MC78M00 Datasheet, PDF (9/12 Pages) Motorola, Inc – THREE-TERMINAL MEDIUM CURRENT POSITIVE FIXED VOLTAGE REGULATORS

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
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 9. Current Regulator
Input
0.33 µF
MC78M05C
R _Constant
Current to
IO
Grounded
Load
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:
IO =
5.0 V
R
+ IIB
IIB = 1.5 mA over line and load changes.
For example, a 500 mA current source would require R to be a
5.0 Ω, 10 W resistor and the output voltage compliance would
be the input voltage less 7.0 V.
Figure 10. Adjustable Output Regulator
MC78M05C
Input
VO, 7.0 V to 20 V
Vin–Vout ≥ 2.0 V
Output
0.33 µF
1.0 k
* ⁷
2
6
3
+
4
MC1741
0.1 µF
10 k
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 11. Current Boost Regulator
MJ2955 or Equiv.
Input
R
MC78MXXC
Output
1.0 µF
1.0 µF
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 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 VBE of the pass transistor.
MOTOROLA ANALOG IC DEVICE DATA
Figure 12. Current Boost with
Short Circuit Protection
MJ2955
Input
RSC ^{or Equiv.}
2N6049
or Equiv.
R
MC78MXXC
1.0 µF
Output
XX = 2 digits of type number indicating voltage.
The circuit of Figure NO TAG 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, a 4.0 A plastic power transistor
is specified.
9