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Texas Instruments Open Load Detection in Intergrated Drivers (Rev. A) Application notes
Application Report
SLVAE49A – April 2019 – Revised January 2020
Open Load Detection in Motor Drivers
Vashist Bist and Hector Hernandez
ABSTRACT
Protection and diagnostics are the highest priorities of any system board. The demand of diagnostic
features to make the system robust and reliable is increasing day by day. Open load detection (OLD) is a
protection diagnostic which determines a load's (e.g. motors, solenoids, relays, LEDs, and resistors)
connectivity to the power-stage of an motor integrated or gate driver. This article presents various types of
OLD diagnostics, features and implementation in TI's motor drivers.
1
2
3
4
5
6
7
Contents
Introduction ................................................................................................................... 2
Passive Open Load Detection ............................................................................................. 6
Active Open Load Detection .............................................................................................. 12
Low-Current Active Open Load Detection .............................................................................. 16
Negative-Current Active Open Load Detection ......................................................................... 17
Summary .................................................................................................................... 19
References .................................................................................................................. 19
List of Figures
1
Passive OLD for Load Connected to VM ................................................................................. 3
2
Active OLD for Load Connected to VM ................................................................................... 3
3
Passive OLD for Load Connected to GND
4
Active OLD for Load Connected to GND
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
.............................................................................. 4
................................................................................ 4
Passive OLD for Load Connected in H-Bridge Configuration.......................................................... 5
Active OLD for Load Connected in H-Bridge Configuration ............................................................ 5
Passive OLD Circuit in an Integrated Driver ............................................................................. 7
Passive OLD Circuit in a Brushed DC Gate Driver ..................................................................... 8
Passive OLD Circuit in a Brushless DC Gate Driver .................................................................... 8
Passive OLD Load Configurations not supported in a Brushless DC Gate Driver .................................. 9
Active OLD Operation ..................................................................................................... 12
Forward Drive and Braking in Asynchronous Rectification .......................................................... 13
Active OLD Circuit in Brushed DC Motor Integrated Drivers ......................................................... 14
Circuit for Active OLD in a DRV8873-Q1 and in a DRV8873 device ................................................ 15
Circuit for Active OLD in a BLDC Gate Driver .......................................................................... 15
Low-Current Active OLD Operation ...................................................................................... 16
Forward Drive and Braking in Synchronous Rectification ............................................................ 17
False OLD Flag with Negative-Current OLD Disabled ............................................................... 17
No False OLD Flag with Negative-Current OLD Enabled ............................................................ 17
Trademarks
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Introduction
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Introduction
The OLD diagnostic detects if the output terminals (OUT1 and OUT2) are disconnected from the loads to
cater to a safer and more robust system. OLD can be done in different diagnostics. Below is a list
describing each OLD diagnostic and in which motor drivers the OLD diagnostics are implemented:
• Passive Open Load Detection: The passive OLD, which is also called offline open load diagnostic, is
carried out before the FETs are in operation. All of the FETs are in Hi-Z state, while a minimal amount
of diagnostic current flows through the load for a short amount of time to test the load's connection to
the FETs. The diagnostic current must be very small to avoid load rotation. For the diagnostic current
to flow, a command is sent by the user to the motor driver to activate the passive OLD and initiate the
diagnostic current flow from four OLD current sources and through either four OLD resistors or internal
blocking diodes. For each FET in each half-bridge, there is one OLD current source and resistor or
internal blocking diode. The passive OLD circuit implementation found in the brushed DC motor
integrated drivers is similar to the implementation found in the stepper motor integrated drivers. In
these two types of drivers, the drivers provide the necessary hardware to conduct passive OLD
diagnostics. In low-side integrated drivers, only the low-side OLD current sources, one for each output,
are required to sense if an OLD event has occurred. Note that there are no OLD resistors or internal
blocking diodes. The passive OLD integrated drivers and in Brushless DC (BLDC) motor gate drivers
operates similarly as both types of drivers use the OLD resistors instead of the internal blocking diodes
found in BDC motor gate drivers. Passive OLD in BLDC gate drivers is dependent on the capacitance
between the load phase pins to ground. Additionally, not all load connections are supported in BLDC
gate driver passive OLD. The details of passive OLD integrated drivers are presented in Section 2.
