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LTC3206
APPLICATIO S I FOR ATIO
Table 4 shows a list of ceramic capacitor manufacturers
and how to contact them:
Table 4. Recommended Capacitor Vendors
PIN 1
AVX
Kemet
Murata
Taiyo Yuden
Vishay
www.avxcorp.com
www.kemet.com
www.murata.com
www.t-yuden.com
www.vishay.com
3206 F07
V IN
CPO
GND
Figure 7. Optimum Single Layer PCB Layout
For  very  light  load  applications,  the  flying  capacitors
may be reduced to save space or cost. The theoretical
minimum output resistance of a 1.5x fractional charge
pump is given by:
Power Efficiency
To calculate the power efficiency ( η ) of a white LED driver
R OL ( MIN ) ≡
1 . 5 V IN – V OUT
I OUT
=
1
2 f OSC C FLY
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
P LED
where f OSC is the switching frequency (960kHz 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 LTC3206, careful board layout is
necessary. A true ground plane and short connections to
all capacitors will improve performance and ensure proper
current sources. Stated mathematically, the power effi-
ciency is given by:
η≡
P IN
The efficiency of the LTC3206 depends upon the mode in
which it is operating. Recall that the LTC3206 operates as
a pass switch, connecting V IN to CPO until one of the LEDs
drops out. This feature provides the optimum efficiency
available for a given input voltage and LED forward volt-
age. When it is operating as a switch, the efficiency is
approximated by:
= LED LED ? LED
regulationunderallconditions.Figure7showstherecom-
mended layout configuration.
η≡
P LED
P IN
V ? I V
V IN ? I IN V IN
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 LTC3206 (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 LTC3206 pins. For a high
since the input current will be very close to the LED
current.
At moderate to high output power, the quiescent current
of the LTC3206 is negligible and the expression above is
valid. For example, with V IN = 3.9V, I OUT = 20mA ? 6 LEDs
and V LED equal to 3.6V, the measured efficiency is 92.2%,
which is very close to the theoretical 92.3% calculation.
quality AC ground, it should be returned to a solid ground
plane that extends all the way to the LTC3206.
3206f
13
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