LTC3561 datasheet(6/16 Pages) LINER

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LTC3561

3561f

APPLICATIO S I FOR ATIO

A general LTC3561 application circuit is shown in

Figure 4. External component selection is driven by the

load requirement, and begins with the selection of the

inductor L1. Once L1 is chosen, CIN and COUT can be

selected.

Operating Frequency

Selection of the operating frequency is a tradeoff between

efficiency and component size. High frequency operation

allows the use of smaller inductor and capacitor values.

Operation at lower frequencies improves efficiency by

reducing internal gate charge losses but requires larger

inductance values and/or capacitance to maintain low

output ripple voltage.

The operating frequency, fO, of the LTC3561 is determined

by an external resistor that is connected between the RT

pin and ground. The value of the resistor sets the ramp

current that is used to charge and discharge an internal

timing capacitor within the oscillator and can be calculated

by using the following equation:

()

Ω

()

−

978 1011

108

.•

or can be selected using Figure 1.

The maximum usable operating frequency is limited by the

minimum on-time and the duty cycle. This can be calcu-

lated as:

fO(MAX) ≈ 6.67 • (VOUT / VIN(MAX)) (MHz)

The minimum frequency is limited by leakage and noise

coupling due to the large resistance of RT.

Inductor Selection

Although the inductor does not influence the operating

frequency, the inductor value has a direct effect on ripple

current. The inductor ripple current

∆IL decreases with

higher inductance and increases with higher VIN or VOUT:

∆ =

−

⎛

⎝⎜

⎞

⎠⎟

OUT

•

•1

Accepting larger values of

∆IL allows the use of low

inductances, but results in higher output voltage ripple,

greater core losses, and lower output current capability.

A reasonable starting point for setting ripple current is

∆IL = 0.4 × IOUT(MAX), where IOUT(MAX) is 1A. The largest

ripple current

∆ILoccursatthemaximuminputvoltage.To

guarantee that the ripple current stays below a specified

maximum, the inductor value should be chosen according

to the following equation:

OUT

IN MAX

∆

−

⎛

⎝⎜

⎞

⎠⎟

•

()

Inductor Core Selection

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Tor-

oid or shielded pot cores in ferrite or permalloy materials

are small and do not radiate much energy, but generally

cost more than powdered iron core inductors with similar

electrical characteristics. The choice of which style induc-

tor to use often depends more on the price vs size require-

ments and any radiated field/EMI requirements than on

what the LTC3561 requires to operate. Table 1 shows some

typical surface mount inductors that work well in LTC3561

applications.

RT (kΩ)

0.5

1.5

2.0

2.5

1000

4.5

TA = 25°C

3561

F02

1.0

500

1500

3.0

3.5

4.0

Figure 1. Frequency vs RT

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