LM350

7

MOTOROLA ANALOG IC DEVICE DATA

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:

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.

+

Vref

Adjust

Vin

Vout

LM350

R1

IPROG

Vout

R2

IAdj

Vref = 1.25 V Typical

Figure 17. 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

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.

D1

Vin

Cin

1N4002

LM350

Vout

R1

+

CO

D2

R2

CAdj

1N4002

Adjust

Figure 18. Voltage Regulator with

Protection Diodes