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LTC3854EMSE 数据表(PDF) 12 Page - Linear Technology

部件名 LTC3854EMSE
功能描述  Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTC3854EMSE 数据表(HTML) 12 Page - Linear Technology

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LTC3854

3854fa
Inductor Core Selection
Once the value for L is determined, the type of inductor
must be selected. High efficiency converters generally
cannot afford the core loss found in low cost powdered
iron cores, forcing the use of more expensive ferrite or
molypermalloy cores. Actual core loss is independent of
core size for a fixed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies; allowing design goals to
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductancecollapsesabruptlywhenthepeakdesigncurrent
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for the
LTC3854 controller: one N-channel MOSFET for the top
(main) switch, and one N-channel MOSFET for the bottom
(synchronous) switch.
Thepeak-to-peakdrivelevelsaresetbytheINTVCCvoltage.
This voltage is 5V during start-up. Consequently, logic-
levelthresholdMOSFETscanbeusedinmostapplications.
The only exception is if low input voltage is expected (VIN
< 5V); then, sub-logic level threshold MOSFETs (VGS(TH)
< 3V) should be used. Pay close attention to the BVDSS
specification for the MOSFETs as well; most of the logic
level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the
on-resistance RDS(ON), Miller capacitance CMILLER, input
voltage and maximum output current. Miller capacitance,
CMILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. CMILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in VDS. This result is
applicaTions inForMaTion
then multiplied by the ratio of the applied VDS to the gate
charge curve specified VDS. When the IC is operating in
continuous mode the duty cycles for the top and bottom
MOSFETs are given by:
Main Switch Duty Cycle
=
VOUT
VIN
=D
Synchronous Switch Duty Cycle
=
VIN − VOUT
VIN
= 1−D
The MOSFET power dissipations at maximum output
current are given by:
PMAIN =
VOUT
VIN
IMAX
(
)2 1+δ
(
)RDS(ON) +
VIN
(
)2 IMAX
2


RDR
(
) CMILLER
(
)
1
VINTVCC − VTH(MIN)
+
1
VTH(MIN)
(f)
PSYNC =
VIN − VOUT
VIN
IMAX
(
)2 1+δ
(
)RDS(ON)
where
δ is the temperature dependency of RDS(ON) and
RDR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. VTH(MIN) is the
typical MOSFET minimum threshold voltage.
BothMOSFETshaveI2RlosseswhilethetopsideN-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. 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
short-circuit when the synchronous switch is on close to
100% of the period.
The term (1 +
δ) is generally given for a MOSFET in the
form of a normalized RDS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.


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