LTC3250 Datasheet, PDF (8/12 Pages) Linear Technology – High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter

LTC3250-1.5/LTC3250-1.2
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OPERATIO (Refer to Simplified Block Diagram)
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or
aluminum should never be used for the flying capacitor
since its voltage can reverse upon start-up of the
LTC3250-1.5/LTC3250-1.2. Ceramic capacitors should
always be used for the flying capacitor.
The flying capacitor controls 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” section). If
only 100mA 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 X7R material will
retain most of its capacitance from –40°C to 85°C whereas
a Z5U or Y5V style capacitor will lose considerable capaci-
tance 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 capacitors it is often more
appropriate to compare the amount of achievable capaci-
tance for a given case size rather than discussing the
specified capacitance value. For example, over rated volt-
age 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 determine what value of capacitor
is needed to ensure minimum 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
1-(803)-448-1943
1-(864)-963-6300
1-(800)-831-9172
1-(800)-348-2496
1-(800)-487-9437
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 LTC3250-1.5/LTC3250-1.2 careful board
layout is necessary for optimal performance. A true ground
plane and short connections to all capacitors will improve
performance and ensure proper regulation under all con-
ditions. Figure 2 shows the recommended layout configu-
ration.
1µF
V_IN
1µF
GND
4.7µF
V_OUT
SHDN
LTC3250-1.5/LTC3250-1.2
VIA TO GROUND PLANE
3250 F02
Figure 2. Recommended Layout
The flying capacitor pins, C⁺ and C^– 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 LTC3250-1.5/LTC3250-1.2
(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 LTC3250-1.5/
LTC3250-1.2 pins. For a high quality AC ground it should
be returned to a solid ground plane that extends all the way
to the LTC3250-1.5/LTC3250-1.2.
3250fa
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