Passive OLD can be found in the following types of drivers:
– Integrated Drivers
• Stepper Motor Drivers
• Brushed DC Motor Drivers
• Low-Side Drivers
– Gate Drivers
• Brushed DC Motor Drivers
• Brushless DC
• Active Open Load Detection: d The active OLD, which is also called online OLD, is carried out while
the FETs driving the load are turned ON. Active OLD ensures that the load is connected to the driver
during the operation. While the load is in operation, the current flowing through the FETs is monitored
to ensure that the load is connected. Active OLD can be found in Integrated Drivers such as Stepper
and BDC Drivers, as well as in Gate Drivers such as BLDC motor gate drivers.In stepper motor
integrated drivers, if the winding current in any coil drops below the open load current threshold (IOLD)
and the current regulation (ITRIP) level set by the indexer, an OLD event is detected. In some BDC
motor drivers, if the current flowing through the load drops below the IOLD during continuous and
PWM operation, an OLD event is detected. In other BDC motor drivers, such as DRV8873-Q1 and the
DRV8873 devices, the active OLD diagnostic monitors the body diode voltage of current re-circulation
only through high-side FETs (asynchronous rectification) to detect an OLD event. In BLDC gate
drivers, the current re-circulation flowing into the body diode of the high-side or low-side FET is
monitored to check the status of the load's connection to the driver. Active OLD is presented in
Section 3. Active OLD can be found in the following types of drivers:
– Integrated Drivers
• Stepper Motor Drivers
• Brushed DC Motor Drivers
– Gate Drivers
• Brushless DC Motor Drivers
• Low-Current Active Open Load Detection: In low-current active OLD, the current OLD threshold is
around 10x less than the active OLD diagnostic. This smaller current threshold gives a flexibility to user
to detect a smaller motor nominal current. The details on the low-current active OLD are presented in
Section 4. Low-current active OLD can be found in the following types of drivers:
– Integrated Gate Drivers
• Brushed DC Motor Drivers
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•
Negative-Current Active Open Load Detection: In negative-current active OLD, the current OLD
threshold is negative. This unique active OLD diagnostic utilizes the current re-circulating through the
body diode of the re-circulation FET (synchronous rectification) to detect an OLD event. In this
diagnostic, the current re-circulation flowing into the FET is monitored to check the status of the load's
connection to the driver. Since it accounts for the negative-current across this FET, it prevents the
false OLD flag seen in active OLD since active OLD does not account for negative current flow. The
details about this OLD diagnostic are presented in Section 5. Negative-current active OLD can be
found in the following types of drivers:
– Integrated Gate Drivers
• Brushed DC Motor Drivers
The OLD diagnostics are dependent on the type of load connection to the output terminal(s). The load
connections can be classified into three configurations:
1.1
Load Connected to Supply
In this configuration, the unidirectional motor or solenoid / relay load is connected between an output (for
example OUT1) and the supply (for example VM) as shown in Figure 1 (Passive OLD) and Figure 2
(Active OLD). This configuration is used for unidirectional control of loads. During the passive OLD, there
is no current flow to the load. During the active OLD, the load's current flows from VM to OUT1 to GND
when the low-side FET is turned ON.
VM
VM
X
VM
X
RL
OUT1
Passive
OLD
X
VM
RL
OUT1
Active
OLD
Figure 1. Passive OLD for Load Connected to VM
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Figure 2. Active OLD for Load Connected to VM
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Load Connected to Ground (GND)
In this configuration, the unidirectional motor or solenoid / relay load is connected between an output (for
example OUT1) and the GND as shown in Figure 3 (Passive OLD) and Figure 4 (Active OLD). This
configuration is used for unidirectional control of loads. During the passive OLD, there is no current flow to
the load. During the active OLD, the load's current flows from OUT1 to GND when the high-side FET is
turned ON.
VM
Passive
OLD
VM
X
Active
OLD
OUT1
X
X
RL
Figure 3. Passive OLD for Load Connected to GND
4
OUT1
Open Load Detection in Motor Drivers
RL
Figure 4. Active OLD for Load Connected to GND
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1.3
Load Connected to H-Bridge
In this configuration, the bidirectional motor or solenoid / relay load is connected between two outputs (for
example OUT1 and OUT2) as shown in Figure 5 (Passive OLD) and Figure 6 (Active OLD). This
configuration is widely used for bidirectional control of loads. This configuration gives flexibility to change
the direction of the load by opposing the voltage polarity at OUT1 and OUT2. During the passive OLD,
there is no current flow to the load. During active OLD, the load's current flows VM to either OUT1 or
OUT2, then to the other output and finally to GND when a half-bridge’s high-side FET and the other driven
half-bridge’s low-side FET are turned ON. To drive in the opposite polarity, those FETs are turned OFF
and the FETs that were OFF are now turned ON.
VM
Passive
OLD
VM
X
Active
OLD
OUT1
OUT1
Passive
OLD
Passive
OLD
X
X
VM
VM
X
X
OUT2
Passive
OLD
X
OUT2
Active
OLD
Figure 5. Passive OLD for Load Connected in H-Bridge
Configuration
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Figure 6. Active OLD for Load Connected in H-Bridge
Configuration
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Passive Open Load Detection
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Passive Open Load Detection
The passive OLD is also referred to as the offline OLD, where the OLD is carried out before the FETs are
turned ON. This diagnostics feature ensures that the load is connected to the driver before driving the
load. Passive OLD cannot be utilized at the same time as other OLD diagnostics. Load connections to
supply, to GND, and in an H-bridge can be detected with passive OLD integrated drivers.
Figure 7 shows the circuit implementation of the passive OLD in BDC and stepper motor integrated
drivers. For gate drivers, the circuit implementation is similar, with the difference being the FETs are not
integrated. The FETs must be in Hi-Z state. Internal source / sink current sources drive current into the
load connected between, for example, OUT1/SH1/DL1 and OUT2/SH2/DL2 during a set deglitch time and
are limited by the load’s resistance. The diagnostic current is very low (~100 µA for DRV8847) such that it
doesn’t rotate the load. If the load is connected between OUT1/SH1/DL1 and OUT2/SH2/DL2, a lowimpedance path is created, causing the diagnostics current to be high and operate in saturation. However,
if the load is disconnected from the either of OUT1/SH1/DL1 and OUT2/SH2/DL2, a high-impedance path
is created, causing the current to be reduced to zero. The voltage at the comparators inputs that share the
same node as the current sources fluctuates with the current variation. When the comparator positive
terminals are greater than the negative reference voltage terminals, the comparator outputs are high.
These comparator outputs are the OLD flags. Passive OLD is not enabled if any other fault other than
OCP/OLD are present.
For BDC gate drivers, the motor driver provides the necessary hardware to conduct passive OLD
diagnostics of the external FETs and the load. The differences between this passive OLD diagnostic and
the passive OLD diagnostic found in integrated and BLDC gate drivers are: BDC gate driver passive OLD
recommends the VDS comparator thresholds should be adjusted to 1-V or greater to ensure enough
headroom for the internal blocking diode forward voltage drop and the internal OLD pull-up and pull-down
circuits replace the OLD resistors with internal blocking diodes, as shown in Figure 8.
Figure 9 shows the circuit implementation of the passive OLD in BLDC gate drivers. The load phase pin to
ground capacitances must discharge before passive OLD is enabled. Single phase high-side and low-side
loads are not supported in BLDC gate drivers, as shown in Figure 10.
In integrated low-side drivers, only low-side OLD current sources are required to detect if a passive OLD
event has occurred.
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MOTOR
OUT1
OUT2
VM
AVDD
VM
AVDD
SW1_HS
X
±
X
ROL_HS
ROL_HS
VOL_HS
+
OL1_HS
VOL_HS
SW2_HS
-
OL2_HS
+
X
IOL_PU
IOL_PU
SW2_LS
SW1_LS
IOL_PD
±
OL1_LS
X
VOL_LS
IOL_PD
X
ROL_LS
X
ROL_LS
VOL_LS
OL2_LS
+
+
OPEN LOAD DETECT
Figure 7. Passive OLD Circuit in an Integrated Driver
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DRAIN
DRV
DRAIN
DRV8
PU_SHx
PU_SHy
GHx
GHy
VDS
VDS
SHx
SHy
BDC
PD_SHx
PD_SHy
VDS
GLx
PGND/SLx
VDS
GLy
PGND/SLy
Figure 8. Passive OLD Circuit in a Brushed DC Gate Driver
Figure 9. Passive OLD Circuit in a Brushless DC Gate Driver
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VDRAIN
HS_VSD
VDRAIN
VDRAIN
+
±
GHx
VM
DLx
IPD
IPU
SHx
LS_VSD
GLx
+
±
SLx
Figure 10. Passive OLD Load Configurations not supported in a Brushless DC Gate Driver
2.1
Circuit Operation and Detection
This section presents the circuit implementation of the passive OLD diagnostic of the DRV8847 device.
