LTC3707-SYNC 查看數據表（PDF） - Linear Technology
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LTC3707-SYNC Datasheet PDF : 32 Pages
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LTC3707-SYNC
APPLICATIO S I FOR ATIO
The inductor value also has secondary effects. The transi-
tion to Burst Mode operation begins when the average
inductor current required results in a peak current below
25% of the current limit determined by RSENSE. Lower
inductor values (higher ∆IL) will cause this to occur at
lower 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.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford 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.
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
that do not increase the height significantly.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for each
controller in the IC: One N-channel MOSFET for the top
(main) switch, and one N-channel MOSFET for the bottom
(synchronous) switch.
Kool Mµ is a registered trademark of Magnetics, Inc.
14
The peak-to-peak drive levels are set by the INTVCC
voltage. This voltage is typically 5V during start-up (see
EXTVCC Pin Connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected
(VIN < 5V); then, sub-logic level threshold MOSFETs
(VGS(TH) < 3V) should be used. Pay close attention to the
BVDSS specification for the MOSFETs as well; most of the
logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance RDS(ON), reverse transfer capacitance CRSS,
input voltage and maximum output current. When the IC
is operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle = VOUT
VIN
Synchronous Switch Duty Cycle = VIN – VOUT
VIN
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
( ) ( ) PMAIN
=
VOUT
VIN
IMAX
2
1+ δ
RDS(ON) +
( ) ( )( )( ) 1
2
VIN
2
IMAX
RDR
C MILLER
( )
1
VINTVCC
–
VTH
+
1
VTH
f
( ) ( ) PSYNC
=
VIN
– VOUT
VIN
2
IMAX 1+ δ RDS(ON)
where δ is the temperature dependency of RDS(ON), RDR is
the effective top driver resistance over the (of approxi-
mately 4Ω at VGS = VMILLER), VIN is the drain potential and
the change in drain potential in the particular application.
VTH is the data sheet specified typical gate threshold
voltage specified in the power MOSFET data sheet. CMILLER
is the calculated capacitance using the gate charge curve
from the MOSFET data sheet. CMILLER is determined by
dividing the increase in charge indicated on the x axis
during the flat, Miller portion of the curve by the stated VDS
transition voltage specified on the curve.
3707sf
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