LTC3251_15 Datasheet, PDF (11/16 Pages) Linear Technology – 500mA High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter

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OPERATIO (Refer to Block Diagram)
Further output noise reduction can be achieved by filtering
the LTC3251 output through a very small series inductor
as shown in Figure 4. A 10nH inductor will reject the fast
output transients caused by the blanking period. The 10nH
inductor can be fabricated on the PC board with about 1cm
(0.4") of 1mm wide PC board trace.
10nH
(TRACE INDUCTANCE)
V_OUT
LTC3251
10µF
1µF
V_OUT
GND
3251 F04
Figure 4. 10nH Inductor Used for
Additional Output Noise Reduction
V_IN Capacitor Selection
The dual phase architecture used by the LTC3251 family
makes input noise filtering much less demanding than
conventional charge pump regulators. The input current
should be continuous at about I_OUT/2. The blanking period
described in the V_OUT section also effects the input. For
this reason it is recommended that a low ESR, 1µF (0.4µF
min) or greater ceramic capacitor be used for C_IN (see
Ceramic Capacitor Selection Guidelines section).
In cases where the supply impedance is high, heavy output
transients can cause significant input transients. These
input transients feed back to the output which slows the
output transient recovery and increases overshoot and
output impedance. This effect can generally be avoided by
using low impedance supplies and short supply connec-
tions. If this is not possible, a ≥4.7µF capacitor is recom-
mended for the input capacitor. Aluminum and tantalum
capacitors are not recommended because of their high
ESR.
Further input noise reduction can be achieved by filtering
the input through a very small series inductor as shown in
Figure 5. A 10nH inductor will reject the fast input tran-
sients caused by the blanking period, thereby presenting
a nearly constant load to the input supply. For economy,
the 10nH inductor can be fabricated on the PC board with
about 1cm (0.4") of 1mm wide PC board trace.
LTC3251/
LTC3251-1.2/LTC3251-1.5
10nH
(TRACE INDUCTANCE)
V_IN
SUPPLY
1µF
V_IN
LTC3251
GND
3251 F05
Figure 5. 10nH Inductor Used for
Additional Input Noise Reduction
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or alumi-
num should never be used for the flying capacitors since
their voltages can reverse upon start-up of the LTC3251.
Ceramic capacitors should always be used for the flying
capacitors.
The flying capacitors control the strength of the charge
pump. In order to achieve the rated output current, it is
necessary for the flying capacitor to have at least 0.4µF of
capacitance over operating temperature with a 2V bias
(see Ceramic Capacitor Selection Guidelines). If only
200mA or less of output current is required for the
application, the flying capacitor minimum can be reduced
to 0.15µF.
Ceramic Capacitor Selection Guidelines
Capacitors of different materials lose their capacitance
with higher temperature and voltage at different rates. For
example, a ceramic capacitor made of X5R or X7R material
will retain most of its capacitance from – 40°C to 85°C,
whereas a Z5U or Y5V style capacitor will lose consider-
able capacitance over that range (60% to 80% loss typ).
Z5U and Y5V capacitors may also have a very strong
voltage coefficient, causing them to lose an additional
60% or more of their capacitance when the rated voltage
is applied. Therefore, when comparing different capaci-
tors, it is often more appropriate to compare the amount
of achievable capacitance for a given case size rather than
discussing the specified capacitance value. For example,
over rated voltage and temperature conditions, a 4.7µF,
10V, Y5V ceramic capacitor in an 0805 case may not
provide any more capacitance than a 1µF, 10V, X5R or X7R
available in the same 0805 case. In fact, over bias and
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