LTC3215 データシートの表示（PDF） - Linear Technology

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LTC3215

700mA Low Noise High Current LED Charge Pump

Linear Technology 

LTC3215

APPLICATIONS INFORMATION

when comparing different capacitors, it is often more ap-

propriate 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 capaci-

tor 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 LTC3215, 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 4.

C1

CCPO

PIN 1

C2

CIN

3215 F04

Figure 4. Example Board Layout

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 PCB runs.

Magnetic fields can also be generated if the flying capacitors

are not close to the LTC3215 (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 LTC3215 pins. For a high quality

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

that extends all the way to the LTC3215.

The following guidelines should be followed when design-

ing a PCB layout for the LTC3215.

• 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 (CIN and CCPO) must also

be placed as close to the part as possible.

• The flying capacitors must also be placed as close to

the part as possible. The traces running from the pins

to the capacitor pads should be as wide as possible.

• VIN, CPO and ILED traces must be made as wide as

possible. This is necessary to minimize inductance,

as well as provide sufficient area for high current ap-

plications.

• LED pads must be large and should be connected to

as much solid metal as possible to ensure proper heat

sinking.

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:

η ≡ PLED

PIN

(4)

3215fc

10

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