LTC3728_15 Datasheet, PDF (16/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIONS INFORMATION
load transient response and sufficient ripple current sig-
nal in the current loop. The maximum ΔI_L occurs at the
maximum input voltage.
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 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
manufacturer is Kool Mμ. Toroids are very space efficient,
especially when you can use several layers of wire. Because
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.
The peak-to-peak drive levels are set by the INTV_CC
voltage. This voltage is typically 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, sublogic level threshold MOSFETs (V_GS(TH)
< 3V) should be used. Pay close attention to the BV_DSS
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 R_DS(ON), reverse-transfer capacitance C_RSS,
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:
Main Switch Duty Cycle = ^VOUT
V_IN
Synchronous Switch Duty Cycle = ^{VIN – VOUT}
V_IN
The MOSFET power dissipations at maximum output
current are given by:
( ) ( ) P_MAIN
=
V_OUT
V_IN
I_MAX
2
1+

R_DS(ON) +
k(V_IN)² (I_MAX )₍C_{RSS )}(f)
( ) ( ) P_SYNC
=
V_IN
– V_OUT
V_IN
I_MAX
2
1+

R_DS(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 I²R losses while the topside N-channel
equation includes an additional term for transition losses,
3728fg
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