LTC3637_15 Datasheet, PDF (13/26 Pages) Linear Technology – 76V, 1A Step-Down Regulator

LTC3637
APPLICATIONS INFORMATION
depends on the price versus size requirements and any
radiated field/EMI requirements. New designs for surface
mount inductors are available from Würth, Coilcraft, TDK,
Toko, 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
>
2
L • I_PEAK2
• V_IN • ∆V_IN
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 LTC3637
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 LTC3637 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 LTC3637 continues to
switch and supply current to the output. The output ripple
can be approximated by:
∆V_OUT
≈ ^{IPE}₂^AK
–
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
requirement 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)
3637fa
For more information www.linear.com/LTC3637
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