Datasheet

LTC3727A-1
14
3727a1fa
The inductor value also has secondary effects. The tran-
sition 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
SENSE
. Lower
inductor values (higher ΔI
L
) will cause this to occur at
lower load currents, which can cause a dip in effi ciency 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 inductance value is determined, the type of
inductor must be selected. Actual core loss is independent
of core size for a fi xed 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 (I
2
R)
losses will increase.
Ferrite designs have very low core loss and are preferred at
high switching frequencies, so designers can concentrate
on reducing I
2
R 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!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated fi eld/EMI requirements. New designs for high cur-
rent surface mount inductors are available from numerous
manufacturers, including Coiltronics, Vishay, TDK, Pulse,
Panasonic, Wuerth, Coilcraft, Toko and Sumida.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for each
controller in the LTC3727A-1: One N-channel MOSFET for
the top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
APPLICATIONS INFORMATION
The peak-to-peak drive levels are set by the INTV
CC
voltage. This voltage is typically 7.5V during start-up
(see EXTV
CC
Pin Connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected
(V
IN
< 5V); then, sub-logic level threshold MOSFETs
(V
GS(TH)
< 3V) should be used. Pay close attention to the
BV
DSS
specifi cation 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 R
DS(ON)
, reverse transfer capacitance C
RSS
,
input voltage and maximum output current. When the
LTC3727A-1 is operating in continuous mode the duty
cycles for the top and bottom MOSFETs are given by:
Main Switch Duty Cycle
V
V
Synchronous Switc
OUT
IN
=
hh Duty Cycle
VV
V
IN OUT
IN
=
–
The MOSFET power dissipations at maximum output
current are given by:
P
V
V
IR
kV I
MAIN
OUT
IN
MAX DS ON
IN MA
=
()
+
()
+
()
2
2
1 δ
()
XXRSS
SYNC
IN OUT
IN
MAX
Cf
P
VV
V
IR
()
()
()
=
()
+
()
–
2
1 δ
DDS ON()
where δ is the temperature dependency of R
DS(ON)
and k
is a constant inversely related to the gate drive current.
Both MOSFETs have I
2
R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
IN
< 20V
the high current effi ciency generally improves with larger
MOSFETs, while for V
IN
> 20V the transition losses rapidly
increase to the point that the use of a higher R
DS(ON)
device
with lower C
RSS
actually provides higher effi ciency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.