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Texas Instruments Reverse battery protection for high side switches Application notes
Application Report
SLVAE55 – January 2019
Reverse battery protection for high side switches
Mahmoud Harmouch, Cameron Phillips
ABSTRACT
Reverse battery, often referred to as reverse polarity, is extremely common in automotive applications.
This application report details the reverse battery mechanism, impact and protection of TI smart high side
switches and the MCU as well. This document also goes through the different circuit topologies in order to
protect the system from reverse polarity events.
1
2
3
4
Contents
Introduction ...................................................................................................................
Methods of Mitigating Reverse Polarity ...................................................................................
Bench Test Verification .....................................................................................................
Conclusion ....................................................................................................................
2
2
6
8
List of Figures
1
Reverse Battery Configuration ............................................................................................. 2
2
Diode-Resistor Ground Network ........................................................................................... 3
3
Ground Network Protection Application
4
5
6
7
8
9
10
11
12
..................................................................................
MCU Current Limiting Resistor Application ..............................................................................
Blocking Diode ...............................................................................................................
TPS4H160EVM Setup ......................................................................................................
INx, GND, DIAG_EN ........................................................................................................
CS, FAULT, and THER Voltage ...........................................................................................
SEL and SEH Voltage ......................................................................................................
TPS1H100EVM Setup ......................................................................................................
IN, CS, DIAG_EN and VBAT at 18 V .....................................................................................
IN, CS, DIAG_EN and VBAT at 24 V .....................................................................................
4
5
6
6
6
6
6
7
7
7
List of Tables
1
Reverse Battery Causes and Mitigation .................................................................................. 2
2
TPS4H160-Q1 Reverse Polarity Measurements ........................................................................ 7
3
TPS1H100-Q1 Reverse Polarity Test Voltages and Currents ......................................................... 8
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1
Introduction
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1
Introduction
Reverse polarity is a common mistake. The way that a reverse polarity event is defined is by connecting
what should be the system ground to a positive potential (voltage) and grounding what should be the
supply port. It can happen after maintenance or during reconnecting the battery when the wires get
crossed. Reverse polarity can lead to catastrophic damage in the electronic circuit even in a short time.
The main cause of damage is the non-controlled reverse current through ESD cells inside the high side
switch and potentially the MCU. Reverse polarity protection consists of limiting the reverse current or
blocking it completely. There are several areas that are susceptible to reverse battery conditions. This
document goes through each of the areas and their mitigation.
VS
MCU
GND
High Side
Switch
Load
GND
Figure 1. Reverse Battery Configuration
Table 1. Reverse Battery Causes and Mitigation
2
Failure Cause During Reverse Battery
Mitigation
Excess Reverse Current Through High Side Switch Ground Pin
Diode + Resistor Ground Network
Excess Current Through MCU Pins into High Side Switch
Current Limit Resistors on all MCU Connected pins
Excess Reverse Current Through Load into High Side Switch
Blocking Diode
Methods of Mitigating Reverse Polarity
There are several different methods of blocking the reverse current: adding a diode resistor ground
network to limit the current flow into the device, adding a blocking diode on the supply to prevent the
current loop, or adding current limiting resistors in between the MCU and the high side switch. Naturally
using all of the methods will have the most protection but each have their own benefits if designers are
constrained to a subset of them. These methods are detailed below and will ensure that the system is
safely protected during reverse polarity events.
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Methods of Mitigating Reverse Polarity
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2.1
Ground Network
High Side
Switch
Load
GND
Ground
Network
RGND
Figure 2. Diode-Resistor Ground Network
The first method consists of current limiting resistor and a diode between high side switch device ground
and system ground (GND network) shown in Figure 2. The GND network is a cheap solution and does not
consume any power during normal operation. This is due to the fact that during normal operation the diode
is forward biased to ground bypassing the resistor. However when there is a reverse current, the resistor
will limit the flow back into the device to a safe level. The GND network limits the current through the
ground pin of the high side switch by Equation 1
IGND = VBATT / RGND
(1)
The datasheet of the high side switch will have the acceptable reverse current allowed. The selection of
the RGND has to also take into consideration that there will be a power dissipation through the resistor
during a reverse battery event. Calculate the power going through the resistor during a reverse battery
event using Equation 2 to appropriately size the resistor for each application .
PRGND = VBAT2 / RGND
(2)
For most applications TI recommends a 1-kΩ resistor. This protects the high side switch from internal
damage due to ESD cell between the supply pin and device ground. As can be seen in Figure 3 the
current is regulated through the GND pin up the ESD cell and out the supply pin. Figure 3 also shows all
of the ESD cells that are on the pins and the pins names. Note that the pin names in parenthesizes apply
to the low RON family of TI high side switches, TPSxHBxx-Q1 such as TPS2HB08-Q1 but function similar
to the higher RON devices, TPSxHxxx-Q1 devices. The device will also only have either RLIM or RCL based
on if it is a low RON device or 100 mΩ and above. Reverse current paths and directions are shown in red.
