Datasheet

LTC3630A
13
3630afa
For more information www.linear.com/LTC3630A
applicaTions inForMaTion
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
>
L •I
PEAK
2
2 • V
IN
• ∆V
IN
It is recommended to use a larger value for C
IN
than calcu-
lated by the above equation since capacitance decreases
with
applied voltage. In general, a 4.7µF X7R ceramic
capacitor is a good choice for C
IN
in most LTC3630A
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
–
1
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 LTC3630A is in sleep. The output ripple has a lower
limit of V
OUT
/160 due to the 5mV typical hysteresis of the
feedback 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 LTC3630A
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
–6
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
160
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
⎛
⎝
⎜
⎞
⎠
⎟
2
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.
Tantalum capacitors have the highest capacitance density
but it is important only to use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR but
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and long-
term reliability. Ceramic capacitors have excellent low ESR
characteristics but can have high voltage coefficient and
audible piezoelectric effects. The high quality factor (Q)
of ceramic capacitors in series with trace inductance can
also lead to significant input voltage ringing.