LTC3205_15 Datasheet, PDF (11/16 Pages) Linear Technology – Multidisplay LED Controller

LTC3205
APPLICATIO S I FOR ATIO
lose considerable capacitance over that range. Z5U and Y5V
capacitors may also have a very strong voltage coefficient
causing them to lose 60% or more of their capacitance when
the rated voltage is applied. Therefore, when comparing dif-
ferent 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 0603 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 4 shows a list of ceramic capacitor manufacturers
and how to contact them:
Table 4. 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
For very light load applications, the flying capacitors may
be reduced to save space or cost. The theoretical mini-
mum output resistance of a 2:3 fractional charge pump is
given by:
R_OL(MIN)
≡
1.5V_IN – V_OUT
I_OUT
=
1
2f_OSCC_FLY
where f_OSC is the switching frequency (800kHz typ) and
C_FLY is the value of the flying capacitors. Note that the
charge pump will typically be weaker than the theoretical
limit due to additional switch resistance, however for very
light load applications, the above expression can be used
as a guideline in determining a starting capacitor value.
Layout Considerations and Noise
Due to its high switching frequency and the transient
currents produced by the LTC3205, 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. Figure 7 shows the recom-
mended layout configuration.
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 printed circuit
board runs. Magnetic fields can also be generated if the
flying capacitors are not close to the LTC3205 (i.e., the loop
area is large). To decouple capacitive energy transfer, a
Faraday shield may be used. This is a grounded PC trace
between the sensitive node and the LTC3205 pins. For a high
quality AC ground, it should be returned to a solid ground
plane that extends all the way to the LTC3205
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 number repre-
sents lost power whether it is in the charge pump or the
current sources. Stated mathematically, the power effi-
ciency is given by:
η ≡ ^PLED
P_IN
PIN 1
GND
V_IN
CPO
3205 F07
Figure 7. Optimum Single Layer PCB Layout
3205f
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