LTC1439CG datasheet(13/32 Pages) LINER

13 / 32 page

LTC1438/LTC1439

APPLICATIONS INFORMATION

Accepting larger values of

∆IL 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

∆IL = 0.4(IMAX). Remember, the

maximum

∆IL occurs at the maximum input voltage.

The inductor value also has an effect on low current

operation. The transition to low current operation begins

when the inductor current reaches zero while the bottom

MOSFET is on. Lower inductor values (higher

∆IL) will

cause this to occur at higher load currents, which can

cause a dip in efficiency in the upper range of low current

operation. In Burst Mode operation (TGS1, 2 pins open),

lower inductance values will cause the burst frequency to

decrease.

The Figure 3 graph gives a range of recommended induc-

tor values vs operating frequency and VOUT.

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

which do not increase the height significantly.

Power MOSFET and D1 Selection

Three external power MOSFETs must be selected for each

controller with the LTC1439: a pair of N-channel MOSFETs

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

the bottom (synchronous) switch. Only one top MOSFET

is required for each LTC1438 controller.

To take advantage of the Adaptive Power output stage, two

topside MOSFETs must be selected. A large [low RSD(ON)]

MOSFET and a small [higher RDS(ON)] MOSFET are re-

quired. The large MOSFET is used as the main switch and

works in conjunction with the synchronous switch. The

smaller MOSFET is only enabled under low load current

conditions. The benefit of this is to boost low to midcurrent

efficiencies while continuing to operate at constant fre-

quency. Also, by using the small MOSFET the circuit will

keep switching at a constant frequency down to lower

currents and delay skipping cycles.

The RDS(ON) recommended for the small MOSFET is

around 0.5

Ω. Be careful not to use a MOSFET with an

RDS(ON) that is too low; remember, we want to conserve

gate charge. (A higher RDS(ON) MOSFET has a smaller gate

capacitance and thus requires less current to charge its

gate). For all LTC1438 and cost sensitive LTC1439 appli-

cations, the small MOSFET is not required. The circuit then

begins Burst Mode operation as the load current drops.

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 increase.

Kool M

µ is a registered trademark of Magnetics, Inc.

OPERATING FREQUENCY (kHz)

100

150

200

1438 F03

250

300

VOUT = 5.0V

VOUT = 3.3V

VOUT = 2.5V

Figure 3. Recommended Inductor Values

LTC1439CG 数据表(PDF) 13 Page - Linear Technology

LTC1439CG 数据表(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

数据表下载