LTC3810-5
19
38105fd
and the bottom MOSFET must be as small as possible,
mandating that these components be placed adjacently.
The diode can be omitted if the efficiency loss is tolerable.
Input Capacitor Selection
In continuous mode, the drain current of the top MOSFET
is approximately a square wave of duty cycle V
OUT
/V
IN
which must be supplied by the input capacitor. To prevent
large input transients, a low ESR input capacitor sized for
the maximum RMS current is given by:
I
CIN(RMS)
≅
I
O(MAX)
V
OUT
V
IN
V
IN
V
OUT
– 1
⎛
⎝⎜
⎞
⎠⎟
1/2
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
O(MAX)
/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that the ripple current ratings from
capacitor manufacturers are often based on only 2000
hours of life. This makes it advisable to further derate
the capacitor or to choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be placed in parallel to meet size or height requirements
in the design.
Because tantalum and OS-CON capacitors are not available
in voltages above 30V, ceramics or aluminum electrolytics
must be used for regulators with input supplies above 30V.
Ceramic capacitors have the advantage of very low ESR
and can handle high RMS current, but ceramics with high
voltage ratings (> 50V) are not available with more than
a few microfarads of capacitance. Furthermore, ceram-
ics have high voltage coefficients which means that the
capacitance values decrease even more when used at the
rated voltage. X5R and X7R type ceramics are recom-
mended for their lower voltage and temperature coef-
ficients. Another consideration when using ceramics is
their high Q which, if not properly damped, may result in
excessive voltage stress on the power MOSFETs. Alumi-
num electrolytics have much higher bulk capacitance,
but they have higher ESR and lower RMS current ratings.
A good approach is to use a combination of aluminum
electrolytics for bulk capacitance and ceramics for low ESR
and RMS current. If the RMS current cannot be handled
by the aluminum capacitors alone, when used together,
the percentage of RMS current that will be supplied by the
aluminum capacitor is reduced to approximately:
% I
RMS,ALUM
≈
1
1
+
(8fCR
ESR
)
2
• 100%
where R
ESR
is the ESR of the aluminum capacitor and C
is the overall capacitance of the ceramic capacitors. Using
an aluminum electrolytic with a ceramic also helps damp
the high Q of the ceramic, minimizing ringing.
Output Capacitor Selection
The selection of C
OUT
is primarily determined by the ESR
required to minimize voltage ripple. The output ripple
(
D
V
OUT
) is approximately equal to:
Δ
V
OUT
≤ Δ
I
L
ESR
+
1
8fC
OUT
⎛
⎝⎜
⎞
⎠⎟
Since
D
I
L
increases with input voltage, the output ripple
is highest at maximum input voltage. ESR also has a sig-
nificant effect on the load transient response. Fast load
transitions at the output will appear as voltage across the
ESR of C
OUT
until the feedback loop in the LTC3810-5 can
change the inductor current to match the new load current
value. Typically, once the ESR requirement is satisfied the
capacitance is adequate for filtering and has the required
RMS current rating.
Manufacturers such as Nichicon, Nippon Chemi-Con
and Sanyo should be considered for high performance
throughhole capacitors. The OS-CON (organic semicon-
ductor dielectric) capacitor available from Sanyo has the
lowest product of ESR and size of any aluminum electroly-
tic at a somewhat higher price. An additional ceramic
capacitor in parallel with OS-CON capacitors is recom-
mended to reduce the effect of their lead inductance.
In surface mount applications, multiple capacitors placed
in parallel may be required to meet the ESR, RMS current
handling and load step requirements. Dry tantalum, special
polymer and aluminum electrolytic capacitors are available
in surface mount packages. Special polymer capacitors
offer very low ESR but have lower capacitance density
applicaTions inForMaTion
