ltc3210 Linear Technology Corporation, ltc3210 Datasheet - Page 13

• Manufacturer Part Number: ltc3210
• Description: Main/cam Led Controller In 3mm ? 3mm Qfn
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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
LTC3210. 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 1.6μF 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. 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 2 shows a list of ceramic capacitor manufacturers
and how to contact them:
Table 2. Recommended Capacitor Vendors
AVX
Kemet
Murata
Taiyo Yuden
Vishay
U
U
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.vishay.com
W
U
Layout Considerations and Noise
Due to the high switching frequency and the transient
currents produced by the LTC3210, 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 will
have high 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 LTC3210 (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 LTC3210 pins. For a high quality
AC ground, it should be returned to a solid ground plane
that extends all the way to the LTC3210.
The following guidelines should be followed when design-
ing a PCB layout for the LTC3210:
• The exposed pad should be soldered to a large copper
• Input and output capacitors must be placed close to
• The fl ying capacitors must be placed close to the part.
• V
• LED pads must be large and connected to other layers
• RM and RC pins are sensitive to noise and capacitance.
plane that is connected to a solid, low impedance ground
plane using plated through-hole vias for proper heat
sinking and noise protection.
the part.
The traces from the pins to the capacitor pad should
be as wide as possible.
and handle high currents.
of metal to ensure proper heat sinking.
The resistors should be placed near the part with mini-
mum line width.
BAT
, CPO traces must be wide to minimize inductance
LTC3210
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3210fa