Note that the values of the parameters utilized as an example are from the device's datasheet. The Hbridge OLD sequence consists of turning ON the high-side switch (SW1_HS) of OUT1's half-bridge and
the low-side switch (SW2_LS) of OUT2's half-bridge together. Depending on how OUT1 and OUT2
connect to the load, there can be three cases that can cause at least one comparator output to be set to
"1"": H-bridge open, H-bridge short, or load connected in the H-bridge. Note that at least one passive OLD
comparator being set to “1” does not mean a passive OLD event has occurred. Only in the H-bridge open
case does a passive OLD flag occur. This case also applicable for drivers that have passive OLD
diagnostic and can drive a unidirectional load tied to supply or GND through a single half-bridge, such as
the DRV8908-Q1, DRV8906-Q1, and DRV8904-Q1 devices of the DRV89XX-Q1 device family. For
integrated drivers, the conditions which set the passive OLD comparator outputs to "1" are when VOLx_HS(+)
> VOLx_HS and when VOLy_LS(-) < VOLy_LS for an H-bridge open case, where x is for half-bridge x and y is for
half-bridge y. For gate drivers, the conditions which set the passive OLD comparator outputs to "1" are
when VOLx_PU(+) > VREF and when VOLy_PD(-) < VREF.
2.1.1
H-Bridge Open
If no load is connected between OUT1 and OUT2, then no current flows from the internal regulator
(AVDD). This OLD example will show why an OLD flag occurs when the H-bridge is open. The
voltages on the positive terminal of high-side comparator of OUT1's half-bridge (OL1_HS) and the
negative terminal of low-side comparator of OUT2's half-bridge (OL2_LS) will be as follows:
High-side comparator of OUT1's half-bridge (OL1_HS)
Since no current is flowing from AVDD, the voltage on OUT1 (which is also the positive terminal of
OL1_HS is clamped to AVDD (for example 4.2 V). OL1_HS has positive input that can be called
VOL1_HS(+) and negative input VOL_HS. The VOL_HS is also called the OLD threshold voltage. Since
VOL1_HS(+) (4.2 V) is greater than VOL_HS (2.3 V), the OL1_HS output is set to "1".
Low-side comparator of OUT2's half-bridge (OL2_LS)
Since no current flows through the SW2_LS switch, the negative terminal of OL2_LS (VOL2_LS(-)) is
pulled down to 0.0 V (GND). OL2_HS has positive input VOL_LS. Since VOL_LS (1.2 V) is greater than
VOL2_LS(-) (0.0 V), the OL2_LS output is set to "1". If both OL1_HS and OL2_LS are set to "1", it
signifies an OLD.
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H-Bridge Short
If there is a short between OUT1 and OUT2, then a short circuit current (ISC) flows from AVDD. A
successful OLD flag in an H-bridge short depends on the ISC. ISC will depend on AVDD, on the highside resistor (ROL_HS = 12 kΩ) and on the low-side resistor (ROL_LS = 15 kΩ). The ISC equation is as
follows:
ISC
VAVDD
15k: 12k:
VAVDD
27k:
(1)
Hence the ISC is calculated using Equation 1 as,
ISC
VAVDD
27k:
4.2V
27k:
155.56PA
(2)
This OLD example will show how an OLD flag cannot occur when there is a short on the H-bridge. The
voltage changes due to an H-bridge short on the positive terminal of OL1_HS (VOL1_HS(+)) and on the
negative terminal of OL2_LS (VOL1_HS) will be as follows:
High-side comparator of OUT1's half-bridge (OL1_HS)
ISC (155.56 µA) is flowing from AVDD. Therefore, the voltage on the positive terminal of OL1_HS is
calculated as,
VOL1_ HS
VAVDD
ISC u 12k:
(3)
Using Equation 3, the VOL1_HS(+) is calculated as,
(4)
Since VOL1_HS(+) (2.33 V) is greater than VOL1_HS (2.3 V), the output of OL1_HS is set to "1".
