LM350
APPLICATIONS INFORMATION
Basic Circuit Operation
The LM350 is a three–terminal floating regulator. In
operation, the LM350 develops and maintains a nominal
1.25 V reference (Vref) between its output and adjustment
terminals. This reference voltage is converted to a
programming current (IPROG) by R1 (see Figure 17), and this
constant current flows through R2 to ground. The regulated
output voltage is given by:
External Capacitors
A 0.1
µF
disc or 1
µF
tantalum input bypass capacitor (Cin)
is recommended to reduce the sensitivity to input line
impedance.
The adjustment terminal may be bypassed to ground to
improve ripple rejection. This capacitor (CAdj) prevents ripple
from being amplified as the output voltage is increased. A
10
µF
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 (CO) in the form of a 1
µF
tantalum or 25
µF
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 (CO > 25
µF,
CAdj > 10
µF).
Diode D1
prevents CO from discharging thru the IC during an input
short circuit. Diode D2 protects against capacitor CAdj
discharging through the IC during an output short circuit. The
combination of diodes D1 and D2 prevents CAdj from
discharging through the IC during an input short circuit.
R2
Vout = Vref (1 +
) + IAdj R2
R1
Since the current from the terminal (IAdj) represents an
error term in the equation, the LM350 was designed to control
IAdj to less than 100
µA
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.
Figure 17. Basic Circuit Configuration
Vin
Vout
+
R1
Adjust
Vref
IPROG
Vout
IAdj
R2
LM350
Figure 18. Voltage Regulator with
Protection Diodes
D1
1N4002
Vref = 1.25 V Typical
Vin
LM350
Vout
+
Load Regulation
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 (R1) 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 R2 can be returned
near the load ground to provide remote ground sensing and
improve load regulation.
Cin
Adjust
R2
R1
D2
CO
1N4002
CAdj
MOTOROLA ANALOG IC DEVICE DATA
7
MOTOROLA [ MOTOROLA, INC ]
