LTC3630
13
3630fd
For more information www.linear.com/LTC3630
applicaTions inForMaTion
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate energy but generally cost more
than powdered iron core inductors with similar charac
-
teristics. The choice of which style inductor to use mainly
depends on the price versus size requirements and any
radiated field/EMI requirements. New designs for sur
face
mount inductors are available from Coiltronics, Coilcraft,
TDK, T
oko, and Sumida.
C
IN
and C
OUT
Selection
The input capacitor, C
IN
, is needed to filter the trapezoidal
current at the source of the top high side MOSFET. C
IN
should be sized to provide the energy required to charge
the inductor without causing a large decrease in input
voltage (∆V
IN
). The relationship between C
IN
and ∆V
IN
is given by:
C
IN
>
PEAK
2 • V
• ∆V
It is recommended to use a larger value for C
IN
than
calculated by the above equation since capacitance de-
creases with applied voltage. In general, a 4.7µF X7R
ceramic capacitor is a good choice for C
IN
in most LTC3630
applications.
To
minimize large ripple voltage, a low ESR input capaci-
tor sized for the maximum RMS current should be used.
RMS current is given by:
I
RMS
= I
OUT(MAX)
•
V
OUT
V
IN
•
V
IN
V
OUT
–
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT
/2. This simple worst-case condition is commonly used
for design because even significant deviations do not offer
much relief. Note that ripple current ratings from capacitor
manufacturers are often based only on 2000 hours of life
which makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design.
The output capacitor, C
OUT
, filters the inductor’s ripple
current and stores energy to satisfy the load current when
the LTC3630 is in sleep. The output ripple has a lower limit
of V
OUT
/160 due to the 5mV typical hysteresis of the feed-
back comparator. The time delay of the comparator adds
an additional ripple voltage that is a function of the load
current. During this delay time, the L
TC3630 continues to
switch and supply current to the output. The output ripple
can be approximated by:
∆V
OUT
≈
I
PEAK
2
–I
LOAD
•
4 • 10
C
OUT
+
V
OUT
160
The output ripple is a maximum at no load and approaches
lower limit of V
OUT
/160 at full load. Choose the output
capacitor C
OUT
to limit the output voltage ripple ∆V
OUT
using the following equation:
C
OUT
≥
I
PEAK
• 2 • 10
–6
∆V
OUT
–
V
OUT
The value of the output capacitor must be large enough
to accept the energy stored in the inductor without a large
change in output voltage during a single switching cycle.
Setting this voltage step equal to 1% of the output voltage,
the output capacitor must be:
C
OUT
> 50 •L •
I
PEAK
V
OUT
Typically, a capacitor that satisfies the voltage ripple re-
quirement is adequate to filter the inductor ripple. To avoid
overheating, the output capacitor must also be sized to
handle the ripple current generated by the inductor. The
worst-case ripple current
in the output capacitor is given
by I
RMS
= I
PEAK
/2. Multiple capacitors placed in parallel
may be needed to meet the ESR and RMS current handling
requirements.
Dry tantalum, special polymer, aluminum electrolytic,
and ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
