LTC3212 Datasheet, PDF (9/12 Pages) Linear Technology – RGB LED Driver and Charge Pump

LTC3212
Applications Information
V_IN, CPO Capacitor Selection
The style and value of the capacitors used with the LTC3212
determine several important parameters such as regulator
control loop stability, output ripple, charge pump strength
and minimum start-up time.
To reduce noise and ripple, it is recommended that low
equivalent series resistance (ESR) ceramic capacitors are
used for both C_VIN and C_CPO. Tantalum and aluminum
capacitors are not recommended due to high ESR.
The value of C_CPO directly controls the amount of output
ripple for a given load current. Increasing the size of C_CPO
will reduce output ripple at the expense of longer start-up
time. The peak-to-peak output ripple of the 2x mode is
approximately given by the expression:
V_RIPPLEP-P
=
I_OUT
2f_OSC • C_CPO
where f_OSC is the LTC3212 oscillator frequency or typically
900kHz and C_CPO is the output storage capacitor.
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
LTC3212. 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 0.6µF of capacitance for flying
capacitor. Capacitors of different materials lose their ca-
pacitance with higher temperature and voltage at different
rates. For example, a ceramic capacitor made of X7R mate-
rial 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 Switching Noise
The LTC3212 has been designed to minimize EMI. How-
ever due to its high switching frequency and the transient
currents produced by the LTC3212, 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 CP and CM will have 5ns to 10ns
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 LTC3212 (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 LTC3212 pins. For a high quality
AC ground, it should be returned to a solid ground plane
that extends all the way to the LTC3212.
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