参数资料
型号: LT3757MPMSE#PBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO10
封装: 3 X 3 MM, LEAD FREE, PLASTIC, MSOP-10
文件页数: 16/36页
文件大小: 436K
代理商: LT3757MPMSE#PBF
LT3757
23
3757fc
Givenanoperatinginputvoltagerange,andhavingchosen
the operating frequency and ripple current in the inductor,
theinductorvalue(L1andL2areindependent)oftheSEPIC
converter can be determined using the following equation:
L1
=L2=
VIN(MIN)
0.5
ISW f
DMAX
For most SEPIC applications, the equal inductor values
will fall in the range of 1H to 100H.
BymakingL1=L2,andwindingthemonthesamecore,the
value of inductance in the preceding equation is replaced
by 2L, due to mutual inductance:
L
=
VIN(MIN)
ISW f
DMAX
This maintains the same ripple current and energy storage
in the inductors. The peak inductor currents are:
IL1(PEAK) = IL1(MAX) + 0.5 IL1
IL2(PEAK) = IL2(MAX) + 0.5 IL2
The RMS inductor currents are:
IL1(RMS) =IL1(MAX) 1+
χ2L1
12
where:
χL1=
IL1
IL1(MAX)
IL2(RMS) =IL2(MAX) 1+
χ2L2
12
where:
χL2 =
IL2
IL2 (MAX)
Based on the preceding equations, the user should choose
the inductors having sufficient saturation and RMS cur-
rent ratings.
In a SEPIC converter, when the power switch is turned on,
the current flowing through the sense resistor (ISENSE) is
the switch current.
Set the sense voltage at ISENSE(PEAK) to be the minimum
of the SENSE current limit threshold with a 20% margin.
The sense resistor value can then be calculated to be:
RSENSE =
80 mV
ISW(PEAK)
SEPIC Converter: Power MOSFET Selection
For the SEPIC configuration, choose a MOSFET with a
VDC rating higher than the sum of the output voltage and
input voltage by a safety margin (a 10V safety margin is
usually sufficient).
The power dissipated by the MOSFET in a SEPIC con-
verter is:
PFET = I2SW(MAX) RDS(ON) DMAX
+ 2 (VIN(MIN) + VOUT)2 IL(MAX) CRSS f/1A
The first term in this equation represents the conduction
losses in the device, and the second term, the switching
loss. CRSS is the reverse transfer capacitance, which is
usually specified in the MOSFET characteristics.
For maximum efficiency, RDS(ON) and CRSS should be
minimized. From a known power dissipated in the power
MOSFET, its junction temperature can be obtained using
the following equation:
TJ = TA + PFET θJA = TA + PFET JC + θCA)
TJ must not exceed the MOSFET maximum junction
temperature rating. It is recommended to measure the
MOSFETtemperatureinsteadystatetoensurethatabsolute
maximum ratings are not exceeded.
applicaTions inForMaTion
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