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: 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, DRV8873-Q1 and the DRV842x-Q1 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:
- 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:
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: