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LTC3563

3563f

APPLICATIO S I FOR ATIO

A general LTC3563 application circuit is shown in Figure1.

External component selection is driven by the load require-

ment and begins with the selection of the inductor L. Once

the inductor is chosen, CIN and COUT can be selected.

the burst clamp. Lower inductor values result in higher

ripple current which causes the transition to occur at lower

load currents. This causes a dip in efﬁciency in the upper

range of low current operation. In Burst Mode operation,

lower inductance values cause the burst frequency to

increase.

Inductor Core Selection

Different core materials and shapes change the size/current

and price/current relationships of an inductor. Toroid or

shielded pot cores in ferrite or permalloy materials are small

and don’t radiate much energy, but generally cost more

than powdered iron core inductors with similar electrical

characteristics. The choice of which style inductor to use

often depends more on the price vs size requirements

and any radiated ﬁeld/EMI requirements than on what the

LTC3563 requires to operate. Table 1 shows some typi-

cal surface mount inductors that work well in LTC3563

applications.

Input Capacitor (CIN) Selection

In continuous mode, the input current of the converter is a

square wave with a duty cycle of approximately VOUT/VIN.

To prevent large voltage transients, a low equivalent series

resistance (ESR) input capacitor sized for the maximum

RMS current must be used. The maximum RMS capacitor

current is given by:

RMS

MAX

OUT

≈

()

–

where the maximum average output current IMAX equals

the peak current minus half the peak-to-peak ripple cur-

rent, IMAX = ILIM – ΔIL/2. This formula has a maximum at

VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case

is commonly used to design because even signiﬁcant

deviations do not offer much relief. Note that capacitor

manufacturer’s ripple current ratings are often based on

only 2000 hours life time. This makes it advisable to further

derate the capacitor, or choose a capacitor rated at a higher

temperature than required. Several capacitors may also be

paralleled to meet the size or height requirements of the

VIN

LTC3563

RUN

3563 F01

CIN

COUT

VIN

2.7V TO 5.5V

VOUT

VSEL

GND

1.28V 1.87V

Figure 1. LTC3563 General Schematic

Inductor Selection

The inductor value has a direct effect on ripple current ΔIL,

which decreases with higher inductance and increases with

higher VIN or VOUT, as shown in following equation:

∆I

OUT

⎛

⎝⎜

⎞

⎠⎟

ƒ •

–

where fO is the switching frequency. A reasonable starting

point for setting ripple current is ΔIL = 0.4 • IOUT(MAX),

where IOUT(MAX) is 500mA. The largest ripple current ΔIL

occurs at the maximum input voltage. To guarantee that

the ripple current stays below a speciﬁed maximum, the

inductor value should be chosen according to the follow-

ing equation:

OUT

IN MAX

⎛

⎝

⎜

⎞

⎠

⎟

ƒ •

–

()

∆

The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple

current to prevent core saturation. Thus, a 600mA rated

inductor should be enough for most applications (500mA

+ 100mA). For better efﬁciency, choose a low DC-resis-

tance inductor.

The inductor value will also have an effect on Burst Mode

operation. The transition to low current operation begins

when the inductor’s peak current falls below a level set by

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