LTC3568_15 Datasheet, PDF (9/18 Pages) Linear Technology – 1.8A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3568
Applications Information
Table 1. Representative Surface Mount Inductors
MANU-
FACTURER PART NUMBER
MAX DC
VALUE 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_RMS ≈ I_MAX
V_OUT (V_IN − V_OUT )
V_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:
ΔV_OUT
≈

ΔI_{L }ESR +
1^
8f_O C_{OUT }
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.
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