LTC3218

9

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APPLICATIONS INFORMATION

performance. MLCCs combined with a tight board layout

current to the LTC3218 will be relatively constant while

the charge pump is on either the input charging phase or

the output charging phase but will drop to zero during

the clock nonoverlap times. Since the nonoverlap time

is small (~15ns), these missing “notches” will result in

only a small perturbation on the input power supply line.

Note that a higher ESR capacitor such as tantalum will

have higher input noise due to the input current change

times the ESR. Therefore, ceramic capacitors are again

recommended for their exceptional ESR performance. Input

noise can be further reduced by powering the LTC3218

through a very small series inductor as shown in Figure 2.

A 10nH inductor will reject the fast current notches,

thereby presenting a nearly constant current load to the

input power supply. For economy, the 10nH inductor can

be fabricated on the PC board with about 1cm (0.4ʺ) of

PC board trace.

Flying Capacitor Selection

Warning: Polarized capacitors such as tantalum or

aluminum should never be used for the flying capaci-

tors since their voltage can reverse upon start-up of the

LTC3218. Ceramic capacitors should always be used for

the flying capacitors.

The flying capacitor controls the strength of the charge

pump. In order to achieve the rated output current it is

necessary to have at least 1.6μF of actual capacitance for

the flying capacitor. 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 capacitance from –40°C

to 85°C whereas a Z5U or Y5V 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 1 shows a list of ceramic capacitor manufacturers

and how to contact them.

Table 1. 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 the high switching frequency and the transient

currents produced by the LTC3218, 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. An example of

such a layout is shown in Figure 3.

The flying capacitor pins, CP and CM, will have 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 LTC3218 (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 LTC3218 pins. For a high quality

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

that extends all the way to the LTC3218.

GND

LTC3218

2.2μF

0.1μF

10nH

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Figure 2. 10nH Inductor Used for Input Noise Reduction

(Approximately 1cm of Wire)