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LM150K

THREE-TERMINAL 3 A ADJUSTABLE VOLTAGE REGULATORS

STMicroelectronics 

LM150-LM250-LM350

TYPICAL APPLICATIONS

+ 1.2V to + 25V ADJUSTABLE REGULATOR

LM350

∆ Needed if device is far from filter capacitors.

* Optional-improves transient response. Output capacitors in the

range of 1µF to 100µF of aluminium or tantalum electrolytic are

commonly used to provide improved output impedance and rejec-

tion of transients.

R2

* * VO = 1.25V (1 + )

R1

* * * R1 = 240Ω for LM150 and LM250

APPLICATION HINTS

In operation, the LM350 develops a nominal 1.25V

reference voltage, V(ref), between the output and

adjustment terminal. The reference voltage is im-

pressed across program resistor R1 and, since the

voltage is constant, a constant current I1 then flows

through the output set resistor R2, giving an output

voltage of

VO = V(ref) ( 1+RR21 ) + IadjR2

Figure 1.

LM350

Since the 50µA current from the adjustment termi-

nal represents an error term, the LM350 was de-

signed to minimize Iadj and make it very constant

with line and load changes. To do this, all quiescent

operating current is returned to the output establi-

shing a minimum load current requirement. If there

is insufficient load on the output, the output will rise.

EXTERNAL CAPACITORS

An input bypass capacitor is recommended. A

0.1µF disc or 1µF solid tantalum on the input is suit-

able input by passing for almost all applications.

The device is more sensitive to the absence of input

bypassing when adjustment or output capacitors

are used byt the above values will eliminate the

possibility of problems.

The adjustment terminal can be bypassed to

ground on the LM350 to improve ripple rejection.

This bypass capacitor prevents ripple form being

amplified as the output voltage is increased. With

a 10µF bypass capacitor 75dB ripple rejection is

obtainable at any output level. Increases over 20µF

do not appreciably improve the ripple rejection at

frequencies above 120Hz. If the bypass capacitor

is used, it is sometimes necessary to include pro-

tection diodes to prevent the capacitor from dis-

charging through internal low current paths and

damaging the device.

In general, the best type of capacitors to use are

solid tantalum. Solid tantalum capacitors have low

impedance even at high frequencies. Depending

upon capacitor construction, it takes about 25µF in

aluminum electrolytic to equal 1µF solid tantalum at

high frequencies. Ceramic capacitors are also

good at high frequencies, but some types have a

large decrease in capacitance at frequencies

around 0.5MHz. For this reason, 0.01µF disc may

seem to work better than a 0.1µF disc as a bypass.

Although the LM350 is stable with no output capa-

citors, like any feedback circuit, certain values of

external capacitance can cause excessive ringing.

This occurs with values between 500pF and

5000pF. A 1µF solid tantalum (or 25µF aluminium

electrolytic) on the output swamps this effect and

insures stability.

LOAD REGULATION

The LM350 is capable of providing extremely good

load regulation but a few precautions are needed

to obtain maximum performance. The current set

resistor connected between the adjustment termi-

nal and the output terminal (usually 240Ω) should

be tied directly to the output of the regulator rather

than near the load. This eliminates line drops from

appearing effectively in series with the reference

and degrading regulation. For example, a 15V

regulator with 0.05Ω resistance between the regu-

lator and load will have a load regulation due to line

resistance of 0.05Ω x IL. If the set resistor is con-

nected near the load the effective line resistance

4/7

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