LTC3210 datasheet(13/16 Pages) LINER

13 / 16 page

LTC3210

3210f

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

LTC3210. 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 1.6µF of 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 capacitance from

– 40°C to 85°C whereas a Z5U or Y5V style capacitor will

lose considerable capacitance over that range. 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

Layout Considerations and Noise

Due to the high switching frequency and the transient

currents produced by the LTC3210, 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.

The ﬂying capacitor pins C1P, C2P, C1M and C2M will

have 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 LTC3210 (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 LTC3210 pins. For a high quality

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

that extends all the way to the LTC3210.

The following guidelines should be followed when design-

ing a PCB layout for the LTC3210:

• The exposed pad should be soldered to a large copper

plane that is connected to a solid, low impedance ground

plane using plated through-hole vias for proper heat

sinking and noise protection.

• Input and output capacitors must be placed close to

the part.

• The ﬂying capacitors must be placed close to the part.

The traces from the pins to the capacitor pad should

be as wide as possible.

• VBAT, CPO traces must be wide to minimize inductance

and handle high currents.

• LED pads must be large and connected to other layers

of metal to ensure proper heat sinking.

• RM and RC pins are sensitive to noise and capacitance.

The resistors should be placed near the part with mini-

mum line width.

APPLICATIO S I FOR ATIO

LTC3210 Datasheet(PDF) 13 Page - Linear Technology

LTC3210 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL