Hardware Reference
In-Depth Information
Figure 2-6.
Regulated battery supply
The 8.4 V battery is formed from seven NiCad cells in series, each producing 1.2 V.
The 8.4 V input allows the battery to drop to a low of 7 V before the minimum headroom
of 2 V is violated.
Depending on the exact 7805 regulator part chosen, a typical heat-sinked parameter
set might be as follows:
•
Input voltage
: 7-25 V
•
Output voltage
: 1.5 A (heat-sinked)
•
Operating temperature
: 125°C
Be sure to use a heat sink on the regulator so that it can dissipate heat energy to the
surrounding air. Without one, the regulator can enter a thermal shutdown state, reducing
current flow to prevent its own destruction. When this happens, the output voltage will
drop below +5 V.
Keep in mind that the amount of power dissipated by the battery is more than
that received by the load. If we assume that the Raspberry Pi is consuming 700 mA, a
minimum of 700 mA is also drawn from the battery through the regulator (and it could
be slightly higher). Realize that the regulator is dissipating additional energy because of
its higher input voltage. The total power dissipated by the regulator and the load is as
follows:
P
d
=
P
L
+
P
R
= 5 V × 0.7
A
+ (8.4
V
− 5
V
) × 0.7
A
= 3.5
W
+ 2.38
W
= 5.88
W
The regulator must dissipate the difference between the input and the output
voltages (2.38 W). This additional energy heats up the regulator with the energy being
given away at the heat sink. Because of this, designers avoid using a high input voltage on
linear regulator circuits.
