Powering the A-Star - 5.0v + USB


Further to superlative’s post, I would like to power the A-Star micro with a 5.0v regulated supply on the VCC line, but also be able to sometimes connect the USB cable to receive commands from the PC, or to reprogram it. The USB is not always connected, so the device must also be able to be powered from the 5.0 supply.

The A-Star is controlling a set of 16 addressable LEDs, so I need 800ma (max) to power the LEDs.These LEDs don’t like more than 5.0v, so my idea is that I power the board + the LEDs from a 5.0v 1A regulated supply. However, from the schematic, it looks like if VIN is not connected, then the USB power (VBUSF) will also be connected to VCC.

(Note: The docs state “The board features a power selection circuit that allows both USB and VIN to be connected at the same time; if this is done, the A-Star will draw power from USB.” and also “It will draw power from VIN only if USB is disconnected.”; but looking at the schematic, I think the P-Channel MOSFET will switch OFF when VIN > VBUSF, and so the board will be powered from the onboard regulator if VIN and VBUS are both connected. Am I correct?)

Is it safe to do this, or is there some way or other components I can use to ensure the board is never powered from the USB port?
(I don’t really want to use a >5v supply plus a hefty 1A regulator for the LEDs.)

Hello, mkeeley.

You are right that the regulator is the preferred power source when both VIN and VBUS are connected. We’ve updated the user’s guide to fix the error; thanks for pointing it out.

I think the best way to safely connect USB and an external 5V supply at the same time is to remove the connection between VBUS and the 5V line. This means that the A-Star can no longer be powered from USB, so it must be powered through the external supply even when USB is connected, but it eliminates any risk of shorting two different power supplies together.

You can remove the connection by desoldering either the fuse or the MOSFET (highlighted in yellow in the diagram below) or by cutting a connection to either of those components (on the red lines in the diagram). The fuse is the easier component to remove or cut, but doing so makes the ATmega32U4 unable to detect whether USB power is present through its VBus pin (it will assume that the USB connection is always active). If USB presence detection is important to your application, you should try removing or cutting the MOSFET instead, which should keep VBUS connected to the sense input while disconnecting it from the rest of the power circuit.

- Kevin