Datasheet

LTC3411
9
sn3411 3411fs
APPLICATIO S I FOR ATIO
WUU
U
typical surface mount inductors that work well in LTC3411
applications.
Table 1. Representative Surface Mount Inductors
MANU- MAX DC
FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Toko A914BYW-2R2M-D52LC 2.2µH 2.05A 49mΩ 2mm
Toko A915AY-2ROM-D53LC 2µH 3.3A 22mΩ 3mm
Coilcraft D01608C-222 2.2µH 2.3A 70mΩ 3mm
Coilcraft LP01704-222M 2.2µH 2.4A 120mΩ 1mm
Sumida CDRH4D282R2 2.2µH 2.04A 23mΩ 3mm
Sumida CDC5D232R2 2.2µH 2.16A 30mΩ 2.5mm
Taiyo Yuden N06DB2R2M 2.2µH 3.2A 29mΩ 3.2mm
Taiyo Yuden N05DB2R2M 2.2µH 2.9A 32mΩ 2.8mm
Murata LQN6C2R2M04 2.2µH 3.2A 24mΩ 5mm
Catch Diode Selection
Although unnecessary in most applications, a small im-
provement in efficiency can be obtained in a few applica-
tions by including the optional diode D1 shown in Figure␣ 5,
which conducts when the synchronous switch is off.
When using Burst Mode operation or pulse skip mode, the
synchronous switch is turned off at a low current and the
remaining current will be carried by the optional diode. It
is important to adequately specify the diode peak current
and average power dissipation so as not to exceed the
diode ratings. The main problem with Schottky diodes is
that their parasitic capacitance reduces the efficiency,
usually negating the possible benefits for LTC3411 cir-
cuits. Another problem that a Schottky diode can intro-
duce is higher leakage current at high temperatures, which
could reduce the low current efficiency.
Remember to keep lead lengths short and observe proper
grounding (see Board Layout Considerations) to avoid
ringing and increased dissipation when using a catch
diode.
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is
a square wave with a duty cycle of approximately V
OUT
/
V
IN
. To prevent large voltage transients, a low equivalent
series resistance (ESR) input capacitor sized for the maxi-
mum RMS current must be used. The maximum RMS
capacitor current is given by:
II
VVV
V
RMS MAX
OUT IN OUT
IN
≈
−()
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– ∆I
L
/2.
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst case is commonly used
to design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple
current ratings are often based on only 2000 hours life-
time. This makes it advisable to further derate the capaci-
tor, or choose a capacitor rated at a higher temperature
than required. Several capacitors may also be paralleled to
meet the size or height requirements of the design. An
additional 0.1µF to 1µF ceramic capacitor is also recom-
mended on V
IN
for high frequency decoupling, when not
using an all ceramic capacitor solution.
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance is
adequate for filtering. The output ripple (∆V
OUT
) is deter-
mined by:
∆≈∆ +






V I ESR
fC
OUT L
O OUT
1
8
where f = operating frequency, C
OUT
= output capacitance
and ∆I
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆I
L
increases
with input voltage. With ∆I
L
= 0.3 • I
LIM
the output ripple
will be less than 100mV at maximum V
IN
and f
O
= 1MHz
with:
ESRC
OUT
< 150mΩ