The DRV8706-Q1 motor driver boasts sophisticated diagnostics for off-line short-circuit and open-load detection. However, understanding the intricacies of these features, particularly for off-line scenarios, can be challenging. This article delves into the functionality of the DRV8706-Q1’s comparators, specifically addressing whether they can detect open gates in off-line situations.
Deciphering the Datasheet: Off-Line Diagnostics
The datasheet (section 7.3.8.11 and figure 7-20) describes a diagnostic procedure involving enabling the OLSC_EN bit after disabling the outputs. Subsequently, enabling the PU_SHx/PD_SHx bits activates internal current sources, creating a voltage drop across the FETs. This voltage is then monitored by four dedicated Vds comparators, which control the VDS_H/L1/2 bits in the VGS_VDS_STAT register.
A key question arises: are these current sources powered by the DRAIN pin (requiring an active supply) or an internal source? Furthermore, does the diagnostic setup generate a higher voltage drop across the comparator inputs compared to normal operation? This higher voltage drop is alluded to in TI literature (Open Load Detection in Motor Drivers – slvae49a), suggesting a need for adjusted Vds comparator thresholds (1V or greater) to accommodate the internal blocking diode’s forward voltage drop.
How Comparators Detect Faults
The core question remains: how do these comparators actually detect open gate faults? If the Vds overcurrent protection (OCP) threshold is set above the diagnostic voltage (e.g., 1.4V or 2V), a fault-free scenario wouldn’t trigger a change in Vds. Similarly, a short-circuited FET would result in a 0V Vds, also not crossing the threshold. However, a fault to DRAIN or GND should theoretically trip a comparator.
Consider enabling PU_SH1. This action generates a voltage across FET H1 and at the SH1 and SH2 pins. Assuming correct operation, the L1 and L2 FETs should block this current due to their off state and reverse-biased body diodes. The same applies to H1 and H2 FETs.
Now, let’s introduce a fault: a short in FET H1 or a short to DRAIN from SH1. This would cause SH1 and SH2 to rise to the DRAIN voltage. While the H1 (and potentially H2) Vds comparator wouldn’t register a change, the L1 and L2 comparators should detect a threshold crossing.
Open Gate Detection: Possibilities and Limitations
The analysis suggests that while the comparators might not directly detect an open gate, they can indirectly infer its presence by monitoring voltage changes at various points in the circuit resulting from the open gate condition. For instance, an open gate in a high-side FET could be detected by the corresponding low-side comparator registering an unexpected voltage rise during the diagnostic test.
However, the datasheet lacks explicit details on how to interpret these indirect indications for definitive open gate detection. Further clarification from Texas Instruments regarding the off-line diagnostic procedure and specific open gate detection mechanisms would be beneficial. This includes details on current source power, diagnostic voltage levels, and expected comparator behavior under various fault conditions.
Conclusion: Further Investigation Needed
While the DRV8706-Q1 incorporates comprehensive diagnostic capabilities, the precise mechanism for detecting open gates in off-line scenarios remains somewhat ambiguous. The available documentation hints at indirect detection methods, relying on voltage changes monitored by the Vds comparators. However, more detailed information from the manufacturer is needed to fully understand and utilize the full potential of the DRV8706-Q1’s diagnostic features for robust open gate detection.
