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LT1959 Datasheet(PDF) 21 Page - Linear Technology

Part No. LT1959
Description  4.5A, 500kHz Step-Down Switching Regulator
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT1959 Datasheet(HTML) 21 Page - Linear Technology

 
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21
LT1959
APPLICATIONS INFORMATION
Figure 13. Loop Stability Test Circuit
TO
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
50
470
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
f
≈ 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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