MC78T00 Datasheet, PDF (9/12 Pages) Motorola, Inc – THREE-AMPERE POSITIVE FIXED VOLTAGE REGULATORS

MC78T00 Series
APPLICATIONS INFORMATION
Design Considerations
The MC78T00 Series of fixed voltage regulators are
designed with Thermal Overload Protection that shuts down
the circuit when subjected to an excessive power overload
condition, Internal Short Circuit Protection that limits the
maximum current the circuit will pass, and Output Transistor
Safe–Area Compensation that reduces the output short
circuit current as the voltage across the pass transistor is
increased.
In many low current applications, compensation
capacitors are not required. However, it is recommended that
the regulator input be bypassed with a capacitor if the
regulator is connected to the power supply filter with long wire
lengths, or if the output load capacitance is large. An input
bypass capacitor should be selected to provide good high
frequency characteristics to insure stable operation under all
load conditions. A 0.33 µF or larger tantalum, mylar, or other
capacitor having low internal impedance at high frequencies
should be chosen. The bypass capacitor should be mounted
with the shortest possible leads directly across the regulator’s
input terminals. Normally good construction techniques
should be used to minimize ground loops and lead resistance
drops since the regulator has no external sense lead.
Figure 14. Current Regulator
Input
MC78T05
0.33µF
R
Constant
Current to
IO
Grounded Load
The MC78T05 regulator can also be used as a current source when
connected as above. In order to minimize dissipation the MC78T05 is
chosen in this application. Resistor R determines the current
as follows:
5.0 V
IO = _R + IB
^ ∆IB 0.7 mA over line, load and Temperature changes
^IB 3.5 mA
For example, a 2.0 A current source would require R to be a 2.5 Ω,
10 W resistor and the output voltage compliance would be the input
voltage less 7.0 V.
Figure 15. Adjustable Output Regulator
MC78T05
Input
Output
0.33µF
1.0k
7
2
–
6
+3
4
MC1741
0.1µF
10k
VO, 8.0 V to 20 V
Vin – VO ≥ 2.5 V
The addition of an operational amplifier allows adjustment to higher or
intermediate values while retaining regulation characteristics. The
minimum voltage obtainable with this arrangement is 3.0 V greater
than the regulator voltage.
Figure 16. Current Boost Regulator
2N4398 or Equiv
R
MC78TXX
Output
1.0µF
0.1µF
XX = 2 digits of type number indicating voltage.
The MC78T00 series can be current boosted with a PNP transistor. The
2N4398 provides current to 15 A. Resistor R in conjuction with the VBE of
the PNP determines when the pass transistor begins conducting; this
circuit is not short circuit proof. Input–output differential voltage
minimum is increased by the VBE of the pass transistor.
Figure 17. Current Boost With
Short Circuit Protection
2N4398
Input
Rsc
or Equiv.
MJ2955
or Equiv.
R
1.0µF
MC78TXX
Output
XX = 2 digits of type number indicating voltage.
The circuit of Figure 17 can be modified to provide supply protection
against short circuits by adding a short circuit sense resistor, RSC, and
an additional PNP transistor. The current sensing PNP must be able to
handle the short circuit current of the three–terminal regulator.
Therefore, an eight–ampere power transistor is specified.
MOTOROLA ANALOG IC DEVICE DATA
9