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參數(shù)資料
| 型號： | LTC1628I-PG |
| 廠商： | Linear Technology Corporation |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 抗輻射高效，5安培開關穩(wěn)壓器 |
| 文件頁數(shù)： | 14/32頁 |
| 文件大小： | 375K |
| 代理商： | LTC1628I-PG |
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14
LTC1628-SYNC
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
SENSE
. Lower
inductor values (higher
I
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 M
μ
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 manu-
facturer is Kool M
μ
. 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 LTC1628-SYNC: 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, sub-logic 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
LTC1628-SYNC is operating in continuous mode the duty
cycles for the top and bottom MOSFETs are given by:
MainSwitchDutyCycle
V
V
OUT
IN
=
SynchronousSwitchDutyCycle
V
V
V
IN
OUT
IN
=
–
The MOSFET power dissipations at maximum output
current are given by:
P
V
V
I
R
k V
(
I
C
f
MAIN
OUT
IN
MAX
DS ON
(
IN
MAX
RSS
=
(
)
)(
+
(
)
+
) (
)( )
2
2
1
δ
)
P
V
V
V
I
R
SYNC
IN
OUT
IN
MAX
DS ON
(
=
(
)
+
(
)
–
)
2
1
δ
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 efficiency 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 efficiency. The
Kool M
μ
is a registered trademark of Magnetics, Inc.
APPLICATIOU
W
U
U
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