MC7800, MC7800A, LM340, LM340A Series

14

MOTOROLA ANALOG IC DEVICE DATA

APPLICATIONS INFORMATION

Design Considerations

The MC7800 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 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.

I

O +

5.0 V

R

) I

B

Figure 7. Current Regulator

Figure 8. Adjustable Output Regulator

Figure 9. Current Boost Regulator

Figure 10. Short Circuit Protection

The MC7800 regulators can also be used as a current source when

connected as above. In order to minimize dissipation the MC7805C is

chosen in this application. Resistor R determines the current as follows:

For example, a 1.0 A 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.

IB ^ 3.2 mA over line and load changes.

Input

0.33

µF

R

IO

MC7805

Constant

Current to

Grounded

Load

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.

Input

MC7805

Output

0.33

µF

10 k

MC1741G

7

6

4

1.0 k

VO = 7.0 V to 20 V

VIN = VO ≥ 2.0 V

0.1

µF

3

2

The MC7800 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.

XX = 2 digits of type number indicating voltage.

MC78XX

Input

Output

R

1.0

µF

MJ2955 or Equiv.

1.0

µF

The circuit of Figure 9 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

four–ampere plastic power transistor is specified.

XX = 2 digits of type number indicating voltage.

1.0

µF

MC78XX

MJ2955

or Equiv.

Output

RSC

R

2N6049

or Equiv.

≥ 10 µF

RSource

0.33

µF

Input

RSource

0.33

µF

≥ 10 µF