Datasheet

LTC3219
16
3219fa
APPLICATIONS INFORMATION
Flying Capacitor Selection
Warning: Polarized capacitors such as tantalum or
aluminum should never be used for the fl ying capaci-
tors since their voltage can reverse upon start-up of the
LTC3219. Ceramic capacitors should always be used for
the fl ying capacitors.
The fl ying capacitors control the strength of the charge
pump. In order to achieve the rated output current it is
necessary to have at least 1μF of capacitance for each of
the fl ying 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 coeffi cient
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 specifi ed 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 1 shows a list of ceramic capacitor manufacturers
and how to contact them:
Table 1. 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 LTC3219 has been designed to minimize EMI. How-
ever due to its high switching frequency and the transient
currents produced by the LTC3219, careful board layout
is necessary. A true ground plane and short connections
to all capacitors will improve performance and ensure
proper regulation under all conditions.
The fl ying 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 fi elds can also be generated if the fl ying capacitors
are not close to the LTC3219 (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 LTC3219 pins. For a high quality
AC ground, it should be returned to a solid ground plane
that extends all the way to the LTC3219.
3219 F04
GND
LTC3219
V
BAT
Figure 4. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Board Trace)