LP2975AIMM-12 National Semiconductor, LP2975AIMM-12 Datasheet - Page 16

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

www.national.com
Application Hints
To estimate the total phase margin, the individual phase shift
contributions of each pole and zero will be calculated assum-
ing f
f
Controller pole shift = −90˚
f
f
f
f
f
Summing the six numbers, the estimate for the total phase
shift is −95˚, which corresponds to a phase margin of 85˚
(a 27˚ improvement over the same application without the
feed-forward capacitor).
For this reason, a feed-forward capacitor is recommended in
all applications. Although not always required, the added
phase margin typically gives faster settling times and pro-
vides some design guard band against C
tions with temperature.
CAUSES AND CURES OF OSCILLATIONS
The most common cause of oscillations in an LDO applica-
tion is the output capacitor ESR. If the ESR is too high or too
low, the zero (f
HIGH ESR: To illustrate the effect of an output capacitor with
high ESR, the previous example will be repeated except that
the ESR will be increased by a factor of 20X. This will cause
the frequency of the zero f
moves it from 5 kHz down to 250 Hz (see graph HIGH ESR
UNSTABLE WITHOUT FEED-FORWARD ).
c
p
z
zf
pf
pg
shift = −arctan (50k/200) = −90˚
shift = arctan (50k/5k) = +84˚
= 50 kHz, and f
shift = arctan (50k/100k) = +79˚
shift = −arctan (50k/40k) = −51˚
shift = −arctan (50k/100k) = −27˚
p
Improved Phase Margin with Feed-Forward
= 200 Hz, f
z
) does not provide enough phase lead.
pg
z
= 100 kHz:
= 5 kHz, f
z
(Continued)
to decrease by 20X, which
zf
= 10 kHz, f
OUT
DS100034-28
and ESR varia-
pf
= 40 kHz,
16
As shown, moving the location of f
tends the bandwidth, pushing the crossover frequency f
to about 200 kHz. In viewing the plot, it can be seen that f
and f
and f
cause enough phase shift to leave very little phase margin.
This application would either oscillate continuously or be
marginally stable (meaning it would exhibit severe ringing on
transient steps).
This can be improved by adding a feed-forward capacitor C
which adds a zero (f
graph HIGH ESR CORRECTED WITH FEED-FORWARD ).
In this case, C
quency as f
to f
frequency, it will add
margin of
LOW ESR: To illustrate how an output capacitor with low
ESR can cause an LDO regulator to oscillate, the same ex-
ample will be shown except that the ESR will be reduced suf-
ficiently to increase the original f
The plot now shows (see graph LOW ESR UNSTABLE
WITHOUT FEED-FORWARD ) that the crossover frequency
f
that the zero f
which is not sufficient for stability.
c
has moved down to about 8 kHz. Since f
pg
z
pg
). Assuming the added pole f
essentially cancel out, leaving only the controller pole
. However, since f
High ESR Unstable without Feed-Forward
High ESR Corrected with Feed-Forward
>
pg
45˚ (adequate for good stability).
z
F
(essentially cancelling out the phase shift due
can only provide about 9˚ of phase lead at f
is selected to place f
zf
) and a pole (f
<
45˚ of phase lag, leaving a phase
pg
now occurs well before f
z
pf
from 5 kHz to 50 kHz.
z
zf
is near or beyond the f
pf
lower in frequency ex-
) to the gain plot (see
at about the same fre-
z
DS100034-29
DS100034-31
is 6X f
c
, it means
c
, it will
c
out
F
c
p
c
,
,