SC4809A 查看數據表（PDF） - Semtech Corporation
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SC4809A Datasheet PDF : 16 Pages
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SC4809A/B/C
POWER MANAGEMENT
Application Information (Cont.)
knowing what the transfer characteristic of the PWM and
switching circuit looks like. Constructing a Bode plot of
the known poles and zeros in the power stage does this.
Bode plots give a visual interpretation of the gain versus
frequency and phase versus frequency characteristics
of a system. In the gain plot, the gain shown at each
frequency represents the amount by which the feedback
loop will reduce a disturbance at that frequency.
The scheme shown below will handle most compensation
requirements. There is a pole at the origin which
contributes a -1 slope in the gain plot, a low frequency
zero, fEAZERO flattens out the slope so the mid-range gain
is equal to Rf/Ri. A high frequency pole, fEAPOLE helps
suppress any high frequency noise from propagating
through the system. Rd forms a voltage divider with Ri
and provides a DC offset.
Besides the RHP zero, the output capacitor and the load
contribute a pole and the output capacitor alone will
contribute a zero based upon its ESR.
fpole
=
1+ D
2 • π • ROUT • COUT
fEAZERO
=
1
2• π •Rf
• Cf
fEAPOLE
=
1
2 • π•Rf
• Cp
fzero
=
1
2π • ESR
• COUT
The control to output gain is calculated by:
GAIN
=
20
•
log•
VC
•
ISC • ROUT • VIN
(1− D) • (2 • N • VO
+
VIN
)
Once the frequency response of the uncompensated
system is determined, the next step is to determine what
compensation is needed around the error amplifier for
optimum performance. As stated earlier, optimum
performance requires a high gain at low frequencies for
good DC regulation and high bandwidth for good transient
response. The crossover frequency, fc, is the frequency
at which the gain magnitude equals 0dB. High bandwidth
is achieved by having the highest possible fc. Because of
the RHP zero, the highest possible crossover frequency
is limited to fRHPZERO/π. The phase margin, or the amount
the phase lag measures at fc less 180°, should be at
least 45° for good transient response with little
overshoot. The magnitude of the gain at the frequency
where the phase plot measures - 180° is referred to as
the gain margin. If the slope of the gain plot is -2, or
-40dB/decade, at low frequencies, it much transition to
a -20dB/decade slope, also known as a -1 slope, one
decade before crossing the 0dB point. If the slope
remains at the -2 slope the resultant gain margin would
be too small causing sever underdamped oscillations at
fc.
By combining the Bode plots of the PWM and power stage
with the error amplifier compensation, a plot of the entire
system is realized.
2005 Semtech Corp.
12
www.semtech.com
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