Adding open-load fault detection to the DRV103 PWM driver

A simple circuit adds a relevant status-reporting bit to the device’s existing already-sizeable capabilities suite.

As described in its datasheet (PDF), the DRV103 is a (mostly) complete, robust, versatile, and efficient solution for solenoid and other power driver applications. However, there’s one trick missing from its sleeve. It lacks the ability to detect and report open-load faults in which the driven load has come adrift or is missing its connection to the power source, or the coil winding has come open or has otherwise lost continuity.

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The 103 falls out of its shrink wrap with built-in over-current and over-temperature detection and protection already in place. The logic-level OK-status flag output goes to zero when an energize command is issued and such an error occurs. So, this variety of mishap and mayhem is already adequately covered. No need for rework there.

But obviously, if the driver attempts to energize the load when the load’s connections are broken, nothing useful will happen. And, in an unmodified 103, neither before nor after the failed attempt will any diagnostic error indication be sent to the controller, which will therefore happily but erroneously assume its orders are continuing to be obeyed. This might not be so good, particularly if the application is critical. And at the end of the day, aren’t they all?

Figure 1 shows a (reasonably) simple modification to address this shortcoming.

Figure 1 The absence of V+ at OUT when Input = 0 causes “wired-or” Status OK bit to be pulled low by Q2, indicating the open-load error condition. A connected GPI pin should be configured for passive pullup.

Here’s how it works.

When Input is pulled low by the controller, typically a µC general-purpose (GP) output bit, Q3 turns off, releasing Q1’s collector. Since Input being low also turns the DRV104 internal transistors off, the driver’s OUT pin will now float high or low depending on whether (or not) an intact connection to V+ is established through a properly connected load. This will turn Q1 on (or off), and Q2’s base along with it.

If a good V+ connection exists, Q1 will now turn on, turning Q2 off, releasing Status OK and causing a connected GP input bit to go true, sharing the good news with the controller.

Otherwise not.

Stephen Woodward‘s relationship with EDN’s DI column goes back quite a long way. Over 200 submissions have been accepted since his first contribution back in 1974.  They have included best Design Idea of the year in 1974 and 2001.

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