LTC3220/LTC3220-1
16
32201fd
For more information www.linear.com/LTC3220
Flying Capacitor Selection
Warning: Polarized capacitors such as tantalum or alumi-
num should never be used for the flying capacitors since
their voltage can reverse upon start-up of the
LTC3220/
LTC3220-1.
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 to have at least 1.6µF of capacitance for each of
the flying capacitors. 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. Z5U and Y5V
capacitors may also have a very poor voltage coefficient
causing them to lose 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 capacitance for a given case size
rather than comparing the specified capacitance value. For
example, over rated voltage and temperature conditions, a
1µF, 10V, Y5V ceramic capacitor in a 0603 case may not
provide any more capacitance than a 0.22µF, 10V, X7R
available in the same case. The capacitor manufacturer’s
data sheet should be consulted to determine what value
of capacitor is needed to ensure minimum capacitances
at all temperatures and voltages.
Table 2 shows a list of ceramic capacitor manufacturers
and how to contact them:
Table 2. Recommended Capacitor Vendors
AVX www.avxcorp.com
Kemet www.kemet.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay www.vishay.com
Layout Considerations and Noise
The LTC3220/LTC3220-1 have been designed to minimize
EMI. However due to their high switching frequency and the
transient currents produced by the LTC3220/LTC3220-1,
careful board layout is necessary. A true ground plane and
short connections to all capacitors will improve perfor
-
mance and ensure proper regulation under all conditions.
The flying capacitor pins C1P,
C2P, C1M and C2M have
controlled edge rate waveforms. The large dV/dt on these
pins can couple energy capacitively to adjacent PCB runs.
Magnetic fields can also be generated if the flying capacitors
are not close to the LTC3220/LTC3220-1 (i.e., the loop
area is large). To decouple capacitive energy transfer, a
Faraday shield may be used. This is a grounded PCB trace
between the sensitive node and the LTC3220/LTC3220-1
pins. For a high quality AC ground, it should be returned
to a solid ground plane that extends all the way to the
LTC3220/LTC3220-1.
applicaTions inForMaTion
