MC78T05C ON, MC78T05C Datasheet - Page 9

• Manufacturer Part Number: MC78T05C
• Description: THREE-AMPERE POSITIVE FIXED VOLTAGE REGULATORS
• Manufacturer: ON

Design Considerations
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.
capacitors are not required. However, it is recommended
that the regulator input be bypassed with a capacitor if the
The MC78T00 Series of fixed voltage regulators are
In many low current applications, compensation
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:
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.
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
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
Input
0.33mF
DI
I
B
B
^ 0.7 mA over line, load and Temperature changes
^ 3.5 mA
2N4398 or Equiv
R
Figure 17. Current Boost Regulator
1.0mF
Figure 15. Current Regulator
MC78T05
BE
MC78TXX
I 0 + 5.0 V
of the pass transistor.
I
R
O
) I B
R
Grounded Load
Current to
Constant
APPLICATIONS INFORMATION
0.1mF
Output
MC78T00 Series
BE
http://onsemi.com
9
Input
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
V
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.
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
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.
O
in
, 8.0 V to 20 V
− V
0.33mF
Figure 16. Adjustable Output Regulator
O
R
1.0k
2.5 V
MC78T05
Figure 18. Current Boost With
or Equiv.
MJ2955
Short Circuit Protection
R
1.0mF
sc
6
or Equiv.
2N4398
7
4
MC1741
−
+
2
3
MC78TXX
10k
SC
Output
, and
Output
0.1mF