LT3510
15
3510fe
voltages and output ripple currents. Also, low inductance
may result in discontinuous mode operation, which is
okay, but further reduces maximum load current. For
details of maximum output current and discontinuous
mode operation, see Linear Technology Application Note
44. Finally, for duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), there is a minimum inductance required to avoid
subharmonic oscillations. See Application Note 19 for
more information.
Input Capacitor Selection
Bypass the inputs of the LT3510 circuit with a 4.7μF or
higher ceramic capacitor of X7R or X5R type. A lower
value or a less expensive Y5V type can be used if there
is additional bypassing provided by bulk electrolytic or
tantalum capacitors. The following paragraphs describe
the input capacitor considerations in more detail.
Step-down regulators draw current from the input supply in
pulses with very fast rise and fall times. The input capaci-
tor is required to reduce the resulting voltage ripple at the
LT3510 and to force this very high frequency switching
current into a tight local loop, minimizing EMI. The input
capacitor must have low impedance at the switching fre-
quency to do this effectively, and it must have an adequate
ripple current rating. With two switchers operating at the
same frequency but with different phases and duty cycles,
calculating the input capacitor RMS current is not simple.
However, a conservative value is the RMS input current for
the channel that is delivering most power (V
OUT
• I
OUT
).
This is given by:
I
CIN(RMS)
=
I
OUT
V
OUT
•V
IN
–V
OUT
()
V
IN
<
I
OUT
2
and is largest when V
IN
= 2V
OUT
(50% duty cycle). As
the second, lower power channel draws input current,
the input capacitor’s RMS current actually decreases as
the out-of-phase current cancels the current drawn by the
higher power channel. Considering that the maximum load
current from a single channel is ~2A, RMS ripple current
will always be less than 1A.
The frequency, V
IN
to V
OUT
ratio, and maximum load cur-
rent requirement of the LT3510 along with the input supply
source impedance, determine the energy storage require-
ments of the input capacitor. Determine the worst-case
condition for input ripple current and then size the input
capacitor such that it reduces input voltage ripple to an
acceptable level. Typical values for input capacitors run
from 10μF at low frequencies to 2.2μF at higher frequencies.
The combination of small size and low impedance (low
equivalent series resistance or ESR) of ceramic capacitors
make them the preferred choice. The low ESR results in
very low voltage ripple and the capacitors can handle plenty
of ripple current. They are also comparatively robust and
can be used in this application at their rated voltage. X5R
and X7R types are stable over temperature and applied
voltage, and give dependable service. Other types (Y5V and
Z5U) have very large temperature and voltage coeffi cients
of capacitance, so they may have only a small fraction of
their nominal capacitance in your application. While they
will still handle the RMS ripple current, the input voltage
ripple may become fairly large, and the ripple current may
end up fl owing from your input supply or from other by-
pass capacitors in your system, as opposed to being fully
sourced from the local input capacitor. An alternative to a
high value ceramic capacitor is a lower value along with
a larger electrolytic capacitor, for example a 1μF ceramic
capacitor in parallel with a low ESR tantalum capacitor.
For the electrolytic capacitor, a value larger than 10μF will
be required to meet the ESR and ripple current require-
ments. Because the input capacitor is likely to see high
surge currents when the input source is applied, tantalum
capacitors should be surge rated. The manufacturer may
also recommend operation below the rated voltage of the
capacitor. Be sure to place the 1μF ceramic as close as
possible to the V
IN
and GND pins on the IC for optimal
noise immunity.
When the LT3510’s input supplies are operated at different
input voltages, an input capacitor sized for that channel
should be placed as close as possible to the respective
V
IN
pins.
A fi nal caution regarding the use of ceramic capacitors
at the input. A ceramic input capacitor can combine with
stray inductance to form a resonant tank circuit. If power
is applied quickly (for example by plugging the circuit
into a live power source) this tank can ring, doubling the
APPLICATIONS INFORMATION
