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參數(shù)資料
| 型號： | LTC1435IG#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | High Efficiency Low Noise Synchronous Step-Down Switching Regulator |
| 中文描述： | 2 A SWITCHING CONTROLLER, 400 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 0.209 INCH, PLASTIC, SSOP-16 |
| 文件頁數(shù)： | 20/20頁 |
| 文件大?。?/td> | 338K |
| 代理商： | LTC1435IG#TRPBF |
9
LTC1435
APPLICATIONS INFORMATION
WU
U
MOSFET is on. Lower inductor values (higher
IL) will
cause this to occur at higher 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.
The Figure 3 graph gives a range of recommended induc-
tor values vs operating frequency and VOUT.
Kool M
is a registered trademark of Magnetics, Inc.
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
which do not increase the height significantly.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for use
with the LTC1435: an N-channel MOSFET for the top
(main) switch and an N-channel MOSFET for the bottom
(synchronous) switch.
The peak-to-peak gate drive levels are set by the INTVCC
voltage. This voltage is typically 5V during start-up (see
EXTVCC Pin Connection). Consequently, logic level thresh-
old MOSFETs must be used in most LTC1435 applica-
tions. The only exception is applications in which EXTVCC
is powered from an external supply greater than 8V (must
(VGS(TH) < 4V) may be used. Pay close attention to the
BVDSS specification for the MOSFETs as well; many of the
logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance RSD(ON), reverse transfer capacitance CRSS,
input voltage and maximum output current. When the
LTC1435 is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
Main Switch Duty Cycle =
V
Synchronous Switch Duty Cycle =
V
OUT
IN
IN
()
V
OUT
IN
The MOSFET power dissipations at maximum output
current are given by:
P
V
IR
IC
f
P
VV
V
IR
MAIN
OUT
IN
MAX
DS ON
MAX
RSS
SYNC
IN
OUT
IN
MAX
DS ON
=
() +
()
+
() (
)(
)( )
=
() +
()
2
185
2
1
δ
k VIN
.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool M
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
Figure 3. Recommended Inductor Values
OPERATING FREQUENCY (kHz)
0
INDUCTOR
VALUE
(
H)
10
20
30
40
60
50
100
150
200
1435 F03
250
300
50
VOUT = 5.0V
VOUT = 3.3V
VOUT = 2.5V
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