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LTC3728

3728fb

The inductor value also has secondary effects. The transi-

tion to Burst Mode operation begins when the average

inductor current required results in a peak current below

25% of the current limit determined by RSENSE. 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 decrease.

Inductor Core Selection

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

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron

cores, forcing the use of more expensive ferrite,

molypermalloy, or Kool M

µ® 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

ferrite. A reasonable compromise from the same manu-

facturer is Kool M

µ. 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

that do not increase the height significantly.

Power MOSFET and D1 Selection

Two external power MOSFETs must be selected for each

controller in the LTC3728: One N-channel MOSFET for the

top (main) switch, and one N-channel MOSFET for the

bottom (synchronous) switch.

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

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

EXTVCC Pin Connection). Consequently, logic-level

threshold MOSFETs must be used in most applications.

The only exception is if low input voltage is expected

(VIN < 5V); then, sub-logic level threshold MOSFETs

(VGS(TH) < 3V) should be used. Pay close attention to the

BVDSS specification for the MOSFETs as well; most of the

logic level MOSFETs are limited to 30V or less.

Selection criteria for the power MOSFETs include the “ON”

resistance RDS(ON), reverse transfer capacitance CRSS,

input voltage and maximum output current. When the

LTC3728 is operating in continuous mode the duty cycles

for the top and bottom MOSFETs are given by:

Main Switch Duty Cycle

OUT

Synchronous Switch Duty Cycle

OUT

–

The MOSFET power dissipations at maximum output

current are given by:

MAIN

OUT

MAX

DS ON

MAX

RSS

() +

()

() ( )( )( )

()

SYNC

OUT

MAX

DS ON

() +

()

–

()

where

δ is the temperature dependency of RDS(ON) and k

is a constant inversely related to the gate drive current.

Both MOSFETs have I2R losses while the topside N-channel

equation includes an additional term for transition losses,

which are highest at high input voltages. For VIN < 20V the

high current efficiency generally improves with larger

MOSFETs, while for VIN > 20V the transition losses rapidly

increase to the point that the use of a higher RDS(ON) device

with lower CRSS actually provides higher efficiency. The

Kool M

µ is a registered trademark of Magnetics, Inc.

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