LM350_08 Datasheet, PDF (8/14 Pages) STMicroelectronics – Three-terminal 3 A adjustable voltage regulators

Application hints
6
Application hints
LM350
Figure 4.
In operation, the LM350 develops a nominal 1.25 V reference voltage, V_(REF), between the
output and adjustment terminal. The reference voltage is impressed across program resistor
R₁ and, since the voltage is constant, a constant current I₁ then flows through the output set
resistor R₂, giving an output voltage of:
V_O = V_(REF) (1+ R₂ / R₁) + I_ADJ x R₂.
Circuit
Since the 50 µA current from the adjustment terminal represents an error term, the LM350
was designed to minimize I_ADJ and make it very constant with line and load changes. To do
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
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