LT3510
16
3510fe
input voltage and damaging the LT3510. 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
Typically step-down regulators are easily compensated with
an output crossover frequency that is 1/10 of the switch-
ing frequency. This means that the time that the output
capacitor must supply the output load during a transient
step is ~2 or 3 switching periods. With an allowable 5%
drop in output voltage during the step, a good starting
value for the output capacitor can be expressed by:
C
VOUT
=
Max Load Step
Frequency • 0.05 • V
OUT
Example:
V
OUT
= 3.3V, Frequency = 1MHz, Max Load Step = 2A
C
VOUT
=
2
1e6 • 0.05 •3.3V
= 12μF
The calculated value is only a suggested starting value.
Increase the value if transient response needs improvement
or reduce the capacitance if size is a priority.
The output capacitor fi lters the inductor current to generate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and to stabilize the LT3510’s
control loop. The switching frequency of the LT3510 deter-
mines the value of output capacitance required. 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
= ∆I
L
/(8f C
OUT
) for ceramic capacitors,
and
V
RIPPLE
= ∆I
L
ESR for electrolytic capacitors (tantalum
and aluminum)
where ∆I
L
is the peak-to-peak ripple current in the
inductor.
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
to the regulation voltage. For a 5% overshoot, this require-
ment becomes:
C
OUT
> 10 L
I
LIM
V
OUT
⎛
⎝
⎜
⎞
⎠
⎟
2
Finally, there must be enough capacitance for good transient
performance. The last equation gives a good starting point.
Alternatively, you can start with one of the designs in this
data sheet and experiment to get the desired performance.
This topic is covered more thoroughly in the section on
loop compensation.
The high performance (low ESR), small size and robustness
of ceramic capacitors make them the preferred type for
LT3510 applications. However, all ceramic capacitors are
not the same. As mentioned above, many of the high value
capacitors use poor dielectrics with high temperature and
voltage coeffi cients. In particular, Y5V and Z5U types lose
a large fraction of their capacitance with applied voltage
and temperature extremes. Because the loop stability and
transient response depend on the value of C
OUT
, you may
not be able to tolerate this loss. Use X7R and X5R types.
You can also use electrolytic capacitors. The ESRs of most
aluminum electrolytics are too large to deliver low output
ripple. Tantalum and newer, lower ESR organic electrolytic
capacitors intended for power supply use, are suitable
and the manufacturers will specify the ESR. The choice of
capacitor value will be based on the ESR required for low
ripple. Because the volume of the capacitor determines
its ESR, both the size and the value will be larger than a
ceramic capacitor that would give you similar ripple per-
formance. One benefi t is that the larger capacitance may
give better transient response for large changes in load
current. Table 2 lists several capacitor vendors.
APPLICATIONS INFORMATION
