LTC4000
24
4000fb
For more information
Figure 12. Empirical Loop Compensation Setup
I
OUT
V
IN
CSN
CSP
IN
CLN
BAT
GND
LTC4000
50Ω
1W
50Ω
GENERATOR
f = 50Hz
R
C
ITH
GND
SWITCHING
CONVERTER
4000 F12
BGATE
ITH
CC
C
C
1000µF
(OBSERVE
POLARITY)
SCOPE
GROUND
CLIP
10k
A
1k
1500pF
0.015µF
B
applicaTions inForMaTion
Generator frequency is set at 50Hz. Lower frequencies
may cause a blinking scope display and higher frequen-
cies may not allow sufficient settling time for the output
transient. Amplitude of the generator output is typically
set at 5V
P-P
to generate a 100mA
P-P
load variation. For
lightly loaded outputs (I
OUT
< 100mA), this level may be
too high for small signal response. If the positive and
negative transition settling waveforms are significantly
different, amplitude should be reduced. Actual amplitude
is not particularly important because it is the shape of
the resulting regulator output waveform which indicates
loop stability.
A 2-pole oscilloscope filter with f = 10kHz is used to
block switching frequencies. Regulators without added
LC output filters have switching frequency signals at their
outputs which may be much higher amplitude than the
low frequency settling waveform to be studied. The filter
frequency is high enough for most applications to pass
the settling waveform with no distortion.
Oscilloscope and generator connections should be made
exactly as shown in Figure 12 to prevent ground loop er-
rors. The oscilloscope is synced by connecting the chan-
nel B probe to the generator output, with the ground clip
of the second probe connected to exactly the same place
as channel A ground. The standard 50Ω BNC sync output
of the generator should not be used because of ground
loop errors. It may also be necessary to isolate either
the generator or oscilloscope from its third wire (earth
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 13), 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.
