LM150-LM250-LM350
TYPICAL APPLICATIONS
+ 1.2V to + 25V ADJUSTABLE REGULATOR
LM350
Since the 50碌A(chǔ) current from the adjustment termi-
nal represents an error term, the LM350 was de-
signed 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 establi-
shing a minimum load current requirement. If there
is insufficient load on the output, the output will rise.
EXTERNAL CAPACITORS
An input bypass capacitor is recommended. A
0.1碌F disc or 1碌F solid tantalum on the input is suit-
able 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 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
a 10碌F bypass capacitor 75dB ripple rejection is
obtainable at any output level. Increases over 20碌F
do not appreciably improve the ripple rejection at
frequencies above 120Hz. If the bypass capacitor
is used, it is sometimes necessary to include pro-
tection diodes to prevent the capacitor from dis-
charging 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碌F in
aluminum electrolytic to equal 1碌F 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 0.5MHz. For this reason, 0.01碌F disc may
seem to work better than a 0.1碌F disc as a bypass.
Although the LM350 is stable with no output capa-
citors, like any feedback circuit, certain values of
external capacitance can cause excessive ringing.
This occurs with values between 500pF and
5000pF. A 1碌F solid tantalum (or 25碌F aluminium
electrolytic) on the output swamps this effect and
insures stability.
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 termi-
nal and the output terminal (usually 240鈩? 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 15V
regulator with 0.05鈩?resistance between the regu-
lator and load will have a load regulation due to line
resistance of 0.05鈩?x I
L
. If the set resistor is con-
nected near the load the effective line resistance
鈭?/div>
Needed if device is far from filter capacitors.
* Optional-improves transient response. Output capacitors in the
range of 1碌F to 100碌F of aluminium or tantalum electrolytic are
commonly used to provide improved output impedance and rejec-
tion of transients.
R2
* * V
O
= 1.25V (1 +
)
R1
* * * R1 = 240鈩?for LM150 and LM250
APPLICATION HINTS
In operation, the LM350 develops a nominal 1.25V
reference voltage, V
(ref)
, between the output and
adjustment terminal. The reference voltage is im-
pressed across program resistor R1 and, since the
voltage is constant, a constant current I
1
then flows
through the output set resistor R2, giving an output
voltage of
R
2
V
O
= V
(ref)
( 1+
) + I
adj
R
2
R
1
Figure 1.
LM350
4/7
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