參數(shù)資料
型號(hào)： LTC3787EGN#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 585 kHz SWITCHING FREQ-MAX, PDSO28
封裝： 0.150 INCH, LEAD FREE, PLASTIC, SSOP-28
文件頁(yè)數(shù)： 10/36頁(yè)
文件大?。?/td> 363K
代理商： LTC3787EGN#TRPBF
LTC3787
18
3787fb
APPLICATIONS INFORMATION
Power MOSFET Selection
Two external power MOSFETs must be selected for each
controller in the LTC3787: one N-channel MOSFET for the
bottom (main) switch, and one N-channel MOSFET for the
top (synchronous) switch.
The peak-to-peak gate drive levels are set by the INTVCC
voltage. This voltage is typically 5.4V 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; many 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 manufacturer’s 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 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
=
V
OUT VIN
V
OUT
Synchronous Switch Duty Cycle
=
V
IN
V
OUT
If the maximum output current is IOUT(MAX) and each chan-
nel takes one half of the total output current, the MOSFET
power dissipations in each channel at maximum output
current are given by:
P
MAIN =
(V
OUT VIN )VOUT
V
2
IN
I
OUT(MAX)
2
2
1
+δ
()
R
DS(ON) + k V
3
OUT
I
OUT(MAX)
2 V
IN
C
MILLER f
P
SYNC =
V
IN
V
OUT
I
OUT(MAX)
2
2
1
+δ
()R
DS(ON)
where
δ is the temperature dependency of RDS(ON) (ap-
proximately 1Ω) is the effective driver resistance at the
MOSFET’s Miller threshold voltage. The constant k, which
accounts for the loss caused by reverse recovery current,
is inversely proportional to the gate drive current and has
an empirical value of 1.7.
Both MOSFETs have I2R losses while the bottom N-channel
equation includes an additional term for transition losses,
which are highest at low input voltages. For high VIN the
high current efficiency generally improves with larger
MOSFETs, while for low VIN the transition losses rapidly
increase to the point that the use of a higher RDS(ON)device
with lower CMILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the bottom switch duty factor is low or dur-
ing overvoltage 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.
相關(guān)PDF資料
PDF描述
LTC3787EUFD#PBF SWITCHING CONTROLLER, 585 kHz SWITCHING FREQ-MAX, PQCC28
LTC3787IGN#PBF SWITCHING CONTROLLER, 585 kHz SWITCHING FREQ-MAX, PDSO28
LTC3788EUH#TRPBF DUAL SWITCHING CONTROLLER, 900 kHz SWITCHING FREQ-MAX, PQCC32
LTC3788EUH#PBF DUAL SWITCHING CONTROLLER, 900 kHz SWITCHING FREQ-MAX, PQCC32
LTC3788IUH#PBF DUAL SWITCHING CONTROLLER, 900 kHz SWITCHING FREQ-MAX, PQCC32
相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
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