LTC1772CS6 datasheet(7/12 Pages) LINER

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LTC1772

1772fb

APPLICATIONS INFORMATION

where f is the operating frequency. Accepting larger values

of IRIPPLE allows the use of low inductances, but results in

higher output voltage ripple and greater core losses. A

reasonable starting point for setting ripple current is

IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE

occurs at the maximum input voltage.

In Burst Mode operation on the LTC1772, the ripple

current is normally set such that the inductor current is

continuous during the burst periods. Therefore, the peak-

to-peak ripple current must not exceed:

RIPPLE

SENSE

≤

003

This implies a minimum inductance of:

MIN

OUT

SENSE

OUT

−

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

003

(Use VIN(MAX) = VIN)

A smaller value than LMIN could be used in the circuit;

however, the inductor current will not be continuous

during burst periods.

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 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 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 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, new designs for surface mount that do

not increase the height significantly are available.

Power MOSFET Selection

An external P-channel power MOSFET must be selected

for use with the LTC1772. The main selection criteria for

the power MOSFET are the threshold voltage VGS(TH) and

the “on” resistance RDS(ON), reverse transfer capacitance

CRSS and total gate charge.

Since the LTC1772 is designed for operation down to low

input voltages, a sublogic level threshold MOSFET (RDS(ON)

guaranteed at VGS = 2.5V) is required for applications that

work close to this voltage. When these MOSFETs are used,

make sure that the input supply to the LTC1772 is less than

the absolute maximum VGS rating, typically 8V.

The required minimum RDS(ON) of the MOSFET is gov-

erned by its allowable power dissipation. For applications

that may operate the LTC1772 in dropout, i.e., 100% duty

cycle, at its worst case the required RDS(ON) is given by:

DS ON

OUT MAX

()

= () +()

100

where PP is the allowable power dissipation and δp is the

temperature dependency of RDS(ON). (1 + δp) is generally

given for a MOSFET in the form of a normalized RDS(ON) vs

temperature curve, but

δp = 0.005/°C can be used as an

approximation for low voltage MOSFETs.

In applications where the maximum duty cycle is less than

100% and the LTC1772 is in continuous mode, the RDS(ON)

is governed by:

DC I

DS ON

OUT

() ≅ ()

()

2 1 δ

where DC is the maximum operating duty cycle of the

LTC1772.

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