LTC4000-1
27
40001fa
For more information www.linear.com/LTC4001-1
applicaTions inForMaTion
determine the complete loop transfer function of each of
the loops. Once these are obtained, it is a matter of analyz-
ing the gain and phase bode plots to ensure that there is
enough phase and gain margin at unity crossover with the
selected values of R
C
and C
C
for all operating conditions.
Even though it is clear that an analytical compensation
method is possible, sometimes certain complications
render this method difficult to tackle. These complica-
tions include the lack of easy availability of the switching
converter transfer function from the ITH or VC control
node to its input or output current, and the variability of
parameter values of the components such as the ESR of
the output capacitor or the R
DS(ON)
of the external PFETs.
Therefore a simpler and more practical way to compensate
the LTC4000-1 is provided here. This empirical method
involves injecting an AC signal into the loop, observing
the loop transient response and adjusting the C
C
and R
C
values to quickly iterate towards the final values. Much
of the detail of this method is derived from Application
Note19 which can be found at www.linear.com using
AN19 in
the search box.
Figure 20 shows the recommended setup to inject an
AC-coupled output load variation into the loop. A function
generator with 50Ω output impedance is coupled through
a 50Ω/1000µF series RC network to the regulator output.
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 20 to prevent ground loop er-
rors. The oscilloscope is synced by connecting the chan-
nelB 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
Figure 20. Empirical Loop Compensation Setup
I
OUT
V
IN
CSN
CSP
IN
CLN
BATGND
LTC4000-1
50Ω
1W
50Ω
GENERATOR
f = 50Hz
R
C
ITHGND
SWITCHING
CONVERTER
40001 F20
BGATE
ITH CC
C
C
1000µF
(OBSERVE
POLARITY)
SCOPE
GROUND
CLIP
10k
A
1k
1500pF0.015µF
B
