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參數(shù)資料
| 型號： | LT3757MPMSE#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | 3 X 3 MM, LEAD FREE, PLASTIC, MSOP-10 |
| 文件頁數(shù)： | 7/36頁 |
| 文件大?。?/td> | 436K |
| 代理商： | LT3757MPMSE#TRPBF |
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LT3757
15
3757fc
applicaTions inForMaTion
Boost Converter: Inductor and Sense Resistor Selection
For the boost topology, the maximum average inductor
current is:
IL(MAX) =IO(MAX)
1
DMAX
Then, the ripple current can be calculated by:
IL = χ IL(MAX) = χ IO(MAX)
1
DMAX
The constant c in the preceding equation represents the
percentage peak-to-peak ripple current in the inductor,
relative to IL(MAX).
The inductor ripple current has a direct effect on the choice
of the inductor value. Choosing smaller values of IL
requires large inductances and reduces the current loop
gain(theconverterwillapproachvoltagemode).Accepting
larger values of IL provides fast transient response and
allowstheuseoflowinductances,butresultsinhigherinput
current ripple and greater core losses. It is recommended
that c fall within the range of 0.2 to 0.6.
Givenanoperatinginputvoltagerange,andhavingchosen
the operating frequency and ripple current in the inductor,
theinductorvalueoftheboostconvertercanbedetermined
using the following equation:
L
=
VIN(MIN)
IL f
DMAX
The peak and RMS inductor current are:
IL(PEAK) =IL(MAX) 1+
χ
2
IL(RMS) =IL(MAX) 1+
χ2
12
Based on these equations, the user should choose the
inductors having sufficient saturation and RMS current
ratings.
Set the sense voltage at IL(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 =
80mV
IL(PEAK)
Boost Converter: Power MOSFET Selection
Important parameters for the power MOSFET include the
drain-source voltage rating (VDS), the threshold voltage
(VGS(TH)), the on-resistance (RDS(ON)), the gate to source
and gate to drain charges (QGS and QGD), the maximum
drain current (ID(MAX)) and the MOSFET’s thermal
resistances (RθJC and RθJA).
The power MOSFET will see full output voltage, plus a
diode forward voltage, and any additional ringing across
its drain-to-source during its off-time. It is recommended
to choose a MOSFET whose BVDSS is higher than VOUT by
a safety margin (a 10V safety margin is usually sufficient).
The power dissipated by the MOSFET in a boost converter is:
PFET=I2L(MAX)RDS(ON)DMAX+2V2OUTIL(MAX)
CRSS f/1A
The first term in the preceding 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 (θJC + θCA)
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