L7809AB datasheet(14/17 Pages) STMICROELECTRONICS

14 / 17 page

APPLICATIONS INFORMATION

DESIGN CONSIDERATIONS

The L7800A 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

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

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.

Figure 4 : Current Regulator.

Figure 5 : Adjustable Output Regulator.

Figure 6 : Current Boost Regulator.

Figure 7 : Short-circuit Protection.

VO, 7.0V to 20V

Vi –VO

≥ 2.0V

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 2.0V

greater than the regulator voltage.

The circuit of figure 6 can be modified to provide

supply protection against short circuit 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,

four-ampere plastic power transistor is specified.

R1 =

V BEQ1

I REQ

−

I Q1

β Q

IO =

I REG

+ Q 1 (I REG

−

V BEQ1

R 1

)

IO =

V XX

R 1

+ I d

L7800AB/AC

14/17

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