LTC3630A Datasheet, PDF (13/24 Pages) Linear Technology – High Efficiency, 76V 500mA Synchronous Step-Down Converter

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
>
2
L • I_PEAK2
• 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 – 1}
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 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:
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_{O U T}
–
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.
∆V_OUT
≈ ^{IPE}₂^AK
–
I_LOAD


•
4 • 10^–6
C_OUT
+
V_OUT
160
For more information www.linear.com/LTC3630A
3630af
13