Datasheet

LTC3766
27
3766fa
For more information www.linear.com/LTC3766
applicaTions inForMaTion
Input Capacitor/Filter Selection
In applications with a low impedance source, or where there
the input voltage is relatively low, a simple capacitive input
filter is generally suitable. This capacitor needs to have a
very low ESR and must be rated to handle a worst-case
RMS input current of:
I
C(RMS)
=
N
S
N
P
⎛
⎝
⎜
⎞
⎠
⎟
I
OUT(MAX)
2
Note that capacitor manufacturers’ ripple current ratings
are often based on only 2000 hours of life. This makes
it advisable to further derate the capacitor, or to choose
a capacitor rated at a higher temperature than required.
Several capacitors may be paralleled to meet size or height
requirements in the design. Due to the high operating
frequency of the LTC3766, ceramic capacitors can also
be used for C
IN
. Always consult the manufacturer if there
is any question.
For higher input voltage applications, however, it can be
very costly to use bulk capacitance that is rated to handle
the required RMS current. Also, if a simple capacitor is
used as an input filter, it is hard to know exactly where
the AC input current will flow when a power supply is
placed into a larger system. To avoid these issues, an LC
filter can be used on the power supply input as shown in
Figure 10. This keeps the large AC currents contained in
relatively small and inexpensive capacitors whose RMS
current rating is known to be adequate. Choosing an LC
filter such that:
1
2π L
F
C
F
<
f
SW
5
will attenuate the AC content of the RMS input current by
a factor of approximately 5×. This greatly alleviates the
RMS current requirements of the bulk input capacitor. The
filter inductor should have a saturation current of at least:
I
SAT(LF)
≥ 1.3•
V
OUT
I
OUT(MAX)
V
IN(MIN)
In order to keep the ripple voltage at the filter output to
a reasonable level, choose a value of L
F
and C
F
that also
satisfies:
L
F
C
F
< 2.9 •
N
S
N
P
⎛
⎝
⎜
⎞
⎠
⎟
V
RIPPLE
I
OUT(MAX)
+
R
ESR
2
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
where V
RIPPLE
is the desired ripple voltage at the output
of the input filter and R
ESR
is the ESR of capacitor C
F
. A
reasonable target for V
RIPPLE
3% of nominal V
IN
.
When using an LC input filter, the output impedance of
the LC filter must never be greater in magnitude than
the input impedance looking into the power stage of the
DC/DC converter. This is necessary to avoid loop insta-
bilities. In most applications, this condition is naturally
satisfied because the ESR
of the bulk input capacitance,
C
BULK
, is high enough to lower the Q of the LC input filter,
thereby reducing the peaking in its output impedance to a
safe level. Also, using a larger value for C
F
reduces the Q,
although this can be expensive in high V
IN
applications.
In some situations, a series damping network must be
added as shown in Figure 10.
L
F
L
D
V
IN
+
V
IN
–
OPTIONAL
C
F
3766 F10
Z
OUT
Z
IN
C
BULK
R
D
MAIN
TRANSFORMER
• •
Figure 10. Input Filter with Optional Damping Network
In order to provide critical damping, choose LD and RD
according to:
L
D
=
L
F
5
andR
D
= 0.8
L
F
C
F