LTC3218 Datasheet, PDF (9/12 Pages) Linear Technology – 400mA Single Wire Camera LED Charge Pump

LTC3218
APPLICATIONS INFORMATION
performance. MLCCs combined with a tight board layout
will yield very good stability. As the value of C_CPO controls
the amount of output ripple, the value of C_VIN controls the
amount of ripple present at the input pin (V_IN). The input
current to the LTC3218 will be relatively constant while
the charge pump is on either the input charging phase or
the output charging phase but will drop to zero during
the clock nonoverlap times. Since the nonoverlap time
is small (~15ns), these missing “notches” will result in
only a small perturbation on the input power supply line.
Note that a higher ESR capacitor such as tantalum will
have higher input noise due to the input current change
times the ESR. Therefore, ceramic capacitors are again
recommended for their exceptional ESR performance. Input
noise can be further reduced by powering the LTC3218
through a very small series inductor as shown in Figure 2.
A 10nH inductor will reject the fast current notches,
thereby presenting a nearly constant current load to the
input power supply. For economy, the 10nH inductor can
be fabricated on the PC board with about 1cm (0.4ʺ) of
PC board trace.
10nH
0.1µF
V_IN
2.2µF
LTC3218
GND
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Figure 2. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Wire)
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
LTC3218. Ceramic capacitors should always be used for
the flying capacitors.
The flying capacitor controls the strength of the charge
pump. In order to achieve the rated output current it is
necessary to have at least 1.6µF of actual capacitance for
the flying capacitor. 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 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
TDK
www.tdk.com
Layout Considerations and Noise
Due to the high switching frequency and the transient
currents produced by the LTC3218, 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. An example of
such a layout is shown in Figure 3.
The flying capacitor pins, CP and CM, will have very 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 LTC3218 (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 LTC3218 pins. For a high quality
AC ground, it should be returned to a solid ground plane
that extends all the way to the LTC3218.
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