LTC3210-1_15 Datasheet, PDF (12/16 Pages) Linear Technology – MAIN/CAM LED Controller with 64-Step Brightness Control in 3mm 3mm QFN

LTC3210-1
Applications Information
the output charging phase but will drop to zero during
the clock nonoverlap times. Since the nonoverlap time
is small (~35ns), 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 higher ESR. Therefore,
ceramic capacitors are recommended for low ESR. Input
noise can be further reduced by powering the LTC3210-1
through a very small series inductor, as shown in Figure 5.
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 (0.4") of
PC board trace.
V_BAT
LTC3210-1
GND
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Figure 5. 10nH Inductor Used for Input Noise
Reduction (Approximately 1cm of Board Trace)
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
LTC3210-1. 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
12
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 LTC3210-1, 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 LTC3210-1 (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 LTC3210-1 pins. For a high quality
AC ground, it should be returned to a solid ground plane
that extends all the way to the LTC3210-1.
The following guidelines should be followed when design-
ing a PCB layout for the LTC3210-1:
• The exposed pad should be soldered to a large copper
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.
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