LT1959CS8 datasheet(21/24 Pages) LINER

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LT1959

APPLICATIONS INFORMATION

Figure 13. 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

1595 F13

TO X1

OSCILLOSCOPE

PROBE

3300pF

330pF

Ω

470

Ω

4.7k

SWITCHING

REGULATOR

0.2ms/DIV

1375/76 F14

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 14. Loop Stability Check

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

adjustment 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.

Keep in mind that this procedure does not take initial

component tolerance into account. You should see fairly

clean response under all load and line conditions to ensure

that component variations will not cause problems. One

note here: according to Murphy, the component most

likely to be changed in production is the output capacitor,

because that is the component most likely to have manu-

facturer variations (in ESR) large enough to cause prob-

lems. It would be a wise move to lock down the sources of

the output capacitor in production.

A possible exception to the “clean response” rule is at very

light loads, as evidenced in Figure 14 with ILOAD = 50mA.

Switching regulators tend to have dramatic shifts in loop

response at very light loads, mostly because the inductor

current becomes discontinuous. One common result is very

slow but stable characteristics. A second possibility is low

phase margin, as evidenced by ringing at the output with

transients. The good news is that the low phase margin at

light loads is not particularly sensitive to component varia-

tion, so if it looks reasonable under a transient test, it will

probably not be a problem in production. Note that

fre-

quency of the light load ringing may vary with component

tolerance but phase margin generally hangs in there.

CURRENT SHARING MULTIPHASE SUPPLY

The circuit in Figure 15 uses multiple LT1959s to produce

a 2.5V, 12A power supply. There are several advantages to

using a multiple switcher approach compared to a single

larger switcher. The inductor size is considerably reduced.

Three 4A inductors store less energy (LI2/2) than one 12A

coil so are far smaller. In addition, synchronizing three

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