LT1534/LT1534-1
APPLICATIONS INFORMATION
where
鈭咺
is the ripple current in the switch, R
CSL
and
R
VSL
are the slew resistors and f
OSC
is the oscillator
frequency.
Power dissipation P
D
is the sum of these three terms. Die
junction temperature is then computed as:
T
J
= T
AMB
+ (P
D
)(胃
JA
)
where T
AMB
is ambient temperature and
胃
JA
is the package
thermal resistance. For the 16-pin SO with fused leads the
胃
JA
is 50擄C/W.
For example, with f
OSC
= 40kHz, 0.4A average current and
0.1A of ripple, the maximum duty cycle is 88%. Assume
slew resistors are both 17k and V
SAT
is 0.26V, then:
P
D
= 0.176W + 0.094W + 0.158W = 0.429W
In an S16 fused lead package the die junction temperature
would be 21擄C above ambient.
Frequency Compensation
Loop frequency compensation is accomplished by way of
a series RC network on the output of the error amplifier (V
C
pin). Referring to Figure 4, the main pole is formed by
capacitor C
VC
and the output impedance of the error
amplifier (approximately 400k鈩?. The series resistor R
VC
creates a 鈥渮ero鈥?which improves loop stability and tran-
sient response. A second capacitor C
VC2
, typically one-
tenth the size of the main compensation capacitor, is
sometimes used to reduce the switching frequency ripple
on the V
C
pin. V
C
pin ripple is caused by output voltage
ripple attenuated by the output divider and multiplied by
the error amplifier. Without the second capacitor, V
C
pin
ripple is:
poor quality (high ESR) output capacitors are used. The
addition of a 0.0047碌F capacitor on the V
C
pin reduces
switching frequency ripple to only a few millivolts. A low
value for R
VC
will also reduce V
C
pin ripple, but loop phase
margin may be inadequate.
R
VC
2k
C
VC
0.01碌F
V
C
PIN
C
VC2
4.7nF
(
1
.
25
)(
V
RIPPLE
)(
g
m
)(
R
VC
)
V
CPIN RIPPLE
=
V
OUT
where V
RIPPLE
= Output ripple (V
P-P
)
g
m
= Error amplifier transconductance
R
VC
= Series resistor on V
C
pin
V
OUT
= DC output voltage
To prevent irregular switching, V
C
pin ripple should be
kept below 50mV
P-P
. Worst-case V
C
pin ripple occurs at
maximum output load current and will also be increased if
U
W
U
U
1534 F03
Figure 4
Capacitors
While the IC reduces the source of switcher noise, it is
essential for the lowest noise, that the filter capacitors
should have low parasitic impedance. Sanyo OS-CON,
Panasonic Specialty Polymer and tantalum capacitors are
the preferred types. Aluminum electrolytics are not suit-
able for this application. In general, ESR is more critical
than capacitance. At higher frequencies, ESL can also be
important. Paralleling capacitors can reduce both ESR and
ESL.
Design Note 95 offers more information about capacitor
selection. The following is a brief summary:
Solid tantalum capacitors have small size and low
impedance. Typically they are available for voltages
below 50V. They may have a problem with surge
currents (AVX TPS line addresses this issue).
OS-CON capacitors have very low impedance but are
only available for 25V or less. Form factor may be a
problem. Sometimes their very low ESR can cause loop
stability problems.
Ceramic capacitors are generally used for high fre-
quency and high voltage bypass. They too can have
such a low ESR as to cause loop stability problems.
Often they can resonate with their ESL before ESR
becomes effective.
Specialty Polymer Aluminum: Panasonic has come out
with their series CD capacitors. While they are only
available for voltages below 16V, they have very low
ESR and good surge capability.
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