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參數(shù)資料
| 型號(hào)： | LT3845EFE#TR |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 600 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-16 |
| 文件頁(yè)數(shù)： | 6/24頁(yè) |
| 文件大?。?/td> | 263K |
| 代理商： | LT3845EFE#TR |
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LT3845
14
3845fb
APPLICATIONS INFORMATION
will increase the peak currents, requiring more ltering
on the input and output of the supply. If
ΔIL is too high,
the slope compensation circuit is ineffective and current
mode instability may occur at duty cycles greater than
50%. To satisfy slope compensation requirements the
minimum inductance is calculated as follows:
LV
DC
R
f
MIN
OUT
MAX
SENSE
SW
>
–
.
21
833
The magnetics vendors specify either the saturation cur-
rent, the RMS current or both. When selecting an inductor
based on inductor saturation current, use the peak current
through the inductor, IOUT(MAX) + ΔIL/2. The inductor
saturation current specication is the current at which
the inductance, measured at zero current, decreases by
a specied amount, typically 30%.
When selecting an inductor based on RMS current rating,
use the average current through the inductor, IOUT(MAX).
The RMS current specication is the RMS current at which
the part has a specic temperature rise, typically 40°C,
above 25°C ambient.
After calculating the minimum inductance value, the
volt-second product, the saturation current and the RMS
current for your design, select an off-the-shelf inductor.
Contact the Application group at Linear Technology for
further support.
For more detailed information on selecting an inductor,
please see the “Inductor Selection” section of Linear
Technology Application Note 44.
MOSFET Selection
The selection criteria of the external N-channel standard
level power MOSFETs include on resistance (RDS(ON)),
reverse transfer capacitance (CRSS), maximum drain
source voltage (VDSS),totalgatecharge(QG)andmaximum
continuous drain current.
For maximum efciency, minimize RDS(ON) and CRSS.
Low RDS(ON) minimizes conduction losses while low CRSS
minimizes transition losses. The problem is that RDS(ON)
is inversely related to CRSS. In selecting the top MOSFET
balancing the transition losses with the conduction losses
is a good idea while the bottom MOSFET is dominated by
the conduction loss, which is worse during a short-circit
condition or at a very low duty cycle.
Calculate the maximum conduction losses of the
MOSFETs:
PI
V
R
P
COND TOP
OUT MAX
OUT
IN
DS ON
COND
()
(
)
(
)
(
=
2
B
BOT
OUT MAX
IN
OUT
IN
DS ON
I
VV
V
R
)(
)
(
)
–
=
2
Note that RDS(ON) has a large positive temperature depen-
dence. The MOSFET manufacturer’s data sheet contains
a curve, RDS(ON) vs Temperature.
In the main MOSFET, transition losses are proportional
to VIN2 and can be considerably large in high voltage ap-
plications (VIN > 20V). Calculate the maximum transition
losses:
PTRAN(TOP) = k VIN2 IOUT(MAX) CRSS fSW
where k is a constant inversely related to the gate driver
current, approximated by k = 2 for LT3845 applications.
The total maximum power dissipations of the MOSFET
are:
PTOP(TOTAL) = PCOND(MAIN) + PTRAN(MAIN)
PBOT(TOTAL) = PCOND(SYNC)
To achieve high supply efciency, keep the total power dis-
sipation in each switch to less than 3% of the total output
power. Also, complete a thermal analysis to ensure that
the MOSFET junction temperature is not exceeded.
TJ = TA + P(TOTAL) θJA
where
θJA is the package thermal resistance and TA is the
ambient temperature. Keep the calculated TJbelowthemax-
imum specied junction temperature, typically 150°C.
Note that when VIN is high and fSW is high, the transition
losses may dominate. A MOSFET with higher RDS(ON)
and lower CRSS may provide higher efciency. MOSFETs
with higher voltage VDSS specication usually have higher
RDS(ON) and lower CRSS.
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