LTC1143CS datasheet(10/20 Pages) LINER

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LTC1143/LTC1143L

LTC1143L-ADJ

Once the frequency has been set by CT, the inductor L must

be chosen to provide no more than 25mV/RSENSE of peak-

to-peak inductor ripple current. This results in a minimum

required inductor value of:

LMIN = 5.1(105)(RSENSE)(CT)VREG

As the inductor value is increased from the minimum

value, the ESR requirements for the output capacitor are

eased at the expense of efficiency. If too small an inductor

is used, the inductor current will become discontinuous

before the LTC1143 series enters Burst Mode operation. A

consequence of this is that the LTC1143 series will delay

entering Burst Mode operation and efficiency will be

degraded at low currents.

Inductor Core Selection

Once the minimum value for L is known, the type of

inductor must be selected. The highest efficiency will be

obtained using Ferrite, Kool M

µ®or Molypermalloy (MPP)

cores. Lower cost powdered iron cores provide suitable

performance, but cut efficiency by 3% to 7%. 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, 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 current

is exceeded. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple that

can cause Burst Mode operation to be falsely triggered. Do

not allow the core to saturate!

Kool M

µ(from Magnetics, Inc.) is a very good, low loss core

material for toroids with a “soft” saturation characteristic.

Molypermalloy is slightly more efficient at high ( > 200 kHz)

switching frequencies but quite a bit more expensive.

Toroids are very space efficient, especially when you can

use several layers of wire, while inductors wound on

bobbins are generally easier to surface mount. New designs

for surface mount are available from Coiltronics, Coilcraft

and Sumida.

APPLICATIONS INFORMATION

Power MOSFET Selection

An external power MOSFET must be selected for use with

each section of the LTC1143 series. The main selection

criteria for the power MOSFETs are the threshold voltage

VGS(TH), maximum VGS rating and on resistance RDS(ON).

Surface mount P-channel power MOSFETs are widely

available in both single and dual configurations. Logic

level MOSFETs are specified for operation up to 20V

maximum VGS and guarantee a maximum RDS(ON) with

VGS = 4.5V. Newer ‘sub’ logic level MOSFETs allow only 8V

maximum VGS but guarantee RDS(ON) with VGS = 2.7V. If

VIN will exceed 8V, logic level MOSFETs must be used; if

conservatively specified, they are generally usable down

to the 3.5V minimum VIN rating of the LTC1143L and

LTC1143L-ADJ.

The maximum output current IMAX determines the RDS(ON)

requirement for the two MOSFETs. When the LTC1143

series is operating in continuous mode, the simplifying

assumption can be made that either the MOSFET or

Schottky diode is always conducting the average load

current. The duty cycles for the MOSFET and diode are

given by:

PCh

Schottky

OUT

Duty Cycle

Diode Duty Cycle =

OUT

≈

−+

()

From the duty cycles the required RDS(ON) for each MOSFET

can be derived:

PCh

OUT MAX

DS(ON)

()









()

where PP is the allowable power dissipation and δP is the

temperature dependencies of RDS(ON). PP will be determined

by efficiency and/or thermal requirements (see Efficiency

Considerations). (1+

δP) is generally given for a MOSFET in

the form of a normalized RDS(ON) vs temperature curve,

Kool M

µ is a registered trademark of Magnetics, Inc.

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