Also note that the resistor diode ground network does not protect the reverse current in the main FET.
This FET current is only limited by the load itself, if the load can not limit the FET current that is electrolytic
capacitor load, a blocking diode is necessary as discussed in Section 2.3.
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Methods of Mitigating Reverse Polarity
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VS (VBB)
OUTx
(VOUTx)
RLIM
(ILIMx)
CS (SNS)
INx (ENx)
High Side
Switch
SEH/L (SELx)
VBATT
Load
Reverse
Battery
FAULT/ST
THER (LATCH)
CL
RCL
GND
Unregulated
DIAG_EN
RGND
IGND
Figure 3. Ground Network Protection Application
2.2
MCU Current Limiting Resistors
In the typical application of high side switches, there is a microcontroller that monitors the diagnostics and
decides to enable or disable the device. The pins being used of the microcontroller are the ADC pins (for
current and fault sensing) and GPIO pins (for enabling device and other setting). The ADC pins of the
microcontroller would go internally to some comparator that would be high impedance inputs, however
there would be an internal ESD diode on the pin that would go to the ground of the MCU. Quite similarly to
the high side switch, this ESD cell is an issue as it will conduct current in the negative direction during a
reverse polarity condition.
Additionally, the GPIO pins of most MCU's have push-pull operation which mean they have pull up FETs
and pull down FETs to enable high or low signal output. This is also an issue in context of a reverse
polarity scenario because the body diode of the pull down FETs will conduct current. The mitigation for
these issues is to have current limiting resistors on the lines connected to the MCU that regulate the
current that flows out of the MCU pins into the high side switch pins. This can be seen in Figure 4.The
current through the resistor can be calculated using Equation 3.
IPROT=(VBATT-VF_MCU-VESD_HSS) / RPROT
(3)
Typically the forward voltage of the body diode and ESD diode, VF_MCU, will be approximately 0.7-1V. The
DC reverse breakdown voltage of the VESD_HSS will normally be shown in the absolute maximum value of
the datasheet, that is TPS4H160-Q1 has –7 V as the maximum value for the control pins. It is important
that the IPROT is acceptable for the MCU to output and for the high side switch to take in. For that reason TI
recommends RPROT to be 10 kΩ or another high impedance resistance. Figure 4 also shows the ground
network as discussed in Section 2.1. As in the previous section though the current flow through the load is
not regulated and has to be accounted for by a blocking diode.
4
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VS (VBB)
VDD
OUTx
(VOUTx)
RLIM
(ILIMx)
RPROT
CS (SNS)
1K
INx (ENx)
RPROT
High Side
Switch
SEH/L (SELx)
VDD
VBATT
RPROT
FAULT/ST
RPROT
DIAG_EN
Load
RPROT
RPROT
IPROT
VF_MCU
THER (LATCH)
CL
VESD
MCU GND
RCL
GND
Unregulated
MCU
RGND
IGND
Figure 4. MCU Current Limiting Resistor Application
2.3
Blocking Diode
A blocking diode placed between the high side switch supply pin prevents any current flow in the system
during a reverse battery condition. The blocking diode blocks the reverse current in any path. The MCU
GND and the device GND are at the same voltage as well as the current through the ESD cells is now
blocked. As can be seen in Figure 5, reverse current is zero and the blocking diode protects the high side
switch, MCU and load from damage.
The issue with the blocking diode is that there is power dissipation associated with the diode. Typically
shottky diodes can have from 300 mV to 1 V of forward voltage with the nominal DC currents of the high
side switch. For instance if the voltage drop is 500 mV at 2 A current the DC power dissipation would be 1
W. This means the diode needs to be appropriately sized for the maximum power dissipation that could be
seen in it.
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Bench Test Verification
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Blocking Diode
VS (VBB)
VDD
OUTx
(VOUTx)
RLIM
(ILIMx)
CS (SNS)
1K
SEH/L (SELx)
VDD
VBATT
High Side
Switch
FAULT/ST
Load
INx (ENx)
MCU
DIAG_EN
THER (LATCH)
CL
RCL
MCU GND
GND
Figure 5. Blocking Diode
3
Bench Test Verification
In order to test the capability and protection of the high side switches during a reverse polarity event, the
EVM for the high side switches was modified to test out the different protection topologies. This section
goes through a couple devices to show that they can survive a high negative voltage on the supply. Note
also that the blocking diode was not tested as by definition it will protect the device from damage since
there is no negative flow of current.
