LTC3215EDD 查看數據表（PDF） - Linear Technology

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LTC3215EDD

700mA Low Noise High Current LED Charge Pump

Linear Technology 

LTC3215

APPLICATIONS INFORMATION

VIN, CPO Capacitor Selection

The value and type of capacitors used with the LTC3215

determine several important parameters such as regulator

control loop stability, output ripple, charge pump strength

and minimum start-up time.

To reduce noise and ripple, it is recommended that low

equivalent series resistance (ESR) ceramic capacitors be

used for both CVIN and CCPO. Tantalum and aluminum ca-

pacitors are not recommended because of their high ESR.

The value of CCPO directly controls the amount of output

ripple for a given load current. Increasing the size of

CCPO will reduce the output ripple at the expense of higher

start-up current. The peak-to-peak output ripple for 1.5x

mode is approximately given by the expression:

VRIPPLE(P-P) = IOUT/(3fOSC • CCPO)

(3)

Where fOSC is the LTC3215’s oscillator frequency (typically

900kHz) and CCPO is the output storage capacitor.

Both the style and value of the output capacitor can sig-

nificantly affect the stability of the LTC3215. As shown in

the block diagram, the LTC3215 uses a control loop to

adjust the strength of the charge pump to match the cur-

rent required at the output. The error signal of this loop

is stored directly on the output charge storage capacitor.

The charge storage capacitor also serves as the dominant

pole for the control loop. To prevent ringing or instability,

it is important for the output capacitor to maintain at least

2.2µF of actual capacitance over all conditions.

Likewise, excessive ESR on the output capacitor will tend

to degrade the loop stability of the LTC3215. The closed

loop output resistance of the LTC3215 is designed to be

76mΩ. For a 100mA load current change, the error signal

will change by about 7.6mV. If the output capacitor has

76mΩ or more of ESR, the closed-loop frequency response

will cease to roll off in a simple one-pole fashion and poor

load transient response or instability could result. Multilayer

ceramic chip capacitors typically have exceptional ESR

performance. MLCCs combined with a tight board layout

will yield very good stability. As the value of CCPO controls

the amount of output ripple, the value of CVIN controls the

amount of ripple present at the input pin (VIN). The input

current to the LTC3215 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 LTC3215

through a very small series inductor as shown in Figure

3. 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 (0.4") of

PC board trace.

10nH

VIN

0.1µF

2.2µF

LTC3215

GND

3215 F03

Figure 3. 10nH Inductor Used for Input Noise Reduction

(Approximately 1cm of Wire)

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 LTC3215.

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 ma-

terials 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 – 40oC to 85oC 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,

3215fc

9

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