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LTC3728L-1

3728l1fc

APPLICATIONS INFORMATION

25% of the current limit determined by RSENSE. Lower

inductor values (higher ΔIL) 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

Usually, high inductance is preferred for small current

ripple and low core loss. Unfortunately, increased induc-

tance requires more turns of wire or a smaller air gap in

the inductor core, resulting in high copper loss or low

saturation current. Once the value of L is known, the actual

inductor must be selected. There are two popular types

of core material of commercial available inductors. Ferrite

core inductors usually have very low core loss and are

preferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing satura-

tion. However, ferrite core saturates “hard”, which means

that inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequent output voltage

ripple. One advantage of the LTC3728L-1 is its current

mode control that detects and limits cycle-by-cycle peak

inductor current. Therefore, accurate and fast protection

is achieved if the inductor is saturated in steady state or

during transient mode.

Powdered iron core inductors usually saturate “soft”, which

means the inductance drops in a linear fashion when the

current increases. However, the core loss of the powder

iron inductor is usually higher than the ferrite inductor.

So designs with high switching frequency should also

address inductor core loss.

Inductor manufacturers usually provide inductance, DCR,

(peak) saturation current and (DC) heating current ratings

in the inductor data sheet. A good supply design should

not exceed the saturation and heating current rating of

the inductor.

Power MOSFET and D1 Selection

Two external power MOSFETs must be selected for each

controller in the LTC3728L-1: 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), Miller capacitance CMILLER, input

voltage and maximum output current. Miller capacitance,

CMILLER, can be approximated from the gate charge curve

usually provided on the MOSFET manufacturers’ data

sheet. CMILLER is equal to the increase in gate charge

along the horizontal axis while the curve is approximately

ﬂat divided by the speciﬁed change in VDS. This result is

then multiplied by the ratio of the application applied VDS

to the Gate charge curve speciﬁed VDS. When the IC 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+

()R

DS(ON) +

VIN

()2 IMAX

RDR

() C

MILLER

()•

VINTVCC –VTHMIN

VTHMIN

()

PSYNC =

VIN –VOUT

VIN

IMAX

()2 1+

()R

DS(ON)

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