LTC3100
19
3100fb
For more information www.linear.com/LTC3100
APPLICATIONS INFORMATION
Low ESR input capacitors reduce input switching noise
and reduce the peak current drawn from the battery. It
follows that ceramic capacitors are also a good choice for
input decoupling and should be located as close as pos-
sible to the device. A 2.2礔 input capacitor on the V
INBST
pin is sufficient for most applications. Larger values may
be used without limitations. For applications where the
power source is more than a few inches away, a larger
bulk decoupling capacitor is recommended on the input
to the boost converter.
Table 2 shows a list of several ceramic capacitor manu-
facturers. Consult the manufacturers directly for detailed
information on their selection of capacitors.
Note that even X5R and X7R type ceramic capacitors have
a DC bias effect which reduces their capacitance with a DC
voltage applied. This effect is particularly bad for capacitors
in the smallest case sizes. Consult the manufacturers data
for the capacitor you select to be assured of having the
necessary capacitance in your application.
Table 2.Capacitor Vendor Information
SUPPLIER
PHONE
WEB SITE
AVX
(803) 448-9411 www.avxcorp.com
Murata
(714) 852-2001 www.murata.com
Taiyo-Yuden
(408) 573-4150 www.t-yuden.com
TDK
(847) 803-6100 www.component.tdk.com
Buck Inductor Selection
The choice of buck inductor value influences both the
efficiency and the magnitude of the output voltage ripple.
Larger inductance values will reduce inductor current ripple
and will therefore lead to lower output voltage ripple. For
a fixed DC resistance, a larger value inductor will yield
higher efficiency by lowering the peak current to be closer
to the average. However, a larger value inductor within the
same family will generally have a greater series resistance,
thereby offsetting this efficiency advantage. Given a desired
peak to peak current ripple, DI
L
, the required inductance
can be calculated via the following expression, where f
represents the switching frequency in MHz:
L =
1
fDI
L
" 1
V
OUT
V
IN
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
礖
( )
A reasonable choice for ripple current is DI
L
= 100mA
which represents 40% of the maximum 250mA load
current. The DC current rating of the inductor should
be at least 450mA to avoid saturation under overload or
short-circuit conditions. To optimize efficiency the inductor
should have a low series resistance. In particularly space
restricted applications it may be advantageous to use
a much smaller value inductor at the expense of larger
ripple current. In such cases, the converter will operate
in discontinuous conduction for a wider range of output
loads and efficiency will be reduced.
In addition, there is a minimum inductor value required
to maintain stability of the current loop (given the fixed
internal slope compensation). Specifically, if the buck
converter is going to be utilized at duty cycles over 40%,
the inductance value must be at least L
MIN
as given by the
following equation:
L
MIN
= 2.5 " V
OUT
(礖)
Table 3 depicts the minimum required inductance for
several common output voltages.
Table 3.Buck Minimum Inductance
OUTPUT VOLTAGE
MINIMUM INDUCTANCE
0.6V
1.5礖
0.8V
2礖
1.2V
3礖
2V
5礖
2.7V
6.8礖
3.3V
8.3礖
Larger values of inductor will also provide slightly greater
output current capability before reaching current limit (by
reducing the peak-to-peak ripple current).
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