參數(shù)資料
型號： LTC3856IFE#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PDSO38
封裝： 4.4 MM, LEAD FREE, PLASTIC, TSSOP-38
文件頁數(shù)： 15/40頁
文件大小： 2222K
代理商： LTC3856IFE#PBF
LTC3856
3856f
applicaTions inForMaTion
The peak-to-peak MOSFET gate drive levels are set by the
voltage, VCC, requiring the use of logic-level threshold
MOSFETs in most applications. Pay close attention to the
BVDSS specification for the MOSFETs as well; many of the
logic-level MOSFETs are limited to 30V or less. Selection
criteria for the power MOSFETs include the on-resistance,
RDS(ON), input capacitance, input voltage and maximum
outputcurrent.MOSFETinputcapacitanceisacombination
of several components but can be taken from the typical
gatecharge curveincludedonmostdatasheets(Figure9).
The curve is generated by forcing a constant input current
into the gate of a common source, current source loaded
stage, then plotting the gate voltage versus time.
mode, the duty cycles for the top and bottom MOSFETs
are given by:
Main Switch Duty Cycle
V
Synchronous Switc
OUT
IN
=
hh Duty Cycle
V
IN
OUT
IN
=
–
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
P
V
I
N
R
V
MAIN
OUT
IN
MAX
DS ON
IN
=
+
( )
+
( )
2
1
δ
( )
22
2
1
I
N
R
C
V
MAX
DR
MILLER
CC
TH IL
(
)(
)
+
–
( )
VV
f
P
V
I
N
TH IL
SYNC
IN
OUT
IN
MAX
( )
–
=
+
( )
2
1
δ RDS ON
( )
where N is the number of output stages,
δ is the tem-
perature dependency of RDS(ON), RDR is the effective top
driver resistance (approximately 2Ω at VGS = VMILLER),
VIN is the drain potential and the change in drain poten-
tial in the particular application. VTH(IL) is the data sheet
specified typical gate threshold voltage specified in the
power MOSFET data sheet at the specified drain current.
CMILLERisthecalculatedcapacitanceusingthegatecharge
curve from the MOSFET data sheet and the technique just
described.
BothMOSFETshaveI2RlosseswhilethetopsideN-channel
equation includes an additional term for transition losses,
which peak at the highest input voltage. For VIN < 20V,
the high current efficiency generally improves with larger
MOSFETs, while for VIN > 20V, the transition losses rapidly
increase to the point that the use of a higher RDS(ON)device
with lower CMILLERactuallyprovideshigherefficiency.The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low, or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
+
–
VDS
VIN
3856 F09
VGS
MILLER EFFECT
QIN
a
b
CMILLER = (QB – QA)/VDS
VGS
V
+
–
Figure 9. Gate Charge Characteristic
The initial slope is the effect of the gate-to-source and
the gate-to-drain capacitance. The flat portion of the
curve is the result of the Miller multiplication effect of the
drain-to-gate capacitance as the drain drops the voltage
across the current source load. The upper sloping line is
due to the drain-to-gate accumulation capacitance and
the gate-to-source capacitance. The Miller charge (the
increase in coulombs on the horizontal axis from a to b
while the curve is flat) is specified for a given VDS drain
voltage, but can be adjusted for different VDS voltages by
multiplying the ratio of the application VDS to the curve
specified VDS values. A way to estimate the CMILLER
term is to take the change in gate charge from points
a and b on a manufacturer’s data sheet and divide by
the stated VDS voltage specified. CMILLER is the most
important selection criteria for determining the transition
loss term in the top MOSFET but is not directly specified
on MOSFET data sheets. CRSS and COS are specified
sometimes but definitions of these parameters are not
included. When the controller is operating in continuous
相關(guān)PDF資料
PDF描述
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