LT1375CS8 datasheet(22/28 Pages) LINER

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LT1375/LT1376

APPLICATIONS INFORMATION

How Do I Test Loop Stability?

The “standard” compensation for LT1376 is a 3.3nF

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

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

compensation components depends, to various extent, on

parameters which are not well controlled. These include

inductor value (

±30% due to production tolerance, load

current and ripple current variations),

output capacitance

(

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

ture, 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.

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 16. 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 17. 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.

The output of the regulator contains both the desired low

frequency transient information and a reasonable amount

of high frequency (500kHz) ripple. The ripple makes it

difficult to observe the small transient, so a two-pole,

100kHz filter has been added. This filter is not particularly

critical; even if it attenuated the transient signal slightly,

this wouldn’t matter because amplitude is not critical.

After verifying that the setup is working correctly, I start

varying load current and input voltage to see if I can find

any combination that makes the transient response look

suspiciously “ringy.” This procedure may lead to an ad-

justment for best loop stability or faster loop transient

response. Nearly always you will find that loop response

looks better if you add in several k

Ω for RC. Do this only

if necessary, because as explained before, RC above 1k

may require the addition of CF to control VC pin ripple. If

everything looks OK, I use a heat gun and cold spray on the

circuit (especially the output capacitor) to bring out any

temperature-dependent characteristics.

Figure 16. Loop Stability Test Circuit

OSCILLOSCOPE

SYNC

ADJUSTABLE

DC LOAD

ADJUSTABLE

INPUT SUPPLY

100Hz TO 1kHz

100mV TO 1VP-P

100

µF TO

1000

µF

RIPPLE FILTER

1375/76 F16

TO X1

OSCILLOSCOPE

PROBE

3300pF

330pF

Ω

470

Ω

4.7k

SWITCHING

REGULATOR

0.2ms/DIV

1375/76 F17

10mV/DIV

VOUT AT IOUT =

500mA

BEFORE FILTER

VOUT AT IOUT =

500mA

AFTER FILTER

VOUT AT IOUT = 50mA

AFTER FILTER

LOAD PULSE

THROUGH 50

Ω

≈ 780Hz

5A/DIV

Figure 17. Loop Stability Check

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