LT1578CS8 datasheet(22/28 Pages) LINER

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LT1578/LT1578-2.5

APPLICATIONS INFORMATION

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.

How Do I Test Loop Stability?

The “standard” compensation for LT1578 is a 100pF

capacitor for CC, with RC = 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.

0.2ms/DIV

1578 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

1578 F13

TO X1

OSCILLOSCOPE

PROBE

3300pF

330pF

Ω

470

Ω

4.7k

SWITCHING

REGULATOR

Figure 13. Loop Stability Test Circuit

LT1578CS8 Datasheet(PDF) 22 Page - Linear Technology

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