LM350

Application hints

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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 LM350 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 aluminium electrolytic) on the

output swamps this effect and insures stability.

6.2

Load regulation

The LM350 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

Figure 5 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.

6.3

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

In the LM350 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 LM350

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 6 shows an LM350 with protection

diodes included for use with outputs greater than 25 V and high values of output

capacitance.