LTC3728 Linear Technology, LTC3728 Datasheet - Page 14

• Manufacturer Part Number: LTC3728
• Description: 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3728
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,
molypermalloy, or Kool Mµ
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 LTC3728: One N-channel MOSFET for the
top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
14
U
U
L
) will cause this to occur at
®
cores. Actual core loss is
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
input voltage and maximum output current. When the
LTC3728 is operating 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:
where δ is the temperature dependency of R
is a constant inversely related to the gate drive current.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
high current efficiency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
Kool Mµ is a registered trademark of Magnetics, Inc.
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
< 3V) should be used. Pay close attention to the
Pin Connection). Consequently, logic-level
=
=
k V
( ) ( )(
V
V
RSS
DS(ON)
V
OUT
IN
IN
IN
–
V
2
actually provides higher efficiency. The
IN
( )
V
I
I
, reverse transfer capacitance C
MAX
MAX
OUT
2
IN
R losses while the topside N-channel
> 20V the transition losses rapidly
( )
2
I
MAX
C
( )
1 δ
RSS
+
=
2
V
)( )
( )
V
OUT
R
1 δ
f
IN
w w w . D a t a S h e e t 4 U . c
+
DS ON
(
=
R
V
)
DS ON
IN
+
(
–
V
DS(ON)
IN
DS(ON)
V
IN
)
OUT
< 20V the
device
and k
RSS
3728fb
CC
,