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The LM105-An Improved Positive Regulator TL H 6906
The
LM105-An
Improved
Positive
Regulator
AN-23
National Semiconductor
Application Note 23
January 1969
The LM105-An Improved
Positive Regulator
Robert J Widlar
Apartado Postal 541
Puerto Vallarta Jalisco
Mexico
introduction
IC voltage regulators are seeing rapidly increasing usage
The LM100 one of the first has already been widely ac-
cepted Designed for versatility this circuit can be used as a
linear regulator a switching regulator a shunt regulator or
even a current regulator The output voltage can be set be-
tween 2V and 30V with a pair of external resistors and it
works with unregulated input voltages down to 7V Dissipa-
tion limitations of the IC package restrict the output current
to less than 20 mA but external transistors can be added to
obtain output currents in excess of 5A The LM100 and an
extensive description of its use in many practical circuits are
described in References 1 3
One complaint about the LM100 has been that it does not
have good enough regulation for certain applications In ad-
dition it becomes difficult to prove that the load regulation is
satisfactory under worst-case design conditions
These
problems prompted development of the LM105 which is
nearly identical to the LM100 except that a gain stage has
been added for improved regulation In the great majority of
applications the LM105 is a plug-in replacement for the
LM100
the improved regulator
The load regulation of the LM100 is about 0 1% no load to
full load without current limiting When short circuit protec-
tion is added the regulation begins to degrade as the output
current becomes greater than about half the limiting current
This is illustrated in
Figure 1 The LM105 on the other hand
gives 0 1% regulation up to currents closely approaching
the short circuit current As shown in
Figure 1b this is partic-
ularly significant at high temperatures
The current limiting characteristics of a regulator are impor-
tant for two reasons First it is almost mandatory that a
regulator be short-circuit protected because the output is
distributed to enough places that the probability of it becom-
ing shorted is quite high Secondly the sharpness of the
limiting characteristics is not improved by the addition of
external booster transistors External transistors can in-
crease the maximum output current but they do not im-
prove the load regulation at currents approaching the short
TL H 6906 1
a T
j
e
25 C
TL H 6906 2
b T
j
e
125 C
Figure 1 Comparison between the load regulation of
the LM100 and LM105 for equal short circuit
currents
circuit current Thus it can be seen that the LM105 provides
more than ten times better load regulation in practical power
supply designs
C1995 National Semiconductor Corporation
RRD-B30M115 Printed in U S A Figure 2 shows that the LM105 also provides better line
regulation than the LM100 These curves give the percent-
age change in output voltage for an incremental change in
the unregulated input voltage They show that the line regu-
lation is worst for small differences between the input and
output voltages The LM105 provides about three times bet-
ter regulation under worst case conditions Bypassing the
internal reference of the regulator makes the ripple rejection
of the LM105 almost a factor of ten better than the LM100
over the entire operating range as shown in the figure This
bypass capacitor also eliminates noise generated in the in-
ternal reference zener of the IC
TL H 6906 3
Figure 2 Comparison between the line regulation char-
acteristics of the LM100 and LM105
The LM105 has also benefited from the use of new IC com-
ponents developed after the LM100 was designed These
have reduced the internal power consumption so that the
LM105 can be specified for input voltages up to 50V and
output voltages to 40V The minimum preload current re-
quired by the LM100 is not needed on the LM105
circuit description
The differences between the LM100 and the LM105 can be
seen by comparing the schematic diagrams in
Figures 3 and
4 Q4 and Q5 have been added to the LM105 to form a
common-collector common-base common-emitter amplifi-
er rather than the single common-emitter differential ampli-
fier of the LM100
In the LM100 generation of the reference voltage starts
with zener diode D1 which is supplied with a fixed current
from one of the collectors of Q2 This regulated voltage
which has a positive temperature coefficient is buffered by
TL H 6906 5
Figure 4 Schematic diagram of the LM105 regulator
Q4 divided down by R1 and R2 and connected in series
with a diode-connected transistor Q7 The negative temper-
ature coefficient of Q7 cancels out the positive coefficient of
the voltage across R2 producing a temperature-compen-
sated 1 8V on the base of Q8 This point is also brought
outside the circuit so that an external capacitor can be add-
ed to bypass any noise from the zener diode
Transistors Q8 and Q9 make up the error amplifier of the
circuit A gain of 2000 is obtained from this single stage by
using a current source another collector on Q2 as a collec-
tor load The output of the amplifier is buffered by Q11 and
used to drive the series-pass transistor Q12 The collector
of Q12 is brought out so that an external PNP transistor or
PNP
NPN combination can be added for increased output
current
Current limiting is provided by Q10 When the voltage
across an external resistor connected between Pins 1 and 8
becomes high enough to turn on Q10 it removes the base
drive from Q11 so the regulator exhibits a constant-current
characteristic Prebiasing the current limit transistor with a
portion of the emitter-base voltage of Q12 from R6 and R7
reduces the current limit sense voltage This increases the
TL H 6906 4
Figure 3 Schematic diagram of the LM100 regulator
2 efficiency of the regulator especially when foldback current
limiting is used With foldback limiting the voltage dropped
across the current sense resistor is about four times larger
than the sense voltage
As for the remaining details the collector of the amplifier
Q9 is brought out so that external collector-base capaci-
tance can be added to frequency-stabilize the circuit when it
is used as a linear regulator This terminal can also be
grounded to shut the regulator off R9 and R4 are used to
start up the regulator while the rest of the circuitry estab-
lishes the proper operating levels for the current source
transistor Q2
The reference circuitry of the LM105 is the same except
that the current through the reference divider R2 R3 and
R4 has been reduced by a factor of two on the LM105 for
reduced power consumption In the LM105 Q2 and Q3 form
an emitter coupled amplifier with Q3 being the emitter-fol-
lower input and Q2 the common-base output amplifier R6 is
the collector load for this stage which has a voltage gain of
about 20 The second stage is a differential amplifier using
Q4 and Q5 Q5 actually provides the gain Since it has a
current source as a collector load one of the collectors of
Q12 the gain is quite high about 1500 This gives a total
gain in the error amplifier of about 30 000 which is ten times
higher than the LM100
It is not obvious from the schematic but the first stage (Q2
and Q3) and second stage (Q4 and Q5) of the error amplifi-
er are closely balanced when the circuit is operating This
will be true regardless of the absolute value of components
and over the operating temperature range The only thing
affecting balance is component matching which is good in a
monolithic integrated circuit so the error amplifier has good
drift characteristics over a wide temperature range
Frequency compensation is accomplished with an external
integrating capacitor around the error amplifier as with the
LM100 This scheme makes the stability insensitive to load-
ing conditions
resistive or reactive
while giving good
transient response However an internal capacitor C1 is
added to prevent minor-loop oscillations due to the in-
creased gain
Additional differences between the LM100 and LM105 are
that a field-effect transistor Q18 connected as a current
source starts the regulator when power is first applied
Since this current source is connected to ground rather
than the output the minimum load current before the regula-
tor drops out of operation with large input-output voltage
differentials is greatly reduced This also minimizes power
dissipation in the integrated circuit when the difference be-
tween the input and output voltage is at the worst-case val-
ue With the LM105 circuit configuration it was also neces-
sary to add Q17 to eliminate a latch-up mechanism which
could exist with l