參數(shù)資料
型號(hào)： LP2975AIMM-12
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 模擬信號(hào)調(diào)理
英文描述： MOSFET LDO Driver/Controller
中文描述： SPECIALTY ANALOG CIRCUIT, PDSO8
封裝： MINI, SOP-8
文件頁(yè)數(shù)： 14/19頁(yè)
文件大小： 715K
代理商： LP2975AIMM-12
Application Hints
(Continued)
As can be seen in the graph, values of C
EFF
in the
500 pF–2500 pF range produce values for f
pg
between
40 kHz and 700 kHz. To determine what effect f
will have
on stability, the bandwidth of the regulator loop must be cal-
culated (see next section CROSSOVER FREQUENCY AND
PHASE MARGIN).
Crossover Frequency and Phase Margin
The term
f
will be used to define the crossover frequency of
the regulator loop (which is the frequency where the gain
curve crosses the 0 dB axis). The importance of this fre-
quency is that it is the point where the loop gain goes below
unity, which marks the usable bandwidth of the regulator
loop.
It is the
phase margin
(or lack of it) at f
that determines
whether the regulator is stable. Phase margin is defined as
the total phase shift subtracted from 180. In general, a
stable loop requires at least 20-30 of phase margin at f
c
.
f
can be approximated by the following equation (all terms
have been previously defined):
This equation assumes that no C
F
is used and f
pg
/f
c
>
1.
If the frequency of the Gate capacitance pole f
has been
calculated (previous section), the amount of added phase
shift may now be determined. As shown in the graph below
(see graph PHASE SHIFT DUE TO f
), the amount of
added phase shift increases as f
pg
approaches f
c
.
The amount of phase shift due to f
that can occur before
oscillation takes place depends on how much added phase
shift is present as a result of the C
OUT
pole (see previous
section OUTPUT CAPACITOR).
Because of this, there is no exact number for f
/f
that can
be given as a fixed limit for stable operation. However, as a
general guideline, it is recommended that f
pg
≥
3 f
c
.
If this is not found to be true after inital calculations, the ratio
of f
/f
can be increased by either reducing C
EFF
(selecting
a different FET) or using a larger value of C
OUT
.
Along with these two methods, another technique for improv-
ing loop stability is the use of a feed-forward capacitor (see
next section FEED-FORWARD COMPENSATION).This can
improve phase margin by cancelling some of the excess
phase shift.
Feed-Forward Compensation
Phase shift in the loop gain of the regulator results from f
p
(the pole from the output capacitor and load resistance), f
pg
(the pole from the FET gate capacitance), as well as the IC’s
internal controller pole (see typical curve). If the total phase
shift becomes excessive, instability can result.
The total phase shift can be reduced using feed-forward
compensation, which places a zero in the loop to reduce the
effects of the poles.
The feed-forward capacitor C
can accomplish this, provided
it is selected to set the zero at the correct frequency. It is im-
portant to point out that the feed-forward capacitor produces
both a zero and a pole The frequency where the zero occurs
will be defined as
f
, and the frequency of the pole will be
defined as
f
pf
. The equations to calculate the frequencies
are:
f
zf
= 6.6 x 10
-6
/ [C
F
x (V
OUT
/1.24 1) ]
f
pf
= 6.6 x 10
-6
/ [C
F
x (1 1.24/V
OUT
)]
In general, the feed-forward capacitor gives the greatest im-
provement in phase margin (provides the maximum reduc-
tion in phase shift) when the zero occurs at a frequency
where the loop gain is
>
1 (before the crossover frequency).
The pole must occur at a higher frequency (the higher the
better) where most of the phase shift added by the new pole
occurs beyond the crossover frequency. For this reason, the
pole-zero pair created by C
become more effective at im-
proving loop stability as they get farther apart in frequency.
In reviewing the equations for f
and f
, it can be seen that
they get closer together in frequency as V
decreases.
For this reason, the use of C
gives greatest benefit at higher
output voltages, declining as V
OUT
approaches 1.24V
(where C
F
has no effect at all).
In selecting a value of feed-forward capacitor, the crossover
frequency f
must first be calculated. In general, the fre-
quency of the zero (f
zf
) set by this capacitor should be in the
range:
0.2 f
c
≤
f
zf
≤
1.0 f
c
The equation to determine the value of the feed-forward ca-
pacitor in fixed-voltage applications is:
C
F
= 6.6 x 10
-6
/ [f
zf
x (V
OUT
/1.24 1) ]
In adjustable applications (using an external resistive di-
vider) the capacitor is found using:
C
C
= 1/(2
π
x R1 x f
zf
)
SUMMARY OF STABILITY INFORMATION
This section will present an explanation of theory and termi-
nology used to analyze loop stability, along with specific in-
formation related to stabilizing LP2975 applications.
BODE PLOTS AND PHASE SHIFT
Loop gain information is most often presented in the form of
a
Bode Plot
, which plots
Gain
(in dB) versus
Frequency
(in
Hertz).
A Bode Plot also conveys phase shift information, which can
be derived from the locations of the
poles
and
zeroes
.
POLE
: A pole causes the slope of the gain curve to
de-
crease by an additional 20 dB/decade
, and it also causes
phase lag
(defined as
negative phase shift
) to occur.
A single pole will cause a maximum 90 of phase lag (see
graph EFFECTS OF A SINGLE POLE). It should be noted
DS100034-23
Phase Shift Due to f
pg
DS100034-22
www.national.com
14
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