參數(shù)資料
型號(hào): LT1578CS8
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: CAP 47UF 50V ELECT TG SMD
中文描述: 3.5 A SWITCHING REGULATOR, 240 kHz SWITCHING FREQ-MAX, PDSO8
封裝: 0.150 INCH, PLASTIC, SO-8
文件頁(yè)數(shù): 22/28頁(yè)
文件大?。?/td> 286K
代理商: LT1578CS8
22
LT1578/LT1578-2.5
APPLICATIO
S I
FOR
ATIO
U
How Do I Test Loop Stability
The “standard” compensation for LT1578 is a 100pF
capacitor for C
C
, with R
C
= 0. While this compensation will
work for most applications, the “optimum” value for loop
compensation components depends, to various extents,
on parameters which are not well controlled. These in-
clude inductor value (
±
30% due to production tolerance,
load current and ripple current variations), output capaci-
tance (
±
20% to
±
50% due to production tolerance,
temperature, 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.
W
U
U
One way to check switching regulator loop stability is by
pulse loading the regulator output while observing the
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 frequency of the loop. Amplitude
of the signal is not particularly important, as long as the
amplitude is not so high that the loop behaves nonlinearly.
0.2ms/DIV
1578 F14
10mV/DIV
V
OUT
AT
I
= 500mA
BEFORE FILTER
V
OUT
AT
I
= 500mA
AFTER FILTER
V
OUT
AT
I
= 50mA
AFTER FILTER
LOAD PULSE
THROUGH 50
f
780Hz
5A/DIV
Figure 14. Loop Stability Check
TO
OSCILLOSCOPE
SYNC
100Hz TO 1kHz
100mV TO 1V
P-P
ADJUSTABLE
DC LOAD
ADJUSTABLE
INPUT SUPPLY
100
μ
F TO
1000
μ
F
RIPPLE FILTER
1578 F13
TO X1
OSCILLOSCOPE
PROBE
3300pF
330pF
50
470
4.7k
SWITCHING
REGULATOR
+
Figure 13. Loop Stability Test Circuit
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