LTC1435I データシートの表示（PDF） - Linear Technology

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LTC1435I

High Efficiency Low Noise Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1435

APPLICATIONS INFORMATION

MOSFET is on. Lower inductor values (higher ∆IL) will

cause this to occur at higher load currents, which can

cause a dip in efficiency in the upper range of low current

operation. In Burst Mode operation, lower inductance

values will cause the burst frequency to decrease.

The Figure 3 graph gives a range of recommended induc-

tor values vs operating frequency and VOUT.

60

VOUT = 5.0V

50

VOUT = 3.3V

VOUT = 2.5V

40

30

20

10

0

0 50 100 150 200 250 300

OPERATING FREQUENCY (kHz)

1435 F03

Figure 3. Recommended Inductor Values

Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. High efficiency converters generally cannot af-

ford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite, molypermalloy

or Kool Mµ® cores. Actual core loss is independent of core

size for a fixed inductor value, but it is very dependent on

inductance selected. As inductance increases, core losses

go down. Unfortunately, increased inductance requires more

turns of wire and therefore copper losses will increase.

Ferrite designs have very low core loss and are preferred

at high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates “hard,” which means that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Molypermalloy (from Magnetics, Inc.) is a very good, low

loss core material for toroids, but it is more expensive than

Kool Mµ is a registered trademark of Magnetics, Inc.

ferrite. A reasonable compromise from the same manu-

facturer is Kool Mµ. Toroids are very space efficient,

especially when you can use several layers of wire. Be-

cause they generally lack a bobbin, mounting is more

difficult. However, designs for surface mount are available

which do not increase the height significantly.

Power MOSFET and D1 Selection

Two external power MOSFETs must be selected for use

with the LTC1435: an N-channel MOSFET for the top

(main) switch and an N-channel MOSFET for the bottom

(synchronous) switch.

The peak-to-peak gate drive levels are set by the INTVCC

voltage. This voltage is typically 5V during start-up (see

EXTVCC Pin Connection). Consequently, logic level thresh-

old MOSFETs must be used in most LTC1435 applica-

tions. The only exception is applications in which EXTVCC

is powered from an external supply greater than 8V (must

be less than 10V), in which standard threshold MOSFETs

(VGS(TH) < 4V) may be used. Pay close attention to the

BVDSS specification for the MOSFETs as well; many of the

logic level MOSFETs are limited to 30V or less.

Selection criteria for the power MOSFETs include the “ON”

resistance RSD(ON), reverse transfer capacitance CRSS,

input voltage and maximum output current. When the

LTC1435 is operating in continuous mode the duty cycles

for the top and bottom MOSFETs are given by:

Main Switch Duty Cycle = VOUT

VIN

Synchronous Switch Duty Cycle = (VIN − VOUT )

VIN

The MOSFET power dissipations at maximum output

current are given by:

PMAIN

=

VOUT

VIN

(IMAX

)2(1+

δ

)RDS(ON)

+

k(VIN)1.85(IMAX )(CRSS)(f)

PSYNC

=

VIN

− VOUT

VIN

(IMAX )2(1+

δ )RDS(ON)

9

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