MC78T00_04 Datasheet, PDF (9/12 Pages) ON Semiconductor – 3.0 A Positive 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 mF 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.
Input
MC78T05
0.33mF
R
Constant
Current to
I_O
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:
I0
+
5.0
R
V
)
IB
DI_B ^ 0.7 mA over line, load and Temperature changes
I_B ^ 3.5 mA
For example, a 2.0 A current source would require R to be a 2.5 W,
10 W resistor and the output voltage compliance would be the input
voltage less 7.0 V.
Figure 15. Current Regulator
MC78T05
Input
Output
0.33mF
1.0k
7
2
−
6
+3
4
MC1741
0.1mF
10k
V_O, 8.0 V to 20 V
V_in − V_O ≥ 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. Adjustable Output Regulator
2N4398 or Equiv
R
MC78TXX
Output
1.0mF
0.1mF
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 conjunction with the V_BE
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 V_BE of the pass transistor.
Figure 17. Current Boost Regulator
2N4398
Input
R_sc
or Equiv.
MJ2955
or Equiv.
R
1.0mF
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, R_SC, 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.
Figure 18. Current Boost With
Short Circuit Protection
http://onsemi.com
9