
*
CIN
GND
R2
COUT
150 m
UVLO
OVP
OUT To Downstream Load
OUT
IN
5V
R4
FLT
VIO
IO
MCU
R5
TPS2660x
> VINMAX/2 mA
GND
R3
dVdT
RTN
CdVdT
ILIM
GND
RILIM
*Optional and only for noise suppression
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Figure 5. Proposed System Fix With Series Current Limiting Resistor
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Fault Handling Using TPS2660 eFuse
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Conclusion
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Sometimes it is not feasible to place a resistor in series to FLTb. Consider the system shown in Figure 1
where FLTb signal provides power good indication to the DC/DC converter. Pullup and pulldown resistors
R4 and R5 must be scaled up if a 40-kΩ resistor is added in series to FLTb. For example, on a 24-V
power supply application with a range of 18 V to 36 V, R4 and R5 are 70 kΩ and 5 kΩ, respectively, for
1.23 V of an enable threshold and 0.5 V of a shutdown threshold. Now with a 40-kΩ series in place to limit
current through FLTb, R4 and R5 must be scaled to 2.8 MΩ and 200 kΩ, respectively, to meet both the
enable and shutdown thresholds. This scaling is not always possible because the current through R4 and
R5 are 12 µA at maximum and can become comparable to a leakage current > 1 µA of the enable pin of
DC/DC converter. Furthermore, a shift in the threshold across temperature can be very wide due to the
variation of leakage current over temperature.
In this case, it is recommended to place D1, a 60-V rated blocking diode like 1N4148WS as shown in
Figure 6, to block the current through FLTb and modify R4 and R5 to accommodate for the forward drop
of D1.
VIN
VIN
COUT
*
CIN
R1
OUT
OUT
IN
150 m
R4
GND
UVLO
OVP
R2
FLT
TPS2660x
SHDN
MODE
MODE
EN
D1
1N4148WS
IMON
dVdT
GND
ILIM
R3
CdVdT
RTN
DC/DC Converter
R5
GND
RILIM
*Optional and only for noise suppression
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Figure 6. Proposed System Fix With Blocking Diode
5
Conclusion
In a PLC system, possible system issues with the TPS2660 fault handling during a specific reverse power
supply sequence are highlighted, and two methods to overcome—using a current limiting resistor or a
blocking diode—are discussed in this application note.
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