Another feature that designers can find useful is the ability to detect an open load or broken wire on the high side switch output. Doing this when the device is disabled is straightforward. Place a weak pullup from VBB to VOUT and measure the voltage drop over the pullup using an integrated comparator. If that voltage drop is less than 2V (that is, VBB – VOUT < 2V), then the device determines that there is an open-load fault. All TI high-side switches and eFuses have integrated open load detection using the scheme presented above.

However, this task becomes more difficult when trying to measure an open load when the device is enabled. Using the same circuit as the off-state open load detection is not an option because VOUT needs to always be close to VBB when the device is enabled. Instead, the load current can be measured, and if this is below a certain threshold, then the device can report an open load fault. A couple TI high-side switches employ this method, but this presents two issues. First, the current that is considered an open load is dictated by the application. In a typical application, the nominal load current is greater than 1A, and an output current of <20mA is considered an open load. However, if the nominal load current is less than 20mA, then the device is always reporting an open load fault, which is unacceptable. Second, accurately measuring low currents becomes exponentially more difficult as the FET ON resistance decreases. Thus, for open load detection when the device is enabled, the current sense functionality is used such that the designer can determine an open load current threshold and recognize a fault depending on the device current sense feedback.

Figure 2-2 High Side Switch Open Load OFF Detection Architecture

Figure 2-3 High Side Switch Current Sense Range Classification