參數(shù)資料
型號(hào)： LTC3891MPFE#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 900 kHz SWITCHING FREQ-MAX, PDSO20
封裝： 4.40 MM, LEAD FREE, PLASTIC, TSSOP-20
文件頁(yè)數(shù)： 9/32頁(yè)
文件大?。?/td> 1931K
代理商： LTC3891MPFE#PBF
LTC3891
17
3891f
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for the
LTC3891 controller: one N-channel MOSFET for the top
(main) switch, and one N-channel MOSFET for the bottom
(synchronous) switch.
The peak-to-peak drive levels are set by the INTVCC
voltage. This voltage is typically 5.1V during start-up
(see EXTVCC Pin Connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
Pay close attention to the BVDSS specification for the
MOSFETs as well.
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’
datasheet. 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
then multiplied by the ratio of the application 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
Synchronous Switch Duty Cycle
=
VIN VOUT
VIN
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 – VTHMIN
+
1
VTHMIN
f
( )
PSYNC =
VIN – VOUT
VIN
IMAX
(
)
2 1+δ
(
)RDS(ON)
APPLICATIONS INFORMATION
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. VTHMIN 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
a 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.
A Schottky diode can be inserted in parallel with the bot-
tom MOSFET to conduct during the dead-time between
the conduction of the two power MOSFETs. This prevents
the body diode of the bottom MOSFET from turning on,
storing charge during the dead-time and requiring a
reverse recovery period that could cost as much as 3%
in efficiency at high VIN. A 1A to 3A Schottky is generally
a good compromise for both regions of operation due to
the relatively small average current. Larger diodes result
in additional transition losses due to their larger junction
capacitance.
CIN and COUT Selection
TheselectionofCINisusuallybasedofftheworst-caseRMS
input current. The highest (VOUT)(IOUT) product needs to
be used in the formula shown in Equation 1 to determine
the maximum RMS capacitor current requirement.
Incontinuousmode,thesourcecurrentofthetopMOSFET
is a square wave of duty cycle (VOUT)/(VIN). To prevent
large voltage transients, a low ESR capacitor sized for the
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