LTC3412 datasheet(9/22 Pages) AD

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LTC3412

3412fc

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OPERATION

APPLICATIONS INFORMATION

current signal at duty cycles in excess of 40%. Normally,

the maximum inductor peak current is reduced when

slope compensation is added. In the LTC3412, however,

slope compensation recovery is implemented to keep the

maximum inductor peak current constant throughout the

range of duty cycles. This keeps the maximum output cur-

rent relatively constant regardless of duty cycle.

Short-Circuit Protection

When the output is shorted to ground, the inductor cur-

rent decays very slowly during a single switching cycle.

To prevent current runaway from occurring, a secondary

current limit is imposed on the inductor current. If the

inductor valley current increases larger than 4.8A, the top

power MOSFET will be held off and switching cycles will

be skipped until the inductor current falls to a safe level.

The basic LTC3412 application circuit is shown in

Figure 1. External component selection is determined by

the maximum load current and begins with the selection

of the inductor value and operating frequency followed

by CIN and COUT.

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 and switching losses but

requires larger inductance values and/or capacitance to

maintain low output ripple voltage.

The operating frequency of the LTC3412 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 calcu-

lated by using the following equation:

ROSC =

3.23•1011

f(Hz)

(Ω)−10kΩ

Although frequencies as high as 4MHz are possible, the

minimum on-time of the LTC3412 imposes a minimum

limit on the operating duty cycle. The minimum on-time

is typically 110ns. Therefore, the minimum duty cycle is

equal to 100 • 110ns • f(Hz).

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current ∆IL increases with higher VIN and decreases

with higher inductance.

ΔIL =

VOUT

⎡

⎣⎢

⎤

⎦⎥

1−

VOUT

VIN

⎡

⎣

⎢

⎤

⎦

⎥

Having a lower ripple current reduces the ESR losses

in the output capacitors and the output voltage ripple.

Highest efficiency operation is achieved at low frequency

with small ripple current. This, however, requires a large

inductor.

A reasonable starting point for selecting the ripple current

is ∆IL = 0.4(IMAX). The largest ripple current occurs at the

highest VIN. To guarantee that the ripple current stays

below a specified maximum, the inductor value should

be chosen according to the following equation:

L =

VOUT

fΔIL(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟ 1−

VOUT

VIN(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

The inductor value will also have an effect on Burst Mode

operation. The transition from low current operation begins

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

burst clamp. Lower inductor values result in higher ripple

current which causes this 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 induc-

tance values will cause the burst frequency to increase.

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