LTC3206

13

3206f

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

VV

If

C

OL MIN

IN

OUT

OUT

OSC FLY

()

.–

≡

=

15

1

2

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.

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.

Figure 7. Optimum Single Layer PCB Layout

GND

CPO

3206 F07

PIN 1

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≡

P

P

LED

IN

The efficiency of the LTC3206 depends upon the mode in

which it is operating. Recall that the LTC3206 operates as

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≡

=

≅

P

P

VI

VI

V

V

LED

IN

LED

LED

IN

IN

LED

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

which is very close to the theoretical 92.3% calculation.