Datasheet

LM138

LM338

SNVS771C –MAY 1998–REVISED DECEMBER 2016

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Product Folder Links: LM138 LM338

Figure 17. Regulator With Line Resistance in Output Lead

With the TO-3 package, it is easy to minimize the resistance 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 regulation.

8.2.1.2.3 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 discharges

into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage

of the regulator, and the rate of decrease of V

. In the LM138 this discharge path is through a large junction that

is 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 LM138 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 18

shows an LM138 with protection diodes included for use with outputs greater than 25 V and high values of output

capacitance.