LTC3552 Datasheet, PDF (15/24 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

LTC3552
APPLICATIO S I FOR ATIO
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_CC.
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:
( ) _{IRMS ≈ IMAX}
V_OUT V_CC − V_OUT
V_CC
where the maximum average output current I_MAX equals
the peak current minus 1/2 the peak-to-peak ripple cur-
rent, I_MAX = I_LIM – ΔI_L/2. This formula has a maximum at
V_CC = 2 • V_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 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_CC 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 ripple voltage 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
⎛
1⎞
∆V_OUT ≈ ∆I_{L ⎝⎜}ESR + _{8fOCOUT ⎠⎟}
where f_O = 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 = 0.3 • I_OUT(MAX) the output
ripple will be less than 100mV at maximum V_CC and f_O =
2.25MHz with ESR_COUT < 150mΩ.
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 cur-
rent handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and solid tantalum
capacitors are all available in surface mount packages.
The OSCON 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,
Panasonic Special Polymer (SP), and Kemet A700, of-
fer very low ESR, but have a lower capacitance density
than other types. Tantalum capacitors have the highest
capacitance density, but they have 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, available in case
heights ranging from 2mm to 4mm. Aluminum electrolytic
capacitors have a significantly larger ESR, and are often
used in extremely cost-sensitive applications provided
that consideration is given to ripple current ratings and
long term reliability. Ceramic capacitors have the low-
est ESR and cost, but also have the lowest capacitance
density, a high voltage and temperature coefficient, and
exhibit audible piezoelectric effects. In addition, the high
Q of ceramic capacitors along with trace inductance can
lead to significant ringing. In most cases, 0.1µF to 1µF of
X5R dielectric ceramic capacitors should also be placed
close to the LTC3552 in parallel with the main capacitors
for high frequency decoupling.
3552f
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