LM138, LM338

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SNVS771B – MAY 1998 – REVISED APRIL 2013

APPLICATION HINTS

terminal. The reference voltage is impressed across program resistor R1 and, since the voltage is constant, a

(1)

Since the 50

μA current from the adjustment terminal represents an error term, the LM138 was designed to

returned to the output establishing 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 1 μF solid tantalum on the input is suitable input

bypassing for almost all applications. The device is more sensitive to the absence of input bypassiing when

adjustment or output capacitors are used but the above values will eliminate the possiblity of problems.

The adjustment terminal can be bypassed to ground on the LM138 to improve ripple rejection. This bypass

capacitor prevents ripple from being amplified as the output voltage is increased. With a 10

μF bypass capacitor

75 dB ripple rejection is obtainable at any output level. Increases over 20

μF do not appreciably improve the

ripple rejection at frequencies above 120 Hz. If the bypass capacitor is used, it is sometimes necessary to

include protection diodes to prevent the capacitor from discharging 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

μF in 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 0.5 MHz. For this reason, 0.01

μF disc may

seem to work better than a 0.1

μF disc as a bypass.

Although the LM138 is stable with no output capacitors, like any feedback circuit, certain values of external

capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1

μF solid

tantalum (or 25

μF aluminum electrolytic) on the output swamps this effect and insures stability.

LOAD REGULATION

The LM138 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 terminal and the output

terminal (usually 240

Ω) should be tied directly to the output of the regulator (case) 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 regulator and load will have a load regulation due to

line resistance of 0.05

0.05

Ω (1 + R2/R1) or in this case, 11.5 times worse.

Figure 16 shows the effect of resistance between the regulator and 240

Ω set resistor.

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