LM138K datasheet(7/12 Pages) STMICROELECTRONICS

Il numero della parte	LM138K

Spiegazioni elettronici	THREE-TERMINAL 5 A ADJUSTABLE VOLTAGE REGULATORS

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will be 0.05

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

worse.

Figure 2 shows the effect of resistance between the

regulator and 140

Ω set resistor.

With the TO-3 package, it is easy to minimize the re-

sistance from the case to the set resistor, by using

2 separate leads to the case. The ground of R2 can

be returned near the ground of the load to provide

remote ground sensing and improve load regula-

tion.

PROTECTION DIODES

When external capacitors are used with any IC regu-

lator it is sometimes necessary to add protection

diodes to prevent the capacitors from discharging

through low current points into the regulator. Most

Figure 2 : Regulator with Line Resistance in Out-

put Lead.

20

µF capacitors have low enough internal series re-

sistance to deliver 20A spikes when shorted. Al-

though the surge is short, there is enough energy to

damage parts of the IC.

When an output capacitor is connected to a regula-

tor and the input is shorted, the output capacitor will

discharge into the output of the regulator. The dis-

charge current depends on the value of the capaci-

tor, the output voltage of the regulator, and the rate

of decrease of VI. In the LM338 this discharge path

is through a large junction that is able to sustain 25A

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 15V or less, there is no

need to use diodes.

The bypass capacitor on the adjustment terminal

can discharge through a low current junction. Dis-

charge 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 protec-

tion is needed for output voltages of 25V or less and

10

µF capacitance. Figure 3 shows an LM338 with

protection diodes included for use with outputs

greater than 25V and high values of output capacit-

ance.

Figure 3 : Regulator with Protection Diodes.

LM138-LM238-LM338

7/12

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