LM340, A Series

16

MOTOROLA ANALOG IC DEVICE DATA

APPLICATIONS INFORMATION

Design Considerations

The LM340, A 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.

Figure 14. Current Regulator

Figure 15. Adjustable Output Regulator

Figure 16. Current Boost Regulator

Figure 17. Short Circuit Protection

5.0 V

R

Input

0.33

µF

LM340–5.0

R

IO

These regulators can also be used as a current source when

connected as above. In order to minimize dissipation the LM340–5.0

is chosen in this application. Resistor R determines the current as

follows:

IO =

+ IQ

IQ ^ 1.5 mA over line and load changes

For example, a 1 A current source would require R to be a 5

Ω,

10 W resistor and the output voltage compliance would be the input

voltage less 7.0 V.

Constant

Current to

Grounded Load

Input

Output

1k

4

6

7

2

3

–

+

0.1

µF

10k

MC1741G

Vout, 7.0 V to 20 V

Vin – VO ≥ 2.0 V

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.

0.33

µF

R

0.1

µF

Output

1.0

µF

The LM340, A series can be current boosted with a PNP transistor. The

MJ2955 provides current to 5.0 A. Resistor R in conjuction 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.

MJ2955 or Equiv

LM340

LM340

Input

RSC

MJ2955

or Equiv.

Output

R

2N6049

or Equiv.

1.0

µF

The circuit of Figure 17 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, 4.0 A plastic power transistor is specified.

LM340–5.0

Input