Hardware Reference
In-Depth Information
Figure 2-3.
3.3 V power
Model A Input Power
Like the Model B, the Model A receives its power from the micro-USB port. The Model
A power requirement is 300 mA, which is easily supported by a powered USB hub or
desktop USB 2 port. A USB 2 port is typically able to supply a maximum of 500 mA unless
the power is divided among neighboring ports. You may find in practice, however, that
not all USB ports will deliver 500 mA.
As with the Model B, factor the power required by your USB peripherals. If your total
nears or exceeds 500 mA, you may need to power your Model A from a separate power
source. Don't try to run a wireless USB adapter from the Model A's USB port if the Pi is
powered by a USB port itself. The total current needed by the Pi and wireless adapter
will likely exceed 500 mA. Supply the wireless adapter power from a USB hub, or power
the Pi from a 1.2 A or better power source. Also be aware that not all USB hubs function
correctly under Linux, so check compatibility if you're buying one for that purpose.
3.3 Volt Power
Since the 3.3 V supply appears at P1-01, P1-17, and P5-02, it is useful to examine Figure
2-3
(shown previously) to note its source. This supply is indirectly derived from the input 5 V
supply, passing through regulator RG2. The maximum excess current that can be drawn
from it is 50 mA; the Raspberry Pi uses up the remaining capacity of this regulator.
When planning a design, you need to budget this 3 V supply carefully. Each GPIO
output pin draws from this power source an additional 3 to 16 mA, depending on how it is
used. For more information about this, see Chapter 10.
Powered USB Hubs
If your power budget is stretched by USB peripherals, you may want to consider the use
of a
powered
USB hub. In this way, the
hub
rather than your Raspberry Pi provides the
necessary power to the downstream peripherals. The hub is especially attractive for the
Model A because it provides additional ports.
