Page 286 - Op Amps Design, Applications, and Troubleshooting

268 POWER SUPPLY CIRCUITS

Although there can be a higher load current under higher input voltage con-
ditions, the computed value of 86 milliamperes is the highest load current that we
can supply under all input voltage conditions and still expect the circuit to remain
in a regulated condition. Ohm's Law can be used to determine the minimum-
value load resistor that can be used with the circuit. That is,

Transistor Q } must be able to safely conduct the difference between the highest
possible input current and the minimum possible load current. For many applica-
tions, we assume that the load can be disconnected and we therefore consider the
minimum load current to be 0. If the regulator were an integral part of a system
that made it impossible for the load to be disconnected (e.g., all part of the same
printed circuit board), then the minimum load current could be greater than 0. For
purposes of this analysis, we assume a worst-case situation, which means that Qi
must be able to handle a value of current given by Equation (6.15),

For the circuit shown in Figure 6.12, the maximum transistor current can be calcu-
1 4 1

We can estimate the worst-case power dissipation in Q^ by applying our bask
power formula.

For our present circuit, the transistor dissipation is estimated as
P D(max) = 126 mA x 9.7 V = 1.22 W

For many applications, the heat dissipated in the transistor will require a heat sink
to keep the transistor within its safe operating range.
The shunt-regulator circuit is inherently current limited. That is, if we try to
draw more current than it is designed to deliver, the output voltage will drop.
Even if we short the output directly to ground, the current will be limited by resis-
tor R 3. If resistor jR 3 has a sufficiently high power rating, then the duration of the
short can be any length. If it has a lower power rating, then the regulator is still
short-circuit proof, but the duration of the overload must be less. If the output of