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LM350

3.0 A, Adjustable Output, Positive Voltage Regulator

ON Semiconductor 

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 19), and

this constant current flows through R2 to ground. The

regulated output voltage is given by:

Vout = Vref (1 + RR21) + IAdj R2

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 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.

Vin

LM350

Adjust

Vout

Vref

+

R1

IPROG

Vout

IAdj

R2

Vref = 1.25 V Typical

Figure 19. Basic Circuit Configuration

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.

External Capacitors

A 0.1 mF disc or 1 mF 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 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 (CO) 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 (CO > 25 mF, CAdj > 10 mF). 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.

D1

1N4002

Vin

Vout

LM350

+

Cin

R1

D2

CO

Adjust

1N4002

R2

CAdj

Figure 20. Voltage Regulator with

Protection Diodes

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