LT1578IS8 Linear Technology, LT1578IS8 Datasheet - Page 22

• Manufacturer Part Number: LT1578IS8
• Description: IC BUCK SYNC ADJ 1.5A 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1578IS8

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 1.21V
• Current - Output: 1.5A
• Frequency - Switching: 200kHz
• Voltage - Input: 4 ~ 15 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

LT1578/LT1578-2.5
APPLICATIONS
How Do I Test Loop Stability?
The “standard” compensation for LT1578 is a 100pF
capacitor for C
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.
22
C
, with R
INPUT SUPPLY
ADJUSTABLE
U
C
= 0. While this compensation will
INFORMATION
U
10mV/DIV
REGULATOR
SWITCHING
W
5A/DIV
ADJUSTABLE
DC LOAD
Figure 13. Loop Stability Test Circuit
Figure 14. Loop Stability Check
U
+
0.2ms/DIV
50
100Hz TO 1kHz
100mV TO 1V
100 F TO
1000 F
TO
OSCILLOSCOPE
SYNC
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.
P-P
3300pF
470
1578 F14
RIPPLE FILTER
4.7k
V
I
BEFORE FILTER
V
I
AFTER FILTER
LOAD PULSE
THROUGH 50
f 780Hz
V
I
AFTER FILTER
OUT
OUT
OUT
OUT
OUT
OUT
= 50mA
= 500mA
= 500mA
AT
AT
AT
330pF
1578 F13
TO X1
OSCILLOSCOPE
PROBE