參數(shù)資料
型號(hào)： LP2975IMMX-3.3/NOPB
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 模擬信號(hào)調(diào)理
英文描述： SPECIALTY ANALOG CIRCUIT, PDSO8
封裝： MINI, SOP-8
文件頁(yè)數(shù)： 8/20頁(yè)
文件大?。?/td> 1135K
代理商： LP2975IMMX-3.3/NOPB
Application Hints (Continued)
Phase Shift Due to f
pg
10003422
Because of this, there is no exact number for f
pg/fc that can
be given as a fixed limit for stable operation. However, as a
general guideline, it is recommended that f
pg
≥ 3f
c.
If this is not found to be true after inital calculations, the ratio
of f
pg/fc can be increased by either reducing CEFF (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
F can accomplish this, provided
it is selected to set the zero at the correct frequency. It is
important to point out that the feed-forward capacitor pro-
duces both a zero and a pole. The frequency where the zero
occurs will be defined as f
zf, and the frequency of the pole
will be defined as f
pf. The equations to calculate the frequen-
cies are:
f
zf =6.6x10
-6/[C
F x(VOUT/1.24 1) ]
f
pf =6.6x10
-6/[C
F x (1 1.24/VOUT)]
In general, the feed-forward capacitor gives the greatest
improvement in phase margin (provides the maximum re-
duction 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
F become more effective at im-
proving loop stability as they get farther apart in frequency.
In reviewing the equations for f
zf and fpf, it can be seen that
they get closer together in frequency as V
OUT decreases.
For this reason, the use of C
F 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
c 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
capacitor in fixed-voltage applications is:
C
F =6.6x10
-6/[f
zf x(VOUT/1.24 1) ]
In adjustable applications (using an external resistive di-
vider) the capacitor is found using:
C
C = 1/(2
π xR1xf
zf)
Summary of Stability Information
This section will present an explanation of theory and termi-
nology used to analyze loop stability, along with specific
information 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
that when the total phase shift at 0 dB reaches (or gets close
to) 180, oscillations will result. Therefore, it can be seen
that at least two poles in the gain curve are required to
cause instability.
ZERO: A zero has an effect that is exactly opposite to a pole.
A zero will add a maximum +90 of phase lead (defined as
positive phase shift). Also, a zero causes the slope of the
gain curve to increase by an additional +20 dB/decade (see
graph EFFECTS OF A SINGLE ZERO).
Effects of a Single Pole
10003425
Total phase shift
The actual test of whether or not a regulator is stable is the
amount of phase shift that is present when the gain curve
crosses the 0 dB axis (the frequency where this occurs was
previously defined as f
c).
The phase shift at f
c can be estimated by looking at all of the
poles and zeroes on the Bode plot and adding up the con-
LP2975
www.national.com
16
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