Datasheet

LTC3568
9
3568fa
applicaTions inForMaTion
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER PA
RT NUMBER VALUE
MAX DC
CURRENT DCR HEIGHT
Toko A914BYW-2R2M (D52LC) 2.2µH 2.05A
49mΩ
2mm
Toko A915Y-2R0M (D53LC-A) 2µH 3.3A
22mΩ
3mm
Toko A918CY-2R0M (D62LCB) 2µH 2.33A
24mΩ
2mm
Coilcraft D01608C-222 2.2µH 2.3A
70mΩ
3mm
Sumida CDRH2D18/HP1R7 1.7µH 1.8A
35mΩ
2mm
Sumida CDRH4D282R2 2.2µH 2.04A
23mΩ
3mm
Sumida CDC5D232R2 2.2µH 2.16A
30mΩ
2.5mm
TDK VLCF4020T-1R8N1R9 1.8µH 1.97A
46mΩ
2mm
Taiyo Yuden N06DB2R2M 2.2µH 3.2A
29mΩ
3.2mm
Taiyo Yuden N05DB2R2M 2.2µH 2.9A
32mΩ
2.8mm
Cooper SD14-2R0 2µH 2.37A
45mΩ
1.45mm
Catch Diode Selection
A catch diode is not necessary.
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 maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
I I
V V V
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
– ΔI
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 cur-
rent ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
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 recommended 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 (ΔV
OUT
) is
determined by:
Δ ≈ Δ +






V I ESR
f C
OUT L
O OUT
1
8
where f = operating frequency, C
OUT
= output capacitance
and ΔI
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI
L
increases
with input voltage. With ΔI
L
= 0.4 • I
OUT
the output ripple
will be less than 100mV at maximum V
IN
and f
O
= 1MHz
with:
ESRC
OUT
< 130mΩ
Once the ESR requirements for C
OUT
have been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantulum
capacitors are all available in surface mount packages.
The OS-CON semiconductor dielectric capacitor avail-
able from Sanyo has the lowest ESR(size) product of any
aluminum electrolytic at a somewhat higher price. Special
polymer capacitors, such as Sanyo POSCAP, offer very
low ESR, but have a lower capacitance density than other
types. Tantalum capacitors have the highest capacitance
density, but it has a larger ESR and it is critical that the
capacitors are surge tested for use in switching power
supplies. An excellent choice is the AVX TPS series of
surface mount tantalums, avalable in case heights ranging
from 2mm to 4mm. Aluminum electrolytic capacitors have
a significantly larger ESR, and is often used in extremely
cost-sensitive applications provided that consideration
is given to ripple current ratings and long term reliability.