LT1375/LT1376
U
W U U
APPLICATIONS INFORMATION
How Do I Test Loop Stability?
of high frequency (500kHz) ripple. The ripple makes it
difficult to observe the small transient, so a two-pole,
100kHz filter has been added. This filter is not particularly
critical; even if it attenuated the transient signal slightly,
this wouldn’t matter because amplitude is not critical.
The “standard” compensation for LT1376 is a 3.3nF
capacitor for CC, with RC = 0. While this compensation will
work for most applications, the “optimum” value for loop
compensationcomponentsdepends,tovariousextent,on
parameters which are not well controlled. These include
inductor value (±30% due to production tolerance, load
current and ripple current variations), output capacitance
(±20% to ±50% due to production tolerance, tempera-
ture, aging and changes at the load), output capacitor ESR
(±200% due to production tolerance, temperature and
aging), and finally, DC input voltage and output load
current . This makes it important for the designer to check
outthefinaldesigntoensurethatitis“robust”andtolerant
of all these variations.
After verifying that the setup is working correctly, I start
varying load current and input voltage to see if I can find
any combination that makes the transient response look
suspiciously “ringy.” This procedure may lead to an ad-
justment for best loop stability or faster loop transient
response. Nearly always you will find that loop response
looks better if you add in several kΩ for RC. Do this only
if necessary, because as explained before, RC above 1k
may require the addition of CF to control VC pin ripple. If
everythinglooksOK, Iuseaheatgunandcoldsprayonthe
circuit (especially the output capacitor) to bring out any
temperature-dependent characteristics.
I check switching regulator loop stability by pulse loading
the regulator output while observing transient response at
the output, using the circuit shown in Figure 16. The
regulator loop is “hit” with a small transient AC load
current at a relatively low frequency, 50Hz to 1kHz. This
causes the output to jump a few millivolts, then settle back
totheoriginalvalue,asshowninFigure17. Awellbehaved
loop will settle back cleanly, whereas a loop with poor
phase or gain margin will “ring” as it settles. The number
ofringsindicatesthedegreeofstability, andthefrequency
of the ringing shows the approximate unity-gain fre-
quency of the loop. Amplitude of the signal is not particu-
larlyimportant, aslongastheamplitudeisnotsohighthat
the loop behaves nonlinearly.
VOUT AT IOUT
500mA
=
BEFORE FILTER
VOUT AT IOUT
500mA
=
10mV/DIV
5A/DIV
AFTER FILTER
VOUT AT IOUT = 50mA
AFTER FILTER
LOAD PULSE
THROUGH 50Ω
f ≈ 780Hz
0.2ms/DIV
1375/76 F17
Figure 17. Loop Stability Check
The output of the regulator contains both the desired low
frequency transient information and a reasonable amount
RIPPLE FILTER
TO X1
OSCILLOSCOPE
PROBE
470Ω
4.7k
SWITCHING
REGULATOR
+
100µF TO
1000µF
3300pF
330pF
50Ω
ADJUSTABLE
INPUT SUPPLY
ADJUSTABLE
DC LOAD
TO
OSCILLOSCOPE
SYNC
100Hz TO 1kHz
100mV TO 1V
P-P
1375/76 F16
Figure 16. Loop Stability Test Circuit
13756fd
22
Linear [ Linear ]
