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

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.

The quiescent current for this regulator is typically 4.3 mA. The 5.0V regulator was

chosen to minimize dissipation and to allow the output voltage to operate to within

6.0 V below the input voltage.

5.0 V

−20 V

Input

MC7905

10

R

−

1.0

mF

Gnd

Gnd

+

1.0

mF

+

R

Figure 9. Current Boost Regulator

(− 5.0 V @ 4.0 A, with 5.0 A Current Limiting)

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

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.

*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

+

Figure 10. Operational Amplifier Supply

(

±15 @ 1.0 A)

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.

+20 V

Input

+15 V

Output

0.33

mF

1N4001

or Equiv

Gnd

Gnd

1.0

mF

+

−20 V

Input

−15 V

Output

MC7815

MC7915

+

+

+

1.0

mF

1.0

mF

1.0

mF