TYPICAL AP PLI CA TI O NS
+ 1.2V to + 25V ADJUSTABLE REGULATOR
APPLICATION HINTS
In operation,the LM350 develops anominal 1.25V
reference voltage, V
(ref)
, between the output and
adjustment terminal. The reference voltage is impressed across program resistor R1and, since the
voltage is constant, aconstant current I1then flows
through theoutput set resistor R2, giving an output
voltage of
VO=V
(ref)
(1+
R
2
R
1
)+I
adjR2
Figure 1.
Since the 50µAcurrent from the adjustment terminal represents an error term, theLM350 was designed to minimize I
adj
and make it very constant
withlineand load changes. To do this, allquiescent
operating current is returned to the output establishing a minimum load currentrequirement. Ifthere
is insufficientloadonthe output, the outputwill rise.
EXTERNAL CAPACITORS
An input bypass capacitor is recommended. A
0.1µFdisc or1µFsolid tantalum on the inputis suitable input by passing for almost all applications.
Thedeviceismore sensitivetothe absenceofinput
bypassing when adjustment or output capacitors
are used byt 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
a10µF bypass capacitor 75dB ripple rejection is
obtainableatanyoutputlevel.Increases over20µF
do not appreciably improve the ripple rejection at
frequencies above 120Hz. 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 tantalumcapacitors have low
impedance even at high frequencies. Depending
upon capacitorconstruction, it takes about 25µFin
aluminumelectrolyticto equal1µFsolid tantalum at
high frequencies. Ceramic capacitors are also
good at high frequencies, but some types have a
large decrease in capacitance at frequencies
around 0.5MHz. For this reason,0.01µF disc may
seem towork better than a0.1µF discas a bypass.
Although the LM350 is stable with no output capacitors, like any feedback circuit, certain values of
external capacitancecan cause excessive ringing.
This occurs with values between 500pF and
5000pF. A 1µF solid tantalum(or 25µF aluminium
electrolytic) on the output swamps this effect and
insures stability.
LOADREGULATION
The LM350is capable ofproviding 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 240Ω) should
be tied directly tothe 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 15V
regulator with 0.05Ωresistance between theregulator and loadwill have a load regulationdue to line
resistance of 0.05Ω xIL. If the set resistor is connected near the load the effective line resistance
∆ Needed if device is far from filter capacitors.
*
Optional-improves transient response. Output capacitors in the
range of 1µF to 100µF of aluminium or tantalum electrolytic are
commonlyused toprovide improvedoutput impedance and rejection of transients.
R2
** VO=1.25V(1+ )
R1
*** R1=240Ωfor LM150 and LM250
LM 350
LM 350
LM150-LM250-LM350
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