3.3.1 Target Regulator

The target voltage regulator is a MIC5353 variable output LDO. The on-board debugger can adjust the voltage output supplied to the board target section by manipulating the MIC5353’s feedback voltage. The hardware implementation is limited to an approximate voltage range from 1.7V to 5.1V. Additional output voltage limits are configured in the debugger firmware to ensure that the output voltage never exceeds the hardware limits of the AVR128DA48 microcontroller. The voltage limits configured in the on-board debugger on AVR128DA48 Curiosity Nano are 1.8–5.5V.

Info: The target voltage is set to 3.3V when the board is manufactured. It can be changed through MPLAB® X IDE project properties and in the Atmel Studio 7 device programming dialog. Any change to the target voltage is persistent, even after a power toggle. The resolution is less than 5 mV but may be limited to 10 mV by the adjustment program.
Info: Voltage settings that are set up in MPLAB® X IDE are not immediately applied to the board. The new voltage setting is applied to the board when the debugger is accessed in any way, like pushing the Refresh Debug Tool Status button in the project dashboard tab, or programming/reading program memory.
Info: There is a simple option to adjust the target voltage with a drag-and-drop command text file to the board. This supports a set of common target voltages. See section Special Commands for further details.

The MIC5353 supports a maximum current load of 500 mA. It is an LDO regulator in a small package, placed on a small printed circuit board (PCB), and the thermal shutdown condition can be reached at lower loads than 500 mA. The maximum current load depends on the input voltage, the selected output voltage, and the ambient temperature. The figure below shows the safe operating area for the regulator, with an input voltage of 5.1V and an ambient temperature of 23°C.

Figure 3-5. Target Regulator Safe Operation Area

The voltage output of the target regulator is continuously monitored (measured) by the on-board debugger. If it is more than 100 mV over/under the set device voltage, an error condition will be flagged, and the target voltage regulator will be turned off. This will detect and handle any short-circuit conditions. It will also detect and handle if an external voltage, which causes VCC_TARGET to move outside of the voltage setting monitoring window of ±100 mV, is suddenly applied to the VTG pin, without setting the VOFF pin low.

Info: If the external voltage is lower than the monitoring window lower limit (target voltage setting – 100 mV), the on-board debugger status LED will blink rapidly. If the external voltage is higher than the monitoring window upper limit (target voltage setting + 100 mV), the on-board debugger status LED will continue to shine. If the external voltage is removed, the status LED will start to blink rapidly until the on-board debugger detects the new situation and turns the target voltage regulator back on.