LT1576CS8 datasheet(22/28 Pages) LINER

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LT1576/LT1576-5

APPLICATIONS INFORMATION

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.

How Do I Test Loop Stability?

The “standard” compensation for LT1576 is a 100pF

capacitor for CC, with RC = 0Ω. While this compensation

will work for most applications, the “optimum” value for

loop compensation components depends, to various ex-

tent, on parameters which are not well controlled. These

include

inductor value (

±30% due to production toler-

ance, load current and ripple current variations),

output

capacitance (

±20% to ±50% due to production tolerance,

temperature, aging and changes at the load),

output

capacitor ESR (

±200% due to production tolerance, tem-

perature 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.

0.2ms/DIV

1576 F14

10mV/DIV

VOUT AT

IOUT = 500mA

BEFORE FILTER

VOUT AT

IOUT = 500mA

AFTER FILTER

LOAD PULSE

THROUGH 50

Ω

≈ 780Hz

5A/DIV

VOUT AT

IOUT = 50mA

AFTER FILTER

Figure 14. Loop Stability Check

OSCILLOSCOPE

SYNC

ADJUSTABLE

DC LOAD

ADJUSTABLE

INPUT SUPPLY

100Hz TO 1kHz

100mV TO 1VP-P

100

µF TO

1000

µF

RIPPLE FILTER

1576 F13

TO X1

OSCILLOSCOPE

PROBE

3300pF

330pF

Ω

470

Ω

4.7k

SWITCHING

REGULATOR

Figure 13. Loop Stability Test Circuit

LT1576CS8 Datasheet(PDF) 22 Page - Linear Technology

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