LTC3216

9

3216fc

APPLICATIONS INFORMATION

Flying Capacitor Selection

Warning: Polarized capacitors such as tantalum or alumi-

num should never be used for the flying capacitors since

their voltage can reverse upon start-up of the LTC3216.

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 2.2μF of actual 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

style capacitor

will lose considerable capacitance over that range. Z5U and

Y5V 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

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

TDK

www.tdk.com

Layout Considerations and Noise

Due to its high switching frequency and the transient

currents produced by the LTC3216, 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.

very 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 LTC3216 (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 LTC3216 pins. For a high quality

AC ground, it should be returned to a solid ground plane

that extends all the way to the LTC3216.

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 numbers

represents lost power whether it is in the charge pump

or the current sources. Stated mathematically, the power

efficiency is given by:

(4)

The efficiency of the LTC3216 depends upon the mode in

which it is operating. Recall that the LTC3216 operates

timum efficiency available for a given input voltage and

Figure 2. 10nH Inductor Used for Input Noise

Reduction (Approximately 1cm of Wire)

GND

LTC3216

2.2μF

0.1μF

10nH

3216 F02