Low-side comparator of OUT2's half-bridge (OL2_LS)
The pull down current of ISC (155.56 µA) is flowing from AVDD to the SW2_LS switch. Therefore, the
voltage on the negative terminal of OL2_LS (VOL2_LS(-)) is calculated as,
(5)
Using Equation 5, the VOL2_LS(-) value is,
(6)
Since VOL2_LS(-) (2.33 V) is greater than VOL_LS (1.2 V), the output of OL2_LS is set to"0". The output of
OL1_HS is set to "1" and the output of OL2_LS is set to "0". Therefore, this case is not considered as
an OLD.
2.1.3
Load Connected in H-Bridge
The OLD monitoring when there is a load to the H-bridge will depend on the load resistance (RL). The
load's current (IL) for a load connected between OUT1 and OUT2 is calculated as,
ILOAD
VAVDD
12k: RL 15k:
VAVDD
RL 27k:
(7)
This OLD example will trigger an OLD event. The voltages at the positive terminal of OL1_HS
(VOL_HS(+)) and the negative terminal of OL2_LS (VOL_HS) will be as follows:
High-side comparator of OUT1's half-bridge (OL1_HS)
If VOL_HS(+) is greater than VOL_HS (2.3 V), the output of OL1_HS is set to "1". The voltage comparison
between VOL_HS(+) and VOL_HS required for the output of OL1_HS to be set to "1" is determined as:
VOL _ HS
VAVDD ILOAD u 12k:
(8)
Putting Equation 7 into Equation 8,
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VOLHS
VAVDD
VAVDD u 12k:
RL 27k:
(9)
Solving Equation 9 for the load resistance (RL), RL is expressed as,
RL !
VAVDD u 12k:
VAVDD VOL _ HS
27k:
(10)
By using the values of VAVDD and VOL_HS in Equation 10, the load resistance (RL) is calculated as ()473.7 Ω. Since the value of the resistance is negative, VOL_HS(+) is greater than VOL_HS (2.3 V) and the
output of OL1_HS is is set to "1".
Low side comparator of OUT2's half-bridge (OL2_LS)
If the voltage at negative terminal of OL2_LS (VOL_LS(-)) is less than VOL_LS (1.2 V), then the output of
OL2_LS is set to "1". Hence, the voltage comparison between VOL_LS(-) and VOL_LS required for the
output of OL2_LS to be set to "1" is calculated as:
VOL _ LS ! ILOAD u 15k:
(11)
Putting Equation 7 to Equation 11,
(12)
Solving Equation 12 for RL, RL is expressed as,
RL !
VAVDD u 15k:
VOL _ LS
27k:
(13)
Using the values of VAVDD and VOL_LS in Equation 13, RL has to be greater than 25.5 kΩ for OL2_LS to
be set to "1". Since the output of OL1_HS is always set to "1", the OLD status is solely dependent on
the output of OL2_LS. If the RL is less than 25.5 kΩ, then an OLD flag will not occur.
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Active Open Load Detection
The active OLD is the diagnostic that is carried out during load operation (current through the load is not
zero). This diagnostic feature ensures that the load is connected to the driver during operation. However, it
cannot detect if the load's terminals are disconnected from the power-stage before the load operation
begins.
Figure 11 shows the operation of the active OLD diagnostic. (Refer to DRV89XX-Q1 device family for
more details). If any of the FETs are turned ON and the current flowing in the particular FET is less than
the OLD's current threshold (IOLD) for time magnitude larger than the OLD deglitch time (tOLD), then an OLD
is detected.
In stepper motor integrated drivers, the active OLD depends on the winding current in any of the coils. If
the winding current in any coil drops below the open load current threshold (IOLD) and also on the ITRIP level
set by the indexer, an open-load condition is detected.
In DRV8873-Q1 and the DRV8873 devices, the active OLD is based on the voltage in a high-side FET's
body diode during the current re-circulation. The current re-circulation occurs through the high-side FET's
body diode in asynchronous rectification. In the gate drivers, the active OLD is based on the voltage in
both the high-side or low-side FET's body diode during the current re-circulation.
Figure 12 shows the flow of current during forward drive and during current re-circulation. The high-side
FET of OUT1 is in operating state. The voltage across the body diode of the current re-circulation highside FET is compared with the fixed reference OLD threshold voltage (VOL_HS) to detect the OLD event.
