LTC3210-2 Datasheet, PDF (13/16 Pages) Linear Technology – MAIN/CAM LED Controllers with 32-Step Brightness Control in 3mm × 3mm QFN

LTC3210-2/LTC3210-3
APPLICATIONS INFORMATION
Flying Capacitor Selection
Warning: Polarized capacitors such as tantalum or
aluminum should never be used for the flying capaci-
tors since their voltage can reverse upon start-up of
the LTC3210-2/LTC3210-3. 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. 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
Due to the high switching frequency and the transient
currents produced by the LTC3210-2/LTC3210-3, 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 flying 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 fields can also be generated if the flying capacitors
are not close to the LTC3210-2/LTC3210-3 (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 LTC3210-2/LTC3210-3
pins. For a high quality AC ground, it should be returned
to a solid ground plane that extends all the way to the
LTC3210-2/LTC3210-3.
The following guidelines should be followed when design-
ing a PCB layout for the LTC3210-2/LTC3210-3:
• The Exposed Pad should be soldered to a large cop-
per plane that is connected to a solid, low impedance
ground plane using plated through-hole vias for proper
heat sinking and noise protection.
• Input and output capacitors must be placed close to the
part.
• The flying capacitors must be placed close to the part.
The traces from the pins to the capacitor pad should
be as wide as possible.
• V_BAT, CPO traces must be wide to minimize inductance
and handle high currents.
• LED pads must be large and connected to other layers
of metal to ensure proper heat sinking.
• RM and RC pins are sensitive to noise and capacitance.
The resistors should be placed near the part with mini-
mum line width.
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