LM138 - LM238 - LM338

Application hints

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Application hints

output and adjustment terminal. The reference voltage is impressed across program resistor

Since the 50 µA current from the adjustment terminal represents an error term, the LM338

this, all quiescent operating current is returned to the output establishing a minimum load

current requirement. If there is insufficient load on the output, the output will rise.

7.1

External capacitors

An input bypass capacitor is recommended. A 0.1 µF disc or 1 µF solid tantalum on the input

is suitable input by passing for almost all applications. The device is more sensitive to the

absence of input bypassing when adjustment or output capacitors are used by the above

values will eliminate the possibility of problems. The adjustment terminal can be bypassed

to ground on the LM338 to improve ripple rejection. This bypass capacitor prevents ripple

form 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

Figure 18.

Application circuit