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EXTERNAL COMPENSATION
Since the purpose of a voltage regulator is to sup-
ply and load variations, the open loop gain of the
regulators must be very high at low frequencies.
This may cause instability as a result of the vari-
ous poles present in the loop. To avoid this insta-
bility dominant pole compensation is used to re-
duce phase shift due to other poles at the unity
gain frequency. The lower the frequency of these
others poles at the unity gain frequency. The
lower the frequency of these other poles, the
greater must be capacitor esed to create the
dominant pole for the same DC gain.
Where the output transistor is a lateral PNP type
there is a pole in the regulation loop at a fre-
quencybtoo low to be compensated by a capaci-
tor which can be integrated. An external compen-
sation is therefore necessary so a very high value
capacitor must be connected from the output to
ground.
The paeassitic equivalent series resistance of the
capacitor used adds a zero to the regulation loop.
This zero may compromise the stability of the
system since its effect tends to cancel the effect
of the pole added. In regulators this ESR must be
less than 3
Ω
and the minimum capacitor value is
47
µ
F.
Figure 1:
Typical Dropout Voltage vs. T
j
(I
o
= 500mA).
L4945
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