In gate drivers, each half-bridge's (phase's) active OLD is enabled independently. For OLD to occur, a
load needs to be connected across an H-bridge configuration. An OLD occurs when the voltage drop
across the body diode of the current re-circulation FET does not exhibit overshoot greater than the VOLA
over VM during the current-re-circulation time. An OLD does not occur if the energy stored in the load is
high enough to cause an overshoot greater than the VOLA over VM. The overshoot is caused by the
negative-current flowing through the body diode of the current re-circulation FET.
Active OLD in DRV8873-Q1, DRV8873 and in BLDC gate drivers, when compared to the negative-current
OLD diagnostic, are different since the former will flag the OLD in asynchronous rectification while the
latter will flag the OLD in synchronous rectification.
Motor Starting Peak
Current
Motor Nominal Current
Motor Open
Motor Connected
Motor
Operation
IOLD
IOUTx
tOLD
Fault Condition
nFAULT Pulled High
nFAULT Released
nFAULT
Time
Fault Cleared
Figure 11. Active OLD Operation
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VM
Continuous
ON
Diode
Recirculation
X
M
OUT1
OUT2
PWM ON
PWM OFF
X
Figure 12. Forward Drive and Braking in Asynchronous Rectification
3.1
Circuit Operation and Detection
Figure 13 shows the circuit implementation of the active OLD in integrated drivers. The high-side FET of
the OUT1 channel and low-side FET of the OUT2 are in the operating state. A reference voltage generator
generates equivalent reference voltages for the OLD comparators' negative input terminals, while the
positive terminals reflect the actual voltage in the FETs. As shown in Figure 13, the outputs of OL1_HS
and OL2_LS are set to "1" when the output voltages (VOUT1_HS and VOUT2_LS) become greater than the
reference voltage (VOL_REF) of the comparators,
(14)
In this example, the reference-FET is any of the four FETs tied to the current reference sources (IOL_REF).
The VOL_REF is determined by IOL_REF and the on-state resistance of the reference-FET (RDS(ON)_REF). Now,
the VOUT1_HS and the VOUT2_LS are determined by the current and on-state resistances (RDS(ON)) through the
FETs that drive the load. When an OLD event occurs, the current through the FETs must be greater than
the OLD current threshold. For these equations, the current through the FETs is called IOL. Hence, by
putting all of these parameters in Equation 14, the equation VOL_REF equation is modified to,
IOL _ REF u RDS(ON) _ REF
IOUTx u RDS(ON)
(15)
Hence, the IOLD can be calculated as,
§ RDS(ON) _ REF
IOUTx ! IOL _ REF u ¨
¨ RDS(ON)
©
·
¸
¸
¹
IOLD
(16)
Equation 16 shows that the OLD depends on the on-state resistance ratio of the reference-FETs to the
FETs driving the load.
Putting the values of on-state resistance ratio (450:1) and the reference generator pull-down current (20
µA), the OLD current threshold can be calculated as,
IOUTx ! 20 u 10
6
§ 450 ·
u¨
¸
© 1 ¹
IOLD
9mA
(17)
Hence, if current through the load is greater than 9 mA while OLD is active, an OLD event is registered.
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Active Open Load Detection
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MOTOR
OUT1
OUT2
VM
VM
X
+
OL1_HS
X
VOL_REF
VOUT1_HS
-
OL2_HS
+
Always
Low
IOL_REF
IOUTx
IOUTx
AVDD
VOUT2_LS
-
Always
Low
IOL_REF
+
OL1_LS
AVDD
VOL_REF
X
X
+
OL2_LS
-
OPEN LOAD DETECT
Figure 13. Active OLD Circuit in Brushed DC Motor Integrated Drivers
As for the circuit implementation in DRV8873-Q1 and the DRV8873 devices, Figure 14 shows the circuit
implementation of the active OLD. For the OL1_HS comparator, if the drain-to-source voltage across the
high-side driving FET is less than the VOL_HS OLD threshold, OLD1_HS output is set to "1" and an OLD is
detected. For the OL2_HS comparator, if the body diode voltage (VD) is less than the VOL_HS OLD
threshold, OLD2_HS output is set to "1" and an OLD is detected. Due to the current re-circulation through
the body diode (IOL), the body diode voltage (VD) depends on IOUTX for an OLD to occur.
There is a dependency on the operating conditions and on external circuitry, such as the output
capacitors, that can cause an OLD to be reported even though the load is present. This case might occur
during a direction change or for small load currents respectively small PWM duty cycles.
