參數(shù)資料
型號(hào): LT3758MPMSE#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): 13/36頁
文件大小: 554K
代理商: LT3758MPMSE#TRPBF
LT3758
20
3758fc
ApplicAtions inForMAtion
LLKistheleakageinductanceoftheprimarywinding,which
is usually specified in the transformer characteristics. LLK
canbeobtainedbymeasuringtheprimaryinductancewith
the secondary windings shorted. The snubber capacitor
value(CCN)canbedeterminedusingthefollowingequation:
CCN =
VSN
VSN RCN f
where VSN is the voltage ripple across CCN. A reasonable
VSN is 5% to 10% of VSN. The reverse voltage rating of
DSN should be higher than the sum of VSN and VIN(MAX).
Flyback Converter: Sense Resistor Selection
In a flyback converter, when the power switch is turned on,
the current flowing through the sense resistor (ISENSE) is:
ISENSE = ILP
Set the sense voltage at ILP(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
ILP(PEAK)
Flyback Converter: Power MOSFET Selection
For the flyback configuration, the MOSFET is selected with
a VDC rating high enough to handle the maximum VIN, the
reflected secondary voltage and the voltage spike due to
theleakageinductance.ApproximatetherequiredMOSFET
VDC rating using:
BVDSS > VDS(PEAK)
where
VDS(PEAK) = VIN(MAX) + VSN
The power dissipated by the MOSFET in a flyback con-
verter is:
PFET = I2M(RMS) RDS(ON) + 2 V2DS(PEAK) 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.
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.
Flyback Converter: Output Diode Selection
The output diode in a flyback converter is subject to large
RMS current and peak reverse voltage stresses. A fast
switching diode with a low forward drop and a low reverse
leakage is desired. Schottky diodes are recommended if
the output voltage is below 100V.
Approximate the required peak repetitive reverse voltage
rating VRRM using:
VRRM >
NS
NP
VIN(MAX) + VOUT
The power dissipated by the diode is:
PD = IO(MAX) VD
and the diode junction temperature is:
TJ = TA + PD RθJA
The RθJA to be used in this equation normally includes the
RθJC for the device, plus the thermal resistance from the
board to the ambient temperature in the enclosure. TJ must
not exceed the diode maximum junction temperature rating.
Flyback Converter: Output Capacitor Selection
The output capacitor of the flyback converter has a similar
operation condition as that of the boost converter. Refer to
the Boost Converter: Output Capacitor Selection section
for the calculation of COUT and ESRCOUT.
The RMS ripple current rating of the output capacitors
in discontinuous operation can be determined using the
following equation:
IRMS(COUT),DISCONTINUOUS ≥ IO(MAX)
4
(3 D2)
3 D2
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