Application hints

LM138 - LM238 - LM338

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0.1 µF disc as a bypass. Although the LM338 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 mF solid tantalum (or 25 µF aluminium

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

7.2

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 terminal 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 15 V

regulator with 0.05

Ω resistance between the regulator and load will have a load regulation

due to line resistance of 0.05

effective line resistance will be 0.05

on page 8 shows the effect of resistance between the regulator and 140

Ω set resistor. With

the TO-3 package, it is easy to minimize the resistance from the case to the set resistor, by

load to provide remote ground sensing and improve load regulation.

7.3

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. Most 20 µF capacitors have low enough internal series resistance to deliver

20 A spikes when shorted. Although the surge is short, there is enough energy to damage

parts of the IC. When an output capacitor is connected to a regulator and the input is

shorted, the output capacitor will discharge into the output of the regulator. The discharge

current depends on the value of the capacitor, the output voltage of the regulator, and the

able to sustain 25 A surge with no problem. This is not true of other types of positive

regulators. For output capacitors of 100 µF or less at output of 15 V or less, there is no need

to use diodes.The bypass capacitor on the adjustment terminal can discharge through a low

current junction. Discharge occurs when either the input or output is shorted. Internal to the

LM338 is a 50

Ω resistor which limits the peak discharge current. No protection is needed

for output voltages of 25 V or less and 10 µF capacitance. Figure 5 on page 8 shows an

LM338 with protection diodes included for use with outputs greater than 25 V and high

values of output capacitance output capacitance an LM338 with protection diodes included

for use with outputs greater than 25 V and high values of output capacitance.