The pins VEXMON, VEXCTRL, and VEXCC are all pins that lead to the integrated LDO controller of the TCAN28XX family. With the addition of a shunt resistor and PNP transistor with a β value between 50 and 500 an additional and adjustable LDO output is available. Connection for these pins is simple with a shunt resistance between VSUP and VEXMON and a PNP with the emitter pin connected to VEXMON, base pin on VEXCTRL, and collector pin on VEXCC can setup a 3rd LDO output for the device.

The controller supports output options of 1.8V, 2.5V, 3.3V, or 5V up to a current of 350mA while still supporting fault detection on this block. The fault detection is very similar to VCC2 in that faults on VEXCC does not cause the SBC to switch modes but only impact VEXCC block. This external LDO has the additional benefit of being able to load share with VCC1 allowing up to 700mA of output current for 3.3V or 5V (depending on TCAN28XX device chosen). These options are all configurable through the internal registers that are accessible through four-wire SPI communication.

The only value that can be a bit confusing to size is that of the shunt resistor between VSUP and VEXMON as this resistor has different purposes depending on usage of the VEXCC module.

If VEXCC is a stand-alone output then this resistance sets the current limit. The voltage drop between VSUP and VEXMON is controlled and specified to be between 150mV and 440mV regardless of VSUP or output configuration on VEXCC. The current produced with the voltage drop between VSUP and VEXMON – which following Ohm’s law is 150mV to 440mV divided by the chosen shunt resistance – is the current limit of the VEXCC output. This can cause some confusion as the wide shunt voltage range leads to wide current limit ranges. To help simplify the 150mV value needs to be used as the point where under normal circumstances the load can never draw 150mV/R_shunt. The 440mV range needs to be used to tell you the max current limit – sometimes the max current limit is above the 350mA of maximum regulated output current– which means that current limiting does not initiate before and undervoltage or short circuit fault occurs. For example – if the application requires 150mA of current during operation this can be a good idea to set 160mA as the lowest possible current limit – which can just be 150mV/160mA which can be around 937.5mΩ as the shunt resistor. However, the lowest current that can trigger a current limit is 160mA – but the largest can be 440mV/937.5mΩ which is around 470mA. That means in this example the current limit can engage as soon as 160mA is reached, but this does not kick in until 470mA which means the device can experience an UV or short-circuit fault before the current is limited.

If VEXCC is being used in a load-sharing application the shunt resistor is used to set the ratio of current between the VEXCC and VCC1 outputs (for example, IEXCC/ICC1). This type of application expects higher total currents as low load currents, roughly less than 100mA, result in VEXCC not outputting current regardless of shunt resistance chosen. If the application load is less than 100mA is suggested not to use load sharing as the VEXCC generally cannot be outputting much – if any current at lower load currents. If the load current is larger the shunt resistance is picked through the following equations:

In systems where the external LDO is not used the VEXMON pin needs to be shorted directly to VEXMON.