LTC1148CS datasheet(9/20 Pages) LINER

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LTC1148

LTC1148-3.3/LTC1148-5

APPLICATIO S I FOR ATIO

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® on 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 induc-

tance increases, core losses go down. Unfortunately,

increased inductance requires more turns of wire and

therefore copper losses 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 cur-

rent is exceeded. This results in an abrupt increase in

inductor ripple current and consequent output voltage

ripple which 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 charac-

teristic. Molypermalloy is slightly more efficient at high

(>200kHz) switching frequencies, but quite a bit more

expensive. Toroids are very space efficient, especially

when you can use several layers of wire. Because they

generally lack a bobbin, mounting is more difficult. How-

ever, new designs for surface mount are available from

Coiltronics and Beckman Industrial Corp. which do not

increase the height significantly.

Power MOSFET and D1 Selection

Two external power MOSFETs must be selected for use

with the LTC1148 series: a P-channel MOSFET for the

main switch, and an N-channel MOSFET for the synchro-

nous switch. The main selection criteria for the power

MOSFETs are the threshold voltage VGS(TH) and on resis-

tance RDS(ON).

The minimum input voltage determines whether standard

threshold or logic-level threshold MOSFETs must be used.

For VIN > 8V, standard threshold MOSFETs (VGS(TH) < 4V)

may be used. If VIN is expected to drop below 8V, logic-

FREQUENCY (kHz)

200

400

600

100

200

LTC1148 • F03

800

1000

300

VSENSE– = VOUT = 5V

VIN=12V

VIN = 10V

VIN = 7V

Figure 3. Timing Capacitor Value

f =

tOFF

)

)1– VOUT

VIN

where:

tOFF = 1.3(104)CT

)

)VREG

VOUT

VREG is the desired output voltage (i.e., 5V, 3.3V). VOUT is

the measured output voltage. Thus VREG/VOUT = 1 in

regulation.

Note that as VIN decreases, the frequency decreases.

When the input to output voltage differential drops

below 1.5V, the LTC1148 series reduces tOFF by in-

creasing the discharge current in CT. This prevents

audible operation prior to dropout.

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 decrease past

zero and change polarity. A consequence of this is that

the LTC1148 series may not enter Burst Mode operation

and efficiency will be severely degraded at low currents.

Kool M

µis a registered trademark of Magnetics, Inc.

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