One of our products uses a standard 1/10-W surface-mount resistor with a high-side current monitor to detect small load changes at the product’s output. Since this circuit is part of the device’s output, it’s vulnerable to field wiring mishaps.
If the field installation results in a shorted output, the resistor, which can’t dissipate much power, will be destroyed. Protecting the resistor with a PTC (positive temperature coefficient) device is difficult because of the product’s wide voltage range, and a fuse is unacceptable because the product is potted.
Our solution was to use a self-protected high-side output driver. We fed the current from the monitoring resistor into the VBB input of the driver and used the driver’s output to provide the product’s output to the outside world (see the figure 1).
|Figure 1.||To protect the current-monitor resistor from a field-wiring error that could cause a short circuit, this circuit
feeds the current from the resistor to a self-protected high-side driver’s VBB input.
Although the current being monitored will now include the small amount of current drawn by the driver itself, in our application that current is inconsequential compared to the output load current, and it may safely be ignored.
While not the typical use of a high-side driver, this approach provides two advantages. First, the driver’s built-in short-circuit protection prevents field wiring errors that cause a shorted output from damaging our product. Also, since the current monitor’s analog output is seen by the product’s control circuitry, that circuitry can also control the enable input of the driver.
Consequently, the driver can be turned off in a situation where the output current might not be high enough to trigger the driver’s internal short-circuit shutdown protection. However, the current would still be high enough to damage the monitoring resistor. A pull-down resistor at the driver’s enable input ensures that the driver remains off until the control circuit wakes up.