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LTC3568
3568fa
applicaTions inForMaTion
Table1.RepresentativeSurfaceMountInductors
MANU-
FACTURER PARTNUMBER
VALUE
MAXDC
CURRENT DCR HEIGHT
Toko
A914BYW-2R2M (D52LC) 2.2H
2.05A 49mΩ 2mm
Toko
A915Y-2R0M (D53LC-A)
2H
3.3A
22m
Ω 3mm
Toko
A918CY-2R0M (D62LCB) 2H
2.33A 24mΩ 2mm
Coilcraft
D01608C-222
2.2H
2.3A
70m
Ω 3mm
Sumida
CDRH2D18/HP1R7
1.7H
1.8A
35m
Ω 2mm
Sumida
CDRH4D282R2
2.2H
2.04A 23mΩ 3mm
Sumida
CDC5D232R2
2.2H
2.16A 30mΩ 2.5mm
TDK
VLCF4020T-1R8N1R9
1.8H
1.97A 46mΩ 2mm
Taiyo Yuden N06DB2R2M
2.2H
3.2A
29m
Ω 3.2mm
Taiyo Yuden N05DB2R2M
2.2H
2.9A
32m
Ω 2.8mm
Cooper
SD14-2R0
2H
2.37A 45mΩ 1.45mm
CatchDiodeSelection
A catch diode is not necessary.
InputCapacitor(CIN)Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately VOUT/VIN.
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
V
RMS
MAX
OUT IN
OUT
IN
≈
(
)
where the maximum average output current IMAX equals
the peak current minus half the peak-to-peak ripple cur-
rent, IMAX = ILIM – ΔIL/2.
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/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.Severalcapacitorsmayalsobeparalleledtomeet
thesizeorheightrequirementsofthedesign.Anadditional
0.1F to 1F ceramic capacitor is also recommended on
VIN for high frequency decoupling, when not using an all
ceramic capacitor solution.
OutputCapacitor(COUT)Selection
The selection of COUT 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 (
ΔVOUT) is
determined by:
Δ
≈ Δ
+
V
I ESR
f C
OUT
L
O OUT
1
8
where f = operating frequency, COUT = output capacitance
and
ΔIL = ripple current in the inductor. The output ripple
is highest at maximum input voltage since
ΔIL increases
with input voltage. With
ΔIL = 0.4 IOUT the output ripple
will be less than 100mV at maximum VIN and fO = 1MHz
with:
ESRCOUT < 130mΩ
Once the ESR requirements for COUT have been met, the
RMS current rating generally far exceeds the IRIPPLE(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
currenthandlingrequirementoftheapplication.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
surfacemounttantalums,avalableincaseheightsranging
from2mmto4mm.Aluminumelectrolyticcapacitorshave
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.