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LT1578/LT1578-2.5
APPLICATIONS INFORMATION
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The output of the regulator contains both the desired low
frequency transient information and a reasonable amount
of high frequency (200kHz) 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, start
varying load current and input voltage to see if you 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 kΩ for R
C
. Do this only
if necessary, because as explained before, R
C
above 1k
may require the addition of C
F
to control V
C
pin ripple.
If everything looks OK, 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. Also, try varying com-
ponent values by a factor of 2 and see if the behavior is still
acceptable. Double and halve the values of R
C
and C
C
and
output capacitors. If the regulator still works correctly, it
will likely be good in production.
A possible exception to the “clean response” rule is at very
light loads, as evidenced in Figure 14 with I
LOAD
= 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.
POSITIVE-TO-NEGATIVE CONVERTER
The circuit in Figure 15 is a classic positive-to-negative
topology using a grounded inductor. It differs from the
standard approach in the way the IC chip derives its
feedback signal. Because the LT1578 accepts only posi-
tive feedback signals, the ground pin must be tied to the
regulated negative output. A resistor divider to ground or,
in this case, the sense pin, then provides the proper
feedback voltage for the chip.
Figure 15. Positive-to-Negative Converter
OUTPUT**
–5V, 0.5A
INPUT
5.5V TO
15V
1578 F15
C2
0.33µF
C
C
R
C
D2
1N5818
C1
100µF
10V TANT
×2
R1
15.8k
R2
4.99k
C3
10µF TO
50µF
D1
1N4148
L1*
15µH
BOOST
LT1578
V
IN
V
SW
FB
GND
V
C
* INCREASE L1 TO 30µH OR 60µH FOR HIGHER CURRENT APPLICATIONS.
SEE APPLICATIONS INFORMATION
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
+
+
Inverting regulators differ from buck regulators in the
basic switching network. Current is delivered to the output
as
square waves with a peak-to-peak amplitude much
greater than load current
. This means that
maximum load
current will be significantly less than the LT1578’s 1.5A
maximum switch current, even with large inductor values
.
The buck converter in comparison, delivers current to the
output as a triangular wave superimposed on a DC level
equal to load current, and load current can approach 1.5A
