LTC1438/LTC1439
13
14389fb
APPLICATIONS INFORMATION
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Accepting larger values of ∆I
L
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 ∆I
L
= 0.4(I
MAX
). Remember, the
maximum ∆I
L
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 ∆I
L
) 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 V
OUT
.
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 Mµ. 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 R
SD(ON)
]
MOSFET and a small [higher R
DS(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 R
DS(ON)
recommended for the small MOSFET is
around 0.5Ω. Be careful not to use a MOSFET with an
R
DS(ON)
that is too low; remember, we want to conserve
gate charge. (A higher R
DS(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 Mµ
®
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 Mµ is a registered trademark of Magnetics, Inc.
OPERATING FREQUENCY (kHz)
0
0
INDUCTOR VALUE (µH)
10
20
30
40
60
50
100 150 200
1438 F03
250 300
50
V
OUT
= 5.0V
V
OUT
= 3.3V
V
OUT
= 2.5V
Figure 3. Recommended Inductor Values
