MC78M00, MC78M00A Series
APPLICATIONS INFORMATION
Design Considerations
The MC78M00/MC78M00A 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
Input
0.33
mF
I
O
MC78M05C
R
Constant
Current to
Grounded
Load
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.
V
O
, 7.0 V to 20 V
V
in
−V
out
≥
2.0 V
Output
Input
MC78M05C
7
The MC78M00 regulators can also be used as a current source
when connected as above. In order to minimize dissipation the
MC78M05C is chosen in this application. Resistor R determines
the current as follows:
5.0 V
+ I
IB
I
O
=
R
I
IB
= 1.5 mA over line and load changes.
For example, a 500 mA current source would require R to be a
10
W,
10 W resistor and the output voltage compliance would be
the input voltage less 7.0 V.
0.33
mF
1.0 k
6
4
*
+
2
0.1
mF
3
10 k
MC1741
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 2.0 V
greater than the regulator voltage.
Figure 10. Current Regulator
Figure 11. Adjustable Output Regulator
MJ2955 or Equiv.
Input
Input
R
SC
MJ2955
or Equiv.
R
MC78MXXC
Output
R
2N6049
or Equiv.
MC78MXXC
1.0
mF
Output
1.0
mF
1.0
mF
XX = 2 digits of type number indicating voltage.
XX = 2 digits of type number indicating voltage.
The MC78M00 series can be current boosted with a PNP
transistor. The MJ2955 provides current to 5.0 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 V
BE
of the pass transistor.
The circuit of Figure 12 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, a 4.0 A plastic power transistor is specified.
Figure 12. Current Boost Regulator
Figure 13. Current Boost with
Short Circuit Protection
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