LT1374C Linear Technology, LT1374C Datasheet - Page 22

• Manufacturer Part Number: LT1374C
• Description: 4.5A/ 500kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1374
APPLICATIONS
How Do I Test Loop Stability?
The “standard” compensation for LT1374 is a 1.5nF
capacitor for C
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.
22
C
, with R
INPUT SUPPLY
ADJUSTABLE
U
C
= 0. While this compensation will
INFORMATION
U
REGULATOR
SWITCHING
W
ADJUSTABLE
DC LOAD
Figure 13. Loop Stability Test Circuit
U
+
50
100Hz TO 1kHz
100mV TO 1V
100 F TO
1000 F
TO
OSCILLOSCOPE
SYNC
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 k for R
if necessary, because as explained before, R
may require the addition of C
10mV/DIV
P-P
5A/DIV
3300pF
470
RIPPLE FILTER
Figure 14. Loop Stability Check
4.7k
330pF
1374 F13
0.2ms/DIV
TO X1
OSCILLOSCOPE
PROBE
F
to control V
C
1374 F14
. Do this only
C
C
pin ripple.
V
I
BEFORE FILTER
V
I
AFTER FILTER
V
I
AFTER FILTER
LOAD PULSE
THROUGH 50
f 780Hz
OUT
OUT
OUT
OUT
OUT
above 1k
OUT
= 500mA
= 50mA
= 500mA
AT
AT
AT