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).
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.
