LP2975AIMM-12 National Semiconductor, LP2975AIMM-12 Datasheet - Page 18

• Manufacturer Part Number: LP2975AIMM-12
• Description: MOSFET LDO Driver/Controller
• Manufacturer: National Semiconductor

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Application Hints
1) Calculate the required value of capacitance for C
that the pole f
PACITOR ). For this calculation, an ESR of about 0.1
be assumed for the purpose of determining C
IMPORTANT: If a smaller value of output capacitor is used
(so that the value of f
control loop will be reduced. This will result in increased ring-
ing on the output voltage during a load transient. If the output
capacitor is made extremely small, oscillations will result.
To illustrate this effect, scope photos have been presented
showing the output voltage of reference design # 2 as the
output capacitor is reduced to approximately 1/30 of the
nominal value (the value which sets f
the effect of deviating from the nominal value is gradual and
the regulator is quite robust in resisting going into oscilla-
tions.
2) Approximate the crossover frequency f
tion in the previous section CROSSOVER FREQUENCY
AND PHASE MARGIN .
3) Calculate the required ESR of the output capacitor so that
the frequency of the zero f
tion OUTPUT CAPACITOR ).
4) Calculate the value of the feed-forward capacitor C
that the zero f
p
zf
200 Hz (see previous section OUTPUT CA-
occurs at the frequency which yields the
p
>
200 Hz), the phase margin of the
z
is set to 0.5 f
(Continued)
p
= 200 Hz). As shown,
c
(see previous sec-
c
using the equa-
OUT
.
OUT
F
can
so
so
18
maximum phase gain for the output voltage selected (see
previous section LOW OUTPUT VOLTAGE AND C
formula for calculating C
FORWARD CAPACITOR .
Lower ESR electrolytics are available which use organic
electrolyte (OSCON types), but are more costly than typical
aluminum electrolytics.
If the calculated value of ESR is higher than what is found in
the selected capacitor, an external resistor can be placed in
series with C
LOW VOLTAGE DESIGNS: Designs which have a low out-
put voltage (where the positive effects of C
may be marginally stable if the C
carefully selected.
Also, if the FET gate capacitance is large (as in the case of
a high-current FET), the pole f
enough in frequency to cause a problem.
The solution in both cases is to increase the amount of out-
put capacitance which will shift f
reduce overall loop bandwidth). The ESR and C
tions should be repeated, since this changes the crossover
frequency f
c
.
OUT
.
F
is in the previous section FEED-
OUT
p
pg
to a lower frequency (and
could possibly get low
and ESR values are not
F
are very small)
F
calcula-
F
). The