LTC1435
U
W U U
APPLICATIONS INFORMATION
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, lower inductance
values will cause the burst frequency to decrease.
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, designsforsurfacemountareavailable
which do not increase the height significantly.
The Figure 3 graph gives a range of recommended induc-
tor values vs operating frequency and VOUT
.
Power MOSFET and D1 Selection
60
Two external power MOSFETs must be selected for use
with the LTC1435: an N-channel MOSFET for the top
(main) switch and an N-channel MOSFET for the bottom
(synchronous) switch.
V
OUT
V
OUT
V
OUT
= 5.0V
= 3.3V
= 2.5V
50
40
30
20
10
0
The peak-to-peak gate drive levels are set by the INTVCC
voltage. This voltage is typically 5V during start-up (see
EXTVCC PinConnection).Consequently,logiclevelthresh-
old MOSFETs must be used in most LTC1435 applica-
tions. The only exception is applications in which EXTVCC
is powered from an external supply greater than 8V (must
be less than 10V), in which standard threshold MOSFETs
(VGS(TH) < 4V) may be used. Pay close attention to the
BVDSS specification for the MOSFETs as well; many of the
logic level MOSFETs are limited to 30V or less.
0
100
150
200
250
300
50
OPERATING FREQUENCY (kHz)
1435 F03
Figure 3. Recommended Inductor Values
Inductor Core Selection
SelectioncriteriaforthepowerMOSFETsincludethe“ON”
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
godown. Unfortunately, increasedinductancerequiresmore
turns of wire and therefore copper losses will increase.
resistance RSD(ON), reverse transfer capacitance CRSS
,
input voltage and maximum output current. When the
LTC1435 is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
V
V
OUT
Main Switch Duty Cycle =
IN
V − V
(
)
IN
OUT
Synchronous Switch Duty Cycle =
V
IN
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!
The MOSFET power dissipations at maximum output
current are given by:
V
V
2
OUT
P
=
I
(
1+δ R
+
) (
)
MAIN
MAX
DS ON
(
)
IN
1.85
k V
I
(
C
f
(
)
)(
)( )
IN
MAX
RSS
Molypermalloy (from Magnetics, Inc.) is a very good, low
losscorematerialfortoroids,butitismoreexpensivethan
V − V
2
IN
OUT
P
=
I
(
1+δ R
) (
)
SYNC
MAX
DS ON
(
)
V
IN
Kool Mµ is a registered trademark of Magnetics, Inc.
9
Linear [ Linear ]
