LM350
APPLICATIONS INFORMATION
Basic Circuit Operation
External Capacitors
The LM350 is a three−terminal floating regulator. In
operation, the LM350 develops and maintains a nominal
1.25 V reference (V
ref
) between its output and adjustment
terminals. This reference voltage is converted to a
programming current (I
PROG
) by R
1
(see Figure 19), and
this constant current flows through R
2
to ground. The
regulated output voltage is given by:
V
out
= V
ref
(1 +
R
2
) + I
Adj
R
2
R
1
Since the current from the terminal (I
Adj
) represents an
error term in the equation, the LM350 was designed to
control I
Adj
to less than 100
mA
and keep it constant. To do
this, all quiescent operating current is returned to the output
terminal. This imposes the requirement for a minimum
load current. If the load current is less than this minimum,
the output voltage will rise.
Since the LM350 is a floating regulator, it is only the
voltage differential across the circuit which is important to
performance, and operation at high voltages with respect to
ground is possible.
V
in
LM350
V
out
+
R
1
Adjust
V
ref
I
PROG
V
out
I
Adj
R
2
A 0.1
mF
disc or 1
mF
tantalum input bypass capacitor
(C
in
) is recommended to reduce the sensitivity to input line
impedance.
The adjustment terminal may be bypassed to ground to
improve ripple rejection. This capacitor (C
Adj
) prevents
ripple from being amplified as the output voltage is
increased. A 10
mF
capacitor should improve ripple
rejection about 15 dB at 120 Hz in a 10 V application.
Although the LM350 is stable with no output
capacitance, like any feedback circuit, certain values of
external capacitance can cause excessive ringing. An
output capacitance (C
O
) in the form of a 1
mF
tantalum or
25
mF
aluminum electrolytic capacitor on the output
swamps this effect and insures stability.
Protection Diodes
When external capacitors are used with any IC regulator,
it is sometimes necessary to add protection diodes to
prevent the capacitors from discharging through low
current points into the regulator.
Figure 18 shows the LM350 with the recommended
protection diodes for output voltages in excess of 25 V or
high capacitance values (C
O
> 25
mF,
C
Adj
> 10
mF).
Diode
D
1
prevents C
O
from discharging thru the IC during an
input short circuit. Diode D
2
protects against capacitor
C
Adj
discharging through the IC during an output short
circuit. The combination of diodes D
1
and D
2
prevents C
Adj
from discharging through the IC during an input short
circuit.
D
1
1N4002
V
in
LM350
C
in
Adjust
R
2
C
Adj
V
out
+
V
ref
= 1.25 V Typical
Figure 19. Basic Circuit Configuration
Load Regulation
R
1
The LM350 is capable of providing extremely good load
regulation, but a few precautions are needed to obtain
maximum performance. For best performance, the
programming resistor (R
1
) should be connected as close to
the regulator as possible to minimize line drops which
effectively appear in series with the reference, thereby
degrading regulation. The ground end of R
2
can be returned
near the load ground to provide remote ground sensing and
improve load regulation.
D
2
C
O
1N4002
Figure 20. Voltage Regulator with
Protection Diodes
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