LTC3252 Datasheet, PDF (9/12 Pages) Linear Technology – Dual, Low Noise, Inductorless Step-Down DC/DC Converter

U
OPERATIO (Refer to Simplified Block Diagram)
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 PC board trace.
V_IN
SUPPLY
10nH
4.7µF
V_IN
LTC3252
GND
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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 LTC3252.
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 the Ceramic Capacitor Selection Guidelines). If 100mA
or less of current is required from an output then its asso-
ciated 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 X7R material will
retain most of its capacitance from – 40°C to 85°C whereas
a Z5U or Y5V style capacitor will lose considerable
capacitance over that range (60% to 80% loss typical).
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
LTC3252
discussing the specified capacitance value. For example,
over rated voltage and temperature conditions, a 4.7µF,
10V, Y5V ceramic capacitor in a 0805 case may not
provide any more capacitance than a 1µF, 10V, X7R
available in the same 0805 case. In fact, over bias and
temperature range, the 1µF, 10V, X7R will provide more
capacitance than the 4.7µF, 10V, Y5V. The capacitor
manufacturer’s data sheet should be consulted to deter-
mine what value of capacitor is needed to ensure mini-
mum capacitance values are met over operating
temperature and bias voltage.
Below is a list of ceramic capacitor manufacturers and
how to contact them:
AVX
Kemet
Murata
Taiyo Yuden
Vishay
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.vishay.com
Layout Considerations
Due to the high switching frequency and transient currents
produced by the LTC3252 careful board layout is neces-
sary for optimal performance. A true ground plane and
short connections to all capacitors will improve perfor-
mance and ensure proper regulation under all conditions.
Figure 7 shows the suggested layout configuration. Note
the exposed paddle of the package is ground (GND) and
must be soldered to the PCB ground.
The flying capacitor pins C1⁺, C1^–, C2⁺ and C2^– will have
very high edge rate wave forms. The large dv/dt on these
pins can couple energy capacitively to adjacent printed
circuit board runs. Magnetic fields can also be generated
if the flying capacitors are not close to the LTC3252 (i.e.,
the loop area is large). To decouple capacitive energy
transfer, a Faraday shield may be used. This is a grounded
PC trace between the sensitive node and the LTC3252
pins. For a high quality AC ground, it should be returned to
a solid ground plane that extends all the way to the
LTC3252. Keep the FB traces away from or shielded from
the flying capacitor traces or degraded performance could
result.
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