SC2434EVB 查看數據表（PDF） - Semtech Corporation
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SC2434EVB Datasheet PDF : 19 Pages
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SC2434
POWER MANAGEMENT
PRELIMINARY
Applications Information (Cont.)
The compensator transfer function has two poles and one
zero:
40
Loop-Gain (dB)
H c(s)
R drp .
R FB 1
s
1
ωz
s .1 s
ω p1
ω p2
To optimize the transient responses, it is recommended
that:
· To use the first compensator pole to cancel the power
stage ESR zero;
· To place the compensator zero at one half of the
switching frequency;
· And to place the second compensator pole at high
frequency.
20
mag_Loop ( i , R )
0
0
20
40
100
180
1 .10 3
1 .10 4
Fi
LoopGain (Degree)
1 .10 5
1 .10 6
90
phase_Loop ( i , R )
0
90
180
100
1 .10 3
1 .10 4
Fi
1 . 10 5
1 .10 6
Fig. 5 - Loop gain Bode plot based on control loop model.
The Bode plots based this model and those obtained from
experiment are depicted in Fig. 5 and Fig. 6, respectively.
It can be seen that the model agrees well with the
experiment. The control model provides us physical
insight of the loop dynamics and helps the designer to
achieve good transient responses and system stability. Here
are few comments:
· The loop crossover frequency (0dB frequency) should
be lower than one fifth (20%) of the switch frequency
to avoid noise pick up and the phase lag introduced
by the complex pole located at one half of the switching
frequency;
· A >20KHz crossover frequency is adequate to assure
good transient response when the VR output
impedance, or droop impedance, is programmed to
be equal to the output capacitor ESR. The ESR
frequency for the output bulk capacitor is usually less
than 20KHz, and beyond that frequency the capacitor
behaves like a resistor up to few hundred KHz, which
is desired for dynamic droop. There is no point to
demand the control loop to have much higher
crossover frequency beyond the ESR zero frequency.
Fig. 6 - Measured loop gain Bode plots.
2005 Semtech Corp.
10
www.semtech.com
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