LT1374IT7(RevA) データシートの表示（PDF） - Linear Technology
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LT1374IT7 Datasheet PDF : 28 Pages
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LT1374
APPLICATIONS INFORMATION
How Do I Test Loop Stability?
The “standard” compensation for LT1374 is a 1.5nF
capacitor for CC, with RC = 0. While this compensation will
work for most applications, the “optimum” value for loop
compensation components depends, to various extent, 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
out the final design to ensure that it is “robust” and tolerant
of all these variations.
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 13. 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
to the original value, as shown in Figure 14. A well behaved
loop will settle back cleanly, whereas a loop with poor
phase or gain margin will “ring” as it settles. The number
of rings indicates the degree of stability, and the frequency
of the ringing shows the approximate unity-gain fre-
quency of the loop. Amplitude of the signal is not particu-
larly important, as long as the amplitude is not so high that
the loop behaves nonlinearly.
The output of the regulator contains both the desired low
frequency transient information and a reasonable amount
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.
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.
10mV/DIV
5A/DIV
0.2ms/DIV
VOUT AT
IOUT = 500mA
BEFORE FILTER
VOUT AT
IOUT = 500mA
AFTER FILTER
VOUT AT
IOUT = 50mA
AFTER FILTER
LOAD PULSE
THROUGH 50Ω
f ≈ 780Hz
1374 F14
Figure 14. Loop Stability Check
ADJUSTABLE
INPUT SUPPLY
SWITCHING
REGULATOR
ADJUSTABLE
DC LOAD
+
100µF TO
1000µF
RIPPLE FILTER
470Ω 4.7k
3300pF
330pF
50Ω
TO
OSCILLOSCOPE
SYNC
100Hz TO 1kHz
100mV TO 1VP-P
TO X1
OSCILLOSCOPE
PROBE
1374 F13
Figure 13. Loop Stability Test Circuit
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