LT1578IS8 データシートの表示（PDF） - Linear Technology

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LT1578IS8

1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1578/LT1578-2.5

APPLICATIONS INFORMATION

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.

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.

ADJUSTABLE

INPUT SUPPLY

SWITCHING

REGULATOR

ADJUSTABLE

DC LOAD

+

100µF TO

1000µF

RIPPLE FILTER

470Ω 4.7k

3300pF

330pF

50Ω

TO

OSCILLOSCOPE

SYNC

100Hz TO 1kHz

100mV TO 1VP-P

TO X1

OSCILLOSCOPE

PROBE

1578 F13

Figure 13. Loop Stability Test Circuit

10mV/DIV

5A/DIV

0.2ms/DIV

VOUT AT

IOUT = 500mA

BEFORE FILTER

VOUT AT

IOUT = 500mA

AFTER FILTER

VOUT AT

IOUT = 50mA

AFTER FILTER

LOAD PULSE

THROUGH 50Ω

f ≈ 780Hz

1578 F14

Figure 14. Loop Stability Check

22

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