Hardware Reference
In-Depth Information
The right side of Figure
2-5
shows the positive DMM probe moved to P1-01 to
measure the +3.3 V pin. Appendix B lists the ATX power supply standard voltage levels,
which include +5 ± 0.25 V and +3.3 ± 0.165 V.
Battery Power
Because of the small size of the Raspberry Pi, it may be desirable to run it from battery
power. Doing so requires a regulator and some careful planning. To meet the Raspberry
Pi requirements, you must form a power budget. Once you know your maximum current,
you can flesh out the rest. The following example assumes that 1 A is required.
Requirements
For clarity, let's list our battery power requirements:
•
Voltage
5 V, within ± 0.25 V
•
Current
1 A
Headroom
The simplest approach is to use the linear LM7805 as the 5 V regulator. But there are some
disadvantages:
•
There must be some headroom above the input voltage
(about 2 V).
•
Allowing too much headroom increases the power dissipation in
the regulator, resulting in wasted battery power.
•
A lower maximum output current can also result.
Your batteries should provide a minimum input of 5+2 V (7 V). Any lower input
voltage to the regulator will result in the regulator “dropping out” and dipping below 5 V.
Clearly, a 6 V battery input will not do.
LM7805 Regulation
Figure
2-6
shows a very simple battery circuit using the LM7805 linear regulator. Resistor
R
L
represents the load (the Raspberry Pi).
