LTC3728LX Linear Technology, LTC3728LX Datasheet - Page 14

• Manufacturer Part Number: LTC3728LX
• Description: 2-Phase Synchronous Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3728L/LTC3728LX
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 R
inductor values (higher I
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, molyper-
malloy, or Kool M
dent of core size for a fixed inductor value, but it is very
dependent on inductance selected. As inductance in-
creases, 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 M . 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 LTC3728L/LTC3728LX: One N-channel
MOSFET for the top (main) switch, and one N-channel
MOSFET for the bottom (synchronous) switch.
14
U
®
cores. Actual core loss is indepen-
U
L
) will cause this to occur at
W
SENSE
U
. Lower
The peak-to-peak drive levels are set by the INTV
voltage. This voltage is typically 5V during start-up (see
EXTV
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected
(V
(V
BV
logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
age and maximum output current. Miller capacitance,
C
usually provided on the MOSFET manufacturers’ data
sheet. C
the horizontal axis while the curve is approximately flat
divided by the specified change in V
multiplied by the ratio of the application applied V
Gate charge curve specified V
in continuous mode the duty cycles for the top and bottom
MOSFETs are given by:
The MOSFET power dissipations at maximum output
current are given by:
Kool M is a registered trademark of Magnetics, Inc.
MILLER
IN
GS(TH)
DSS
P
P
Main Switch Duty Cycle
Synchronous Switch Duty Cycle
MAIN
SYNC
< 5V); then, sub-logic level threshold MOSFETs
CC
specification for the MOSFETs as well; most of the
MILLER
, can be approximated from the gate charge curve
< 3V) should be used. Pay close attention to the
Pin Connection). Consequently, logic-level
V
V
V
DS(ON)
V
IN
INTVCC
V
OUT
IN
IN
is equal to the increase in gate charge along
2
–
V
IN
I
V
, Miller capacitance C
I
MAX
MAX
OUT
2
1
–
V
THMIN
2
I
R
MAX
1
DR
DS
2
V
C
. When the IC is operating
V
OUT
R
1
V
IN
MILLER
w w w . D a t a S h e e t 4 U . c
DS ON
THMIN
DS
1
(
. This result is then
R
MILLER
V
)
DS ON
IN
•
f
(
–
V
IN
V
)
, input volt-
OUT
DS
to the
3728lxfa
CC