Finally, when using active OLD in gate drivers, capacitors must be placed between the load phase nodes
and GND. These capacitor are required for BLDC motors and both bi-directional and unidirectional BDC
motors at the phase nodes. If a solenoid load is connected, the capacitors are not required. Capacitors
must be sized as:
14
Open Load Detection in Motor Drivers
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Active Open Load Detection
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VM
VM
SH2
SH2
SH1
+
SH1
Detects OLD
if the diode VF
drop < VOLA
±
SH2
SH1
VM
No OLD detected
if the diode VF
drop > VOLA
Figure 14. Circuit for Active OLD in a DRV8873-Q1 and in a DRV8873 device
VM
SH2
DL2
SH1
DL1
+
SH2
DL2
SH1
DL1
VM
±
VM
SH1
DL1
SH2
DL2
±
+
Detects OLD if the No OLD detected if the
diode VF drop < VOLA diode VF drop > VOLA
Figure 15. Circuit for Active OLD in a BLDC Gate Driver
Cphase t
VTH u Crss
VOLA(min)
Cos s
(18)
Where VTH is the threshold voltage of the FETs and VOLA(min) in the DRV8343-Q1 and DRV8340-Q1 is 150
mV. The values of the FET Crss and Coss should be used for 0 V VDS. Derating of Cphase must be considered
when selecting the capacitance.
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15
Low-Current Active Open Load Detection
4
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Low-Current Active Open Load Detection
Low-current active OLD, found in the DRV89xx-Q1 devices, is designed for loads with a smaller motor
nominal current than the IOLD of active OLD. Figure 16 shows that the low-current active OLD threshold
(IOLD_LOW) replaces the IOLD from the active OLD. In low-current active OLD, IOLD_LOW is around 10 times less
than the active OLD's current OLD threshold. With IOLD_LOW being 10 times less than IOLD permits for more
flexibility in OLD when utilizing a load that requires a small nominal current to trigger an OLD event. If a
low-side FET is turned ON and the current flowing in that low-side FET is less than the IOLD_LOW for at least
the OLD deglitch time (tOLD), then an OLD event has occurred.
Motor Starting Peak
Current
Motor Nominal Current
Motor Open
Motor Connected
Motor
Operation
IOLD_LOW
IOUTx
tOLD
Fault Condition
nFAULT Pulled High
nFAULT Released
nFAULT
Time
Fault Cleared
Figure 16. Low-Current Active OLD Operation
The low-current active OLD has trade offs that must be considered:
• In the DRV89xx-Q1 devices, this OLD is only applicable for the current flowing in the low-side FETs,
meaning it cannot be detected using the high-side FETs.
• If low-current active OLD is used, the overcurrent protection threshold for the low-side FET is also
reduced by 11 times.
• The RDS(ON) of the low-side FET will increase by 11 times, hence the thermal performance has to be
monitored. However, given the current across the low-side FET is expected to be low, the thermal
dissipation of the low-side FET is expected to be limited.
16
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Negative-Current Active Open Load Detection
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5
Negative-Current Active Open Load Detection
In negative-current active OLD, found in the DRV89xx-Q1 devices, the current OLD threshold is negative
(IOLD_NEG). The negative current re-circulation occurs through the current re-circulation FET in synchronous
rectification. Figure 17 shows the operation of the H-bridge when forward driving and when using
synchronous rectification. If negative-current active OLD is not utilized, the device can show a false OLD
since the current re-circulating through the current re-circulation FET is negative and it is less than the
positive current OLD threshold the other active OLD diagnostics use. When the negative-current active
OLD is enabled, this negative flow of current through the current re-circulation FET does not show false
OLD event due now utilizing a negative-current active OLD current setting (IOLD_NEG).
VM
Continuous
ON
FET
Recirculation
OUT1
X
M
OUT2
PWM ON
PWM OFF
Figure 17. Forward Drive and Braking in Synchronous Rectification
Figure 18 shows the waveforms of false OLD when the negative-current active OLD is not active in an Hbridge configuration. In synchronous rectification with high-side current re-circulation, one of the driving
FETs is always turned ON and the other driven half-bridge's low-side and high-side FETs operate in a
complimentary manner. Initially, for the first PWM cycle, all OLD diagnostics are disabled to show the
currents in the different FETs during the load operation. If the active OLD diagnostic is enabled in the
second PWM cycle, then the device registers a false OLD flag during the high-side FET current recirculation due to the negative current. This false flag will cause both of the switching high-side and lowside FETs to be turned OFF. Since the fault causes OUT2_HS to turn OFF, the high-side FET's body
diode instead of the high-side FET conducts to complete the current re-circulation path back to the supply.
