Application hints

LM350

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6

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 LM350

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.

6.1

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 LM350 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 aluminium electrolytic to equal 1 µF solid tantalum at high frequencies.

Ceramic capacitors are also good at high frequencies, but some types have a large

Figure 4.

Circuit