LTC3206 Datasheet, PDF (13/16 Pages) Linear Technology – I2C Multidisplay LED Controller

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
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
minimum output resistance of a 1.5x 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 (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
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 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
quality AC ground, it should be returned to a solid ground
plane that extends all the way to the LTC3206.
PIN 1
V_IN
CPO
GND
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Figure 7. Optimum Single Layer PCB Layout
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
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:
η ≡ ^PLED = ^{VLED • ILED} ≅ ^VLED
P_IN V_IN • I_IN V_IN
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.
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