LT3758EDD データシートの表示（PDF） - Linear Technology

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LT3758EDD

High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller

Linear Technology 

LT3758/LT3758A

Applications Information

The constant c in the preceding equation represents the

percentage peak-to-peak ripple current in the inductor,

relative to IL(MAX).

The inductor ripple current has a direct effect on the choice

of the inductor value. Choosing smaller values of ∆IL

requires large inductances and reduces the current loop

gain (the converter will approach voltage mode). Accepting

larger values of ∆IL provides fast transient response and

allows the use of low inductances, but results in higher input

current ripple and greater core losses. It is recommended

that c fall within the range of 0.2 to 0.6.

Given an operating input voltage range, and having chosen

the operating frequency and ripple current in the inductor,

the inductor value of the boost converter can be determined

using the following equation:

L

=

VIN(MIN)

∆IL • f

•

DMAX

The peak and RMS inductor current are:

IL(PEAK )

=

IL(MAX )

•





1+

c

2





IL(RMS) = IL(MAX) •

1+ c2

12

Based on these equations, the user should choose the

inductors having sufficient saturation and RMS current

ratings.

Set the sense voltage at IL(PEAK) to be the minimum of the

SENSE current limit threshold with a 20% margin. The

sense resistor value can then be calculated to be:

RSENSE

=

80mV

IL(PEAK )

Boost Converter: Power MOSFET Selection

Important parameters for the power MOSFET include the

drain-source voltage rating (VDS), the threshold voltage

(VGS(TH)), the on-resistance (RDS(ON)), the gate to source

and gate to drain charges (QGS and QGD), the maximum

drain current (ID(MAX)) and the MOSFET’s thermal

resistances (RθJC and RθJA).

16

The power MOSFET will see full output voltage, plus a

diode forward voltage, and any additional ringing across

its drain-to-source during its off-time. It is recommended

to choose a MOSFET whose BVDSS is higher than VOUT by

a safety margin (a 10V safety margin is usually sufficient).

The power dissipated by the MOSFET in a boost converter is:

PFET = I2L(MAX) • RDS(ON) • DMAX + 2 • V2OUT • IL(MAX)

• CRSS • f/1A

The first term in the preceding equation represents the

conduction losses in the device, and the second term, the

switching loss. CRSS is the reverse transfer capacitance,

which is usually specified in the MOSFET characteristics.

For maximum efficiency, RDS(ON) and CRSS should be

minimized. From a known power dissipated in the power

MOSFET, its junction temperature can be obtained using

the following equation:

TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA)

TJ must not exceed the MOSFET maximum junction

temperature rating. It is recommended to measure the

MOSFET temperature in steady state to ensure that absolute

maximum ratings are not exceeded.

Boost Converter: Output Diode Selection

To maximize efficiency, a fast switching diode with low

forward drop and low reverse leakage is desirable. The

peak reverse voltage that the diode must withstand is

equal to the regulator output voltage plus any additional

ringing across its anode-to-cathode during the on-time.

The average forward current in normal operation is equal

to the output current, and the peak current is equal to:

ID(PEAK )

= IL(PEAK)

=





1+

c

2 

• IL(MAX)

It is recommended that the peak repetitive reverse voltage

rating VRRM is higher than VOUT by a safety margin (a 10V

safety margin is usually sufficient).

The power dissipated by the diode is:

PD = IO(MAX) • VD

and the diode junction temperature is:

TJ = TA + PD • RθJA

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