LTC1773EMS データシートの表示（PDF） - Linear Technology

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LTC1773EMS

Synchronous Step-Down DC/DC Controller

Linear Technology 

LTC1773

APPLICATIONS INFORMATION

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. However, oper-

ating at a higher frequency generally results in lower

efficiency because of external MOSFET gate charge losses.

The inductor value has a direct effect on ripple current. The

ripple current, ∆IL, decreases with higher inductance or

frequency and increases with higher VIN or VOUT.

( )( ) ∆IL =

1

fL

⎛

VOUT ⎝⎜1–

VOUT ⎞

VIN ⎠⎟

(1)

Accepting larger values of ∆IL allows the use of lower

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is 30% to 40% of IMAX. Remember,

the maximum ∆IL occurs at the maximum input voltage.

The inductor value also has an effect on Burst Mode

operation. The transition to low current operation begins

when the inductor current peaks fall to approximately 1/3

its original value. Lower inductor values (higher ∆IL) will

cause this to occur at lower 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 increase.

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 in-

crease. Ferrite designs have very low core losses and are

preferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard”, which means that

inductance collapses abruptly when the peak design cur-

rent 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

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, new designs for surface mount are

available which do not increase the height significantly.

Power MOSFET and Schottky Diode Selection

Two external power MOSFETs must be selected for use

with the LTC1773: a P-channel MOSFET for the top (main)

switch, and an N-channel MOSFET for the bottom (syn-

chronous) switch.

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

voltage. Therefore, for VIN > 5V, logic-level threshold

MOSFETs should be used. But, for VIN < 5V, sub-logic

level threshold MOSFETs (VGS(TH) < 3V) should be used.

In these applications, make sure that the VIN to the

LTC1773 is less than 8V because the absolute maximum

VGS rating of the majority of these sub-logic threshold

MOSFETs is 8V.

Selection criteria for the power MOSFETs include the “ON”

resistance RDS(ON), reverse transfer capacitance CRSS,

input voltage, maximum output current, and total gate

charge. When the LTC1773 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 RDSON +

K(VIN )2 (IMAX )(C RSS )(f)

1773fb

8

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