MC7900 Series
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9
APPLICATIONS INFORMATION
Design Considerations
The MC7900 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 capacitor chosen
should have an equivalent series resistance of less than
0.7 W. The bypass capacitor should be mounted with the
shortest possible leads directly across the regulators 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. Bypassing the
output is also recommended.
Figure 8. Current Regulator
-20 V
Input
MC7905
10
R
I
O
= 200 mA
-
V
O
≤ 10 V
1.0 mF
Gnd Gnd
+
1.0 mF
+
The MC7905, −5.0 V regulator can be used as a constant
current source when connected as above. The output current
is the sum of resistor R current and quiescent bias current as
follows:
I
O
+
5.0 V
R
) I
B
The quiescent current for this regulator is typically 4.3
mA. The 5.0 V regulator was chosen to minimize dissipation
and to allow the output voltage to operate to within 6.0 V
below the input voltage.
Figure 9. Current Boost Regulator
(−5.0 V @ 4.0 A, with 5.0 A Current Limiting)
*Mounted on heatsink.
-10 V
Input
0.56
0.56
0.56
MJE200*
or Equiv
5.6
2N3055*
or Equiv
MC7905*
1.0 mF
-5.0 V
Output
Gnd Gnd
+
+
1.0 mF
10 mF
+
When a boost transistor is used, short circuit currents are
equal to the sum of the series pass and regulator limits, which
are measured at 3.2 A and 1.8 A respectively in this case.
Series pass limiting is approximately equal to 0.6 V/R
SC
.
Operation beyond this point to the peak current capability of
the MC7905C is possible if the regulator is mounted on a
heatsink; otherwise thermal shutdown will occur when the
additional load current is picked up by the regulator.
Figure 10. Operational Amplifier Supply
+20 V
Input
+15 V
Output
0.33 mF
1N4001G
or Equiv
Gnd
Gnd
1.0 mF
+
-20 V
Input
-15 V
Output
MC7815
MC7915
+
+
+
1.0 mF
1.0 mF
1.0 mF
1N4001G
or Equiv
Clamp diode
1N4001G or Equiv
1N4001G or Equiv
The MC7815 and MC7915 positive and negative
regulators may be connected as shown to obtain a dual
power supply for operational amplifiers. A clamp diode
should be used at the output of the MC7815 to prevent
potential latch−up problems whenever the output of the
positive regulator (MC7815) is drawn below ground with an
output current greater than 200 mA.
