LT3506/LT3506A
10
3506afc
tor must have low impedance at the switching frequency
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, cal-
culating 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:
II
VVV
V
I
INRMS OUT
OUT IN OUT
IN
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 ~1.6A, RMS ripple
current will always be less than 0.8A.
The high frequency of the LT3506 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is less than 22F (less than 10F
for the LT3506A). The combination of small size and low
impedance (low equivalent series resistance or ESR) of
ceramic capacitors makes 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 tem-
perature 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 bypass 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 1F ceramic capacitor in parallel with a low ESR tantalum
capacitor. For the electrolytic capacitor, a value larger than
22F (10F for the LT3506A) will be required to meet the
ESR and ripple current requirements. 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 1F ceramic as close as possible to the V
IN
and GND
pins on the IC for optimal noise immunity.
A fi nal caution is in order 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 plug-
ging the circuit into a live power source) this tank can ring,
doubling the input voltage and damaging the LT3506. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details, see Application Note 88.
Output Capacitor Selection
The output capacitor fi lters the inductor current to gen-
erate an output with low voltage ripple. It also stores
energy in order satisfy transient loads and to stabilize the
LT3506’s control loop. Because the LT3506 operates at a
high frequency, you don’t need much output capacitance.
Also, the current mode control loop doesn’t require the
presence of output capacitor series resistance (ESR). For
these reasons, you are free to use ceramic capacitors to
achieve very low output ripple and small circuit size.
Estimate output ripple with the following equations:
V
RIPPLE
= ΔI
L
/(8 • f • C
OUT
) for ceramic capacitors
V
RIPPLE
= ΔI
L
• ESR for electrolytic capacitors (tantalum
and aluminum)
where ΔI
L
is the peak-to-peak ripple current in the induc-
tor. The RMS content of this ripple is very low, and the
RMS current rating of the output capacitor is usually not
of concern.
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor is transferred to the output, you
would like the resulting voltage step to be small compared
APPLICATIONS INFORMATION
