TYPICAL APPLICATIONS

+ 1.2V to + 25V ADJUSTABLE REGULATOR

APPLICATION HINTS

In operation, the LM338 develops a nominal 1.25V

adjustment terminal. The reference voltage is im-

pressed across program resistor R1 and, since the

through the output set resistor R2, giving an output

voltage of

Figure 1.

Since the 50

µA current from the adjustment termi-

nal represents an error term, the LM338 was de-

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

0.1

µF 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 LM338 to improve ripple rejection.

This bypass capacitor prevents ripple form being

amplified as the output voltage is increased. With

a10

µF bypass capacitor 75dB ripple rejection is

obtainable at any output level. Increases over 20

µF

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 25

µFin

aluminum electrolytic to equal 1

µF 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, 0.01

µF disc may

seem to work better than a 0.1

µF disc as a bypass.

Although the LM338 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 1

µF solid tantalum (or 25µF aluminium

electrolytic) on the output swamps this effect and

insures stability.

LOAD REGULATION

The LM338 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 240

Ω) 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 0.05

Ω resistance between the regu-

lator and load will have a load regulation due to line

resistance of 0.05

nected near the load the effective line resistance

∆ Needed if device is far from filter capacitors.

* Optional-improves transient response. Output capacitors in the

range of 1

µF to 100µF of aluminium or tantalum electrolytic are

commonly used to provide improved output impedance and rejec-

tion of tran-sients.

R2

)

R1

*** R1 =240

Ω for LM138 and LM238

LM138-LM238-LM338

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