Datasheet

LT3507A
12
3507af
To maintain output regulation, this peak current must be
less than the LT3507A’s switch current limit, I
LIM
. For SW1,
I
LIM
is typically 5.1A at low duty cycles and decreases
linearly to 3.7A at DC = 0.8. For SW2 and SW3, I
LIM
is
typically 3.3A for at low duty cycles and decreases linearly
to 2.4A at DC = 0.8.
The minimum inductance can now be calculated as:
L
MIN
=
1
−
DC
MIN
2 • f
•
V
OUT
+
V
F
I
LIM
–I
OUT
However, it’s generally better to use an inductor larger
than the minimum value. The minimum inductor has large
ripple currents which increase core losses and require
large output capacitors to keep output voltage ripple low.
Select an inductor greater than L
MIN
that keeps the ripple
current below 30% of I
LIM
.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should
be greater than I
LPK
. For highest efficiency, the series
resistance (DCR) should be less than 0.1Ω. Table 2 lists
several vendors and series that are suitable.
Table 2. Inductors
MANUFACTURER SERIES
INDUCTANCE
RANGE
CURRENT
RANGE
Würth WE-HC 1µH to 6.5µH 6A to 15A
Coilcraft XAL40xx 0.22µH to 15µH 3.3A to 21.5A
Sumida CDRH103R 0.8µH to 10µH 2.8A to 8.3A
TDK VLF 2.2µH to 10µH 3.8A to 7.7A
Vishay IHLP-2525CZ-11 1µH to 10µH 2.5A to 9.5A
This analysis is valid for continuous mode operation
(I
OUT
> I
LIM
/2). For details of maximum output current in
discontinuous mode operation, see Linear Technology’s
Application Note AN44. Finally, for duty cycles greater
than 50% (V
OUT
/V
IN
> 0.5), a minimum inductance is
required to avoid subharmonic oscillations. This minimum
inductance is:
SW1:L
MIN
= V
OUT
+ V
F
( )
•
0.4
f
SW
SW2, SW3:L
MIN
= V
OUT
+ V
F
( )
•
0.75
f
SW
with L
MIN
in µH and f
SW
in MHz.
Output Capacitor Selection
The output capacitor filters the inductor current to generate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilize the LT3507A’s
control loop. Because the LT3507A 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.
You can estimate output ripple with the following
equations:
V
RIPPLE
=
ΔI
L
8 • f • C
OUT
for ceramic capacitors
and
V
RIPPLE
= ΔI
L
• ESR for electrolytic capacitors
(tantalum and aluminum)
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:
C
OUT
> 10 • L •
I
LIM
V
OUT






2
The low ESR and small size of ceramic capacitors make
them the preferred type for LT3507A applications. Not all
ceramic capacitors are the same, however. Many of the
higher value capacitors use poor dielectrics with high
temperature and voltage coefficients. In particular, Y5V and
Z5U types lose a large fraction of their capacitance with
applied voltage and at temperature extremes. Because loop
stability and transient response depend on the value of C
OUT
,
this loss may be unacceptable. Use X7R and X5R types.
APPLICATIONS INFORMATION