This false OLD flag is eliminated by enabling the negative-current OLD. Figure 19 the negative-current
active OLD current setting (IOLD_NEG) is enabled for the current re-circulation FET of the switching halfbridge. Since the current threshold is negative and greater than the current re-circulating through the
current re-circulation FET, the false OLD flag is prevented and the FETs of the switching half-bridge are
not disabled in the second PWM cycle. With setting the current threshold to a negative value, the OLD
flags where the load is disconnected during synchronous rectification can be detected. There is no
tradeoff to enabling the negative-current active OLD.
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Negative-Current Active Open Load Detection
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OUT1_HS
OUT1_HS
OUT1_LS
OUT1_LS
OUT2_LS
OUT2_LS
OUT2_HS
OUT2_HS
IOUT1_HS
IOLD
IOUT1_HS
IOLD
Body Diode
Conduction
IOUT2_LS
IOLD
IOUT2_LS
OLD Disabled
IOUT2_HS
OLD Disabled
OLD Enabled
IOLD
IOLD_NEG
tOLD
Fault
Condition
nFAULT Pulled High
No FAULT
nFAULT Pulled High
nFAULT
Time
Time
Figure 18. False OLD Flag with Negative-Current OLD
Disabled
18
OLD Enabled
IOUT2_HS
(FET Current)
nFAULT
IOLD
Open Load Detection in Motor Drivers
Figure 19. No False OLD Flag with Negative-Current OLD
Enabled
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Summary
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6
Summary
This article has presented different load connections found in OLD diagnostics, such as the half-bridge
load connected to supply, the half-bridge load connected to ground and the H-bridge load, along with five
types of OLD diagnostics: passive, active, low-current active, and negative-current active OLDs.
The passive OLD diagnostic is suited for the applications which require to check the connectivity of the
driver to the load before powering the driver. In such applications, there can be hazard of the open wire
connecting to other low-voltage circuitry, which can cause potential damage to this circuitry, or to power
lines, which can cause an overcurrent event. Passive OLD in low-side integrated drivers only requires lowside OLD current sources to detect a passive OLD event. BLDC gate drivers require the load phase pin to
ground capacitances to discharge before the passive OLD is enabled.
The active, low-current active, and negative-current active OLD diagnostics are suited for applications
where the driver current is to be monitored while the load is running. The active OLD can operate in both
continuous and in PWM operation. The DRV8873-Q1 and the DRV8873 devices can only detect an active
OLD during high-side asynchronous rectification. BLDC gate drivers require the active load to be
connected in an H-bridge configuration. The low-current active OLD sets a 10x current OLD threshold to
give a larger flexibility to detect loads with smaller load nominal currents than the IOLD. The negativecurrent active OLD changes the IOLD to IOLD_NEG to account for the negative high-side or low-side FET
current re-circulation in synchronous rectification and prevent a false active OLD flag.
7
References
•
•
•
•
•
•
•
•
•
•
Texas
Texas
Texas
Texas
Texas
Texas
Texas
Texas
Texas
Texas
Instruments, DRV8847 Dual H-Bridge Motor Driver Datasheet.
Instruments, DRV8889-Q1 Automotive H-Bridge Motor Driver Datasheet.
Instruments, DRV8899-Q1 Automotive H-Bridge Motor Driver Datasheet.
Instruments, DRV8860 Eight Serial Interface Low-Side Driver Datasheet.
Instruments, DRV8806 Quad Serial Interface Low-Side Driver Datasheet.
Instruments, DRV89xx-Q1 Automotive Multi-Channel Half-Bridge Motor Driver Datasheet.
Instruments, DRV8873-Q1 Automotive H-Bridge Motor Driver Datasheet.
Instruments, DRV8873 Automotive H-Bridge Motor Driver Datasheet.
Instruments, DRV8343-Q1 Automotive H-Bridge Motor Driver Datasheet.
Instruments, DRV8340-Q1 Automotive H-Bridge Motor Driver Datasheet.
SLVAE49A – April 2019 – Revised January 2020
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Copyright © 2019–2020, Texas Instruments Incorporated
19
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