參數(shù)資料
型號： LTC3728LZEUH
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 3 A DUAL SWITCHING CONTROLLER, 590 kHz SWITCHING FREQ-MAX, PQCC28
封裝： 5 X 5 MM, PLASTIC, MO-220WHHD, QFN-28
文件頁數(shù)： 6/32頁
文件大?。?/td> 467K
代理商： LTC3728LZEUH
LTC3728LZ
14
3728lzf
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 RSENSE. Lower
inductor values (higher ΔIL) will cause this to occur at
lower load currents, which can cause a dip in efciency 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 efciency 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 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 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 efcient,
especially when you can use several layers of wire. Because
they generally lack a bobbin, mounting is more difcult.
However, designs for surface mount are available that do
not increase the height signicantly.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for each
controller in the LTC3728LZ: 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 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 specication 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), Miller capacitance CMILLER, input
voltage and maximum output current. Miller capacitance,
CMILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. CMILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
at divided by the specied change in VDS. This result is
then multiplied by the ratio of the application applied VDS
to the Gate charge curve specied VDS. When the IC is
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle
V
OUT
IN
=
Synchronous Switch Duty Cycle
VV
V
IN
OUT
IN
=
–
The MOSFET power dissipations at maximum output
current are given by:
P
V
IR
V
I
MAIN
OUT
IN
MAX
DS ON
IN
MAX
=
() +
()
+
()
2
1
δ
()
2
11
()(
)
+
RC
VV
V
DR
MILLER
INTVCC
THMIN
TH
–
M
MIN
f
()
P
VV
V
IR
SYNC
IN
OUT
IN
MAX
DS ON
=
() +
()
–
()
2
1
δ
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