LTC3216 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3216
Linear Technology
'LTC3216' PDF : 12 Pages View PDF
LTC3216
APPLICATIONS INFORMATION
0.1μF
10nH
2.2μF
VIN
LTC3216
GND
3216 F02
Figure 2. 10nH Inductor Used for Input Noise
Reduction (Approximately 1cm of Wire)
Flying Capacitor Selection
Warning: Polarized capacitors such as tantalum or alumi-
num should never be used for the flying capacitors since
their voltage can reverse upon start-up of the LTC3216.
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 2.2μF of actual 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 – 40oC to 85oC 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
TDK
www.tdk.com
Layout Considerations and Noise
Due to its high switching frequency and the transient
currents produced by the LTC3216, 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 C1+, C2+, C1– and C2– 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 LTC3216 (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 LTC3216 pins. For a high quality
AC ground, it should be returned to a solid ground plane
that extends all the way to the LTC3216.
Power Efficiency
To calculate the power efficiency (η) of a white LED
driver chip, the LED power should be compared to the
input power. The difference between these two numbers
represents lost power whether it is in the charge pump
or the current sources. Stated mathematically, the power
efficiency is given by:
PLED
(4)
PIN
The efficiency of the LTC3216 depends upon the mode in
which it is operating. Recall that the LTC3216 operates
as a pass switch, connecting VIN to CPO, until dropout
is detected at the ILED pin. This feature provides the op-
timum efficiency available for a given input voltage and
3216fc
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