Datasheet

LT3509
16
3509fc
You can estimate output ripple with the following
equations.
For ceramic capacitors where low capacitance value is
more signifi cant than ESR:
V
RIPPLE
= ΔI
L
/(8 •f
SW
•C
OUT
)
For electrolytic capacitors where ESR is high relative to
capacitive reactance:
V
RIPPLE
= ΔI
L
•ESR
where ΔI
L
is the peak-to-peak ripple current in the inductor.
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
I
C(RMS)
= ΔI
L
/12
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor transfers to the output, the resulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
CLIV
OUT LIM OUT
> 10
2
••( / )
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3509 applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coeffi cients. In particular, Y5V
APPLICATIONS INFORMATION
75kR
C
COMP-
NODE
R1
R2
1.73M
95pF
V
IN
V
REF
= 0.8V
C
PL
260µS
LT3509
1.1S
V
OUT
3509 F10
+
–
C
OUT
Figure 10. Small-Signal Equivalent Circuit
or from R
C
. This model works well as long as the inductor
current ripple is not too low (ΔI
RIPPLE
> 5% I
OUT
) and the
loop crossover frequency is less than f
SW
/5. An optional
phase lead capacitor (CPL) across the feedback divider
may improve the transient response.
Output Capacitor Selection
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 stabilize the LT3509’s
control loop. Because the LT3509 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.