Since the MCU just provides a body diode or ESD drop and allows the current to flow through as
discussed in Section 2.2, to test this on the bench the CS, INx, SEH/L, DIAG_EN and THER pins were
connected to ground through a 10kΩ resistor effective bypassing the MCU voltage drop and putting the full
negative voltage on the protection resistors. This can be thought as absolute worst case.
3.1
TPS4H160-Q1 Reverse Polarity
Using the TPS4H160EVM and connecting the control pins to ground through a 10 kΩ, we are able to
measure the values of the pins. Table 2 shows the voltages and currents value on each I/O pin at 36-V
reverse voltage. Figure 6 shows the EVM setup and connection. The test was done with putting a –36-V
supply to simulate worst case negative battery transient. Note that the VBAT waveform in the oscilloscope
screen shots is the System GND from the setup and all waveforms are with respect to the VS pin.
One very important note: unlike the rest of the pins, the CS pin has a lower impedance to it's ESD diode
than any of the rest. That is due to the fact that the CS pin in all devices has to be in an acceptable range
to reflect the output current during normal operation. Because of this, one of the big keys when running
this test is to make sure the current through that resistor is not too high to break the internal ESD diode.
This also will come into use when going to the higher current devices such as the low RON high side
switches that will require lower resistor values on the current sensing output which in turn means more
current flowing through in reverse polarity.
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Bench Test Verification
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VS
OUTx
36V
THER
THER
10 k
INx
10 k
SEL/SEH
INx
SEH/L
TPS4H160-Q1
10 k
FAULT
FAULT/ST
DIAG_EN
DIAG_EN
10 k
10 k
CS
10 k
CS
CL
Figure 7. INx, GND, DIAG_EN
RCS
RCL
GND
GND
1k
System GND
VBAT
Figure 6. TPS4H160EVM Setup
Figure 8. CS, FAULT, and THER Voltage
Figure 9. SEL and SEH Voltage
Table 2. TPS4H160-Q1 Reverse Polarity Measurements
3.2
Device GND
VS to
to System
System GND
GND
Device GND
to VS
INx to
Device GND
DIAG_EN to
Device GND
CS to Device FAULT to
GND
Device GND
THER to
Device GND
SEL to
Device GND
SEH to
Device GND
–36 V
35.3 V
0.7 V
12 V
18 V
0.9 V
11.3 V
18.3 V
12 V
12 V
Current
35.3 mA
N/A
2.4 mA
1.8 mA
13 mA
2.47 mA
1.8 mA
2.4 mA
2.4 mA
TPS1H100-Q1 Reverse Polarity
TPS1H100-Q1 is a single channel high side switch and has less I/O pins than TPS4H160-Q1. Figure 10
shows the test setup for TPS1H100-Q1 in reverse polarity. Notice it is similar to Figure 6 but with less I/O
pins. Table 3 shows the voltages and currents during the reverse polarity testing.
TPS1H100-Q1 was tested at 18-V and 24-V reverse voltage. Since the TPS1H100-Q1 is a lower RON and
single channel, it can have higher nominal currents than the TPS4H160-Q1. This means to have a current
sense range large enough for the full range of nominal currents, the RCS resistor must be smaller than it
was on the TPS4H160-Q1. This means more current will be flowing through that resistor during a reverse
polarity event. For this reason the TPS1H100-Q1 was tested at 18 V and 24 V so that the current would
be at a safe level. This trade off of how high reverse polarity voltage can be taken by the device must be
taken into consideration when designing the system as a whole.
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Conclusion
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VS
OUT
18V/24V
THER
10 k
IN
IN
10 k
DIAG_EN
TPS1H100-Q1
DIAG_EN
10 k
CS
10 k
CS/ST
CL
RCL
GND
Figure 11. IN, CS, DIAG_EN and VBAT at 18 V
RCS
1k
System GND
VBAT
Figure 10. TPS1H100EVM Setup
Figure 12. IN, CS, DIAG_EN and VBAT at 24 V
Table 3. TPS1H100-Q1 Reverse Polarity Test Voltages and Currents
4
VS to System GND
Device GND to System
Device GND to VS
GND
INx to Device GND
DIAG_EN to Device
GND SCR
CS to Device GND
–18 V
17.3 V
0.7 V
11.3 V
11.6 V
2V
Current
17.3 mA
N/A
0.67 mA
0.64 mA
24 mA
–24 V
23.3 V
0.72 V
11.31 V
11.85 V
2.11 V
Current
23.7 mA
N/A
1.27 mA
1.26 mA
33 mA
Conclusion
TI Smart High Side Switches have robust protection and capability of protecting during reverse battery
events with the use of several techniques. As laid out in Table 1 a ground network, protection resistors
and blocking diodes all help to provide system level protection during reverse polarity. Using these tips
during the design phase will save the MCU and high side switch from damage while still enabling normal
functionality.
8
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