LTC4000-1
28
40001fa
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applicaTions inForMaTion
ground) connection in the power plug to prevent ground
loop errors in the scope display. These ground loop errors
are checked by connecting channel A probe tip to exactly
the same point as the probe ground clip. Any reading on
channel A indicates a ground loop problem.
Once the proper setup is made, finding the optimum
values for the frequency compensation network is fairly
straightforward. Initially, C
C
is made large (≥1μF) and R
C
is made small (≈10k). This nearly always ensures that the
regulator will be stable enough to start iteration. Now, if
the regulator output waveform is single-pole over damped
(see the waveforms in Figure 21), the value of C
C
is re-
duced in steps of about 2:1 until the response becomes
slightly under damped. Next, R
C
is increased in steps of
2:1 to introduce a loop zero. This will normally improve
damping and allow the value of C
C
to be further reduced.
Shifting back and forth between R
C
and C
C
variations will
allow one to quickly find optimum values.
If the regulator response is under damped with the initial
large value of C
C
, R
C
should be increased immediately before
larger values of C
C
are tried. This will normally bring about
the over damped starting condition for further iteration.
The optimum values for R
C
and C
C
normally means the
smallest value for C
C
and the largest value for R
C
which
still guarantee well damped response, and which result in
the largest loop bandwidth and hence loop settling that is
as rapid as possible. The reason for this approach is that
it minimizes the variations in output voltage caused by
input ripple voltage and output load transients.
A switching regulator which is grossly over damped will
never oscillate, but it may have unacceptably large output
transients following sudden changes in input voltage or
output loading. It may also suffer from excessive overshoot
problems on startup or short circuit recovery. To guarantee
acceptable loop stability under all conditions, the initial
values chosen for R
C
and C
C
should be checked under all
conditions of input voltage and load current. The simplest
way of accomplishing this is to apply load currents of
minimum, maximum and several points in between. At
each load current, input voltage is varied from minimum
to maximum while observing the settling waveform.
If large temperature variations are expected for
the system,
stability checks should also be done at the temperature
extremes. There can be significant temperature varia-
tions in several key component parameters which affect
stability; in particular, input and output capacitor value
and their ESR, and inductor permeability. The external
converter parametric variations also need some consid-
eration especially the transfer function from the ITH/VC
pin voltage to the output variable (voltage or current). The
LTC4000-1 parameters that vary with temperature include
the transconductance and the output resistance of the
error amplifiers (A4-A7). For modest temperature varia-
tions, conservative over damping under worst-case room
temperature conditions is usually sufficient to guarantee
adequate stability at all temperatures.
One measure of stability margin is to vary the selected
values of both R
C
and C
C
by 2:1 in all four possible com-
binations. If the regulator response remains reasonably
well damped under all conditions, the regulator can be
considered fairly tolerant of parametric variations. Any
tendency towards an under damped (ringing) response
indicates that a more conservative compensation may
be needed.
Figure 21. Typical Output Transient Response at Various
Stability Level
REGULATOR OUTPUT
WITH LARGE C
C
, SMALL R
C
WITH REDUCED C
C
, SMALL R
C
FURTHER REDUCTION IN C
C
MAY BE POSSIBLE
IMPROPER VALUES WILL
CAUSE OSCILLATIONS
EFFECT OF INCREASED R
C
40001 F21
