5.4. A-Star 32U4 Prime SV regulator

The A-Star 32U4 Prime SV can be powered from a 5 V to 36 V external source. The input voltage is regulated to 5 V by a 1 A ISL85410/ISL854102 switching step-down (buck) converter from Intersil. (We also make a standalone regulator based on this integrated circuit.)

As shown in the left graph below, the SV’s switching regulator has an efficiency – defined as (Power out)/(Power in) – of 80% to 95% for most combinations of input voltage and load.

The A-Star’s components, including the microcontroller and LEDs, draw 30 mA to 40 mA in typical applications (without the buzzer, microSD card, or an LCD). The rest of the regulator’s achievable output current, which depends on input voltage as well as ambient conditions, can be used to power other devices. The right graph above shows the output currents where the regulator’s output voltage drops below 4.75 V. These currents are close to the limits of the regulator’s capability and generally cannot be sustained for long periods; under typical operating conditions, a safe limit for the maximum continuous regulator output current is approximately 1 A.

The dropout voltage of a step-down regulator is defined as the minimum amount by which the input voltage much exceed the regulator’s target output voltage in order to assure the target output can be achieved. As can be seen in the graph below, the dropout voltage of the Prime SV’s regulator increases approximately linearly with the output current. For light loads where the dropout voltage is small, the board can operate down to 5 V. However, for larger loads, the dropout voltage should be taken into consideration when selecting a power supply; operating above 6 V will ensure the full output current is available.

Typical dropout voltage of the 5 V regulator on the A-Star 32U4 Prime SV.

Note: Although the ISL85410/ISL854102 is rated for a maximum operating input voltage of 36 V, it is not appropriate to power the Prime SV with a 36 V battery, as battery voltages can be much higher than nominal voltages when they are charged. The maximum nominal battery voltage we recommend is 24 V, and if you approach that limit, you should take extra precautions to prevent LC voltage spikes from damaging the board (see this application note for more information). A good practice is to ensure that the A-Star’s power switch is off before connecting it to a voltage source.