LTC3350
27
3350fc
For more information
www.linear.com/LTC3350
Using the above equation, the inductor may be too large
to provide a fast enough transient response to hold up
V
OUT
when input power goes away. This occurs in cases
where the maximum V
IN
can be high (e.g. 25V) and the
backup voltage low (e.g. 6V). In these situations it would
be best to choose an inductor that is smaller resulting in
maximum peak-to-peak ripple as high as 40% of I
CHG(MAX)
.
Once the value for L is known, the type of inductor core
must be selected. Ferrite cores are recommended for their
very low core loss. Selection criteria should concentrate
on minimizing copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design current
is exceeded. This causes an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate. The saturation current
for the inductor should be at least 80% higher than the
maximum regulated current, I
CHG(MAX)
. A list of inductor
suppliers is provided in Table 3.
Table 3. Inductor Vendors
VENDOR
URL
Coilcraft
www.coilcraft.com
Murata
www.murata.com
Sumida
www.sumida.com
TDK
www.tdk.com
Toko
www.toko.com
Vishay
www.vishay.com
Würth Electronic
www.we-online.com
C
OUT
and C
CAP
Capacitance
V
OUT
serves as the input to the synchronous controller in
step-down mode and as the output in step-up (backup)
mode. If step-up mode is used, place 100µF of bulk
(aluminum electrolytic, OS-CON, POSCAP) capacitance
for every 2A of backup current desired. For 5V system
applications, 100µF per 1A of backup current is recom-
mended. In addition, a certain amount of high frequency
bypass capacitance is needed to minimize voltage ripple.
The voltage ripple in step-up mode is:
∆
V
OUT
=
1–
V
CAP
V
OUT
⎛
⎝⎜
⎞
⎠⎟
1
C
OUT
•
f
SW
+
V
OUT
V
CAP
•
R
ESR
⎡
⎣
⎢
⎤
⎦
⎥
I
OUT(BACKUP)
Maximum ripple occurs at the lowest V
CAP
that can supply
I
OUT(BACKUP)
. Multilayer ceramics are recommended for
high frequency filtering.
If step-up mode is unused, then the specification for
C
OUT
will be determined by the desired ripple voltage in
step-down mode:
∆
V
OUT
=
V
CAP
V
OUT
1–
V
CAP
V
OUT
⎛
⎝⎜
⎞
⎠⎟
I
CHG(MAX)
C
OUT
•
f
SW
+
I
CHG(MAX)
•
R
ESR
In continuous conduction mode, the source current of the
top MOSFET is a square wave of duty cycle V
CAP
/V
OUT
.
To prevent large voltage transients, a low ESR capacitor
sized for the maximum RMS current must be used. The
maximum RMS capacitor current is given by:
I
RMS
≅
I
CHG(MAX)
V
CAP
V
OUT
V
OUT
V
CAP
–1
This formula has a maximum at V
OUT
= 2V
CAP
, where
I
RMS
= I
CHG(MAX)
/2. This simple worst-case condition is
commonly used for design because even significant devia-
tions do not offer much relief.
Medium voltage (20V to 35V) ceramic, tantalum, OS-CON,
and switcher-rated electrolytic capacitors can be used as
input capacitors. Sanyo OS-CON SVP, SVPD series, Sanyo
POSCAP TQC series, or aluminum electrolytic capacitors
from Panasonic WA series or Cornel Dublilier SPV series
in parallel with a couple of high performance ceramic
capacitors can be used as an effective means of achieving
low ESR and high bulk capacitance.
V
CAP
serves as the input to the controller in step-up mode
and as the output in step-down mode. The purpose of the
V
CAP
capacitor is to filter the inductor current ripple. The
V
CAP
ripple (
∆
V
CAP
) is approximated by:
∆
V
CAP
≈ ∆
I
PP
1
8C
CAP
•
f
SW
+
R
ESR
⎛
⎝⎜
⎞
⎠⎟
where f
SW
is the switching frequency, C
CAP
is the ca-
pacitance on V
CAP
and
∆
I
PP
is the ripple current in the
inductor. The output ripple is highest at maximum input
voltage since
∆
I
PP
increases with input voltage.
applicaTions inForMaTion
