LTC4002 datasheet(15/24 Pages) LINER

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LTC3783

3783f

Boost Converter: Inductor Selection

Given an operating input voltage range, and having chosen

the operating frequency and ripple current in the inductor,

the inductor value can be determined using the following

equation:

where

IN MIN

MAX

OUT

∆

⎛

⎝⎜

⎞

⎠⎟

∆ =

()

(

•

MAX

)

–

Remember that most boost converters are not short-

circuit protected. Under a shorted output condition, the

inductor current is limited only by the input supply capa-

bility. For applications requiring a step-up converter that is

short-circuit protected, please refer to the applications

section covering SEPIC converters.

The minimum required saturation current of the inductor

can be expressed as a function of the duty cycle and the

load current, as follows:

L SAT

OUT MAX

MAX

()

•

–

⎛

⎝⎜

⎞

⎠⎟

The saturation current rating for the inductor should be

checked at the minimum input voltage (which results in the

highest inductor current) and maximum output current.

Boost Converter: Operating in Discontinuous Mode

Discontinuous mode operation occurs when the load

current is low enough to allow the inductor current to run

out during the off-time of the switch, as shown in Figure 7.

Once the inductor current is near zero, the switch and

diode capacitances resonate with the inductance to form

damped ringing at 1MHz to 10MHz. If the off-time is long

enough, the drain voltage will settle to the input voltage.

Depending on the input voltage and the residual energy in

the inductor, this ringing can cause the drain of the power

MOSFET to go below ground where it is clamped by the

body diode. This ringing is not harmful to the IC and it has

not been shown to contribute significantly to EMI. Any

attempt to damp it with a snubber will degrade the

efficiency.

Boost Converter: Power MOSFET Selection

The power MOSFET can serve two purposes in the LTC3783:

it represents the main switching element in the power

path, and its RDS(ON) can represent the current sensing

element for the control loop. Important parameters for the

power MOSFET include the drain-to-source breakdown

voltage BVDSS, the threshold voltage VGS(TH), the on-

resistance RDS(ON) versus gate-to-source voltage, the

gate-to-source and gate-to-drain charges QGS and QGD,

respectively, the maximum drain current ID(MAX) and the

MOSFET’s thermal resistances

θJC and θJA.

The gate drive voltage is set by the 7V INTVCC low drop

regulator. Consequently, 6V rated MOSFETs are required

in most high voltage LTC3783 applications. If low input

voltage operation is expected (e.g., supplying power from

a lithium-ion battery or a 3.3V logic supply), then sublogic-

level threshold MOSFETs should be used. Pay close atten-

tion to the BVDSS specifications for the MOSFETs relative

to the maximum actual switch voltage in the application.

Many logic-level devices are limited to 30V or less, and the

switch node can ring during the turn-off of the MOSFET

due to layout parasitics. Check the switching waveforms

of the MOSFET directly across the drain and source

terminals using the actual PC board layout for excessive

ringing.

OPERATIO

OUTPUT

VOLTAGE

200mV/DIV

INDUCTOR

CURRENT

1A/DIV

µs/DIV

3783 F07

MOSFET

DRAIN

VOLTAGE

20V/DIV

Figure 7. Discontinuous Mode Waveforms

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