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
µ
F
LM340–5.0
R
I
O
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:
I
O
=
+ I
Q
I
Q
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
µ
F
10k
MC1741G
V
out
, 7.0 V to 20 V
V
in
– V
O
≥
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
µ
F
R
0.1
µ
F
Output
1.0
µ
F
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 V
BE
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 V
BE
of the pass transistor.
MJ2955 or Equiv
LM340
LM340
Input R
SC
MJ2955
or Equiv.
Output
R
2N6049
or Equiv.
1.0
µ
F
The circuit of Figure 17 can be modified to provide supply protection
against short circuits by adding a short circuit sense resistor, R
SC
, 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
