參數(shù)資料
型號(hào)： LTC3833EUDC#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 2000 kHz SWITCHING FREQ-MAX, PQCC20
封裝： 3 X 3 MM, LEAD FREE, PLASTIC, QFN-20
文件頁(yè)數(shù)： 9/36頁(yè)
文件大小： 513K
代理商： LTC3833EUDC#PBF
LTC3833
17
3833f
APPLICATIONS INFORMATION
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
RSENSE sensing. Light load power loss can be modestly
higher with a DCR network than with a sense resistor due
totheextraswitchinglossesincurredthroughR1.However,
DCR sensing eliminates a sense resistor, reduces conduc-
tion losses and provides higher efficiency at heavy loads.
Peak efficiency is about the same with either method.
To maintain a good signal-to-noise ratio for the current
sense signal, use a minimum
VSENSE of 10mV. For a
DCR sensing application, the actual ripple voltage will be
determined by:
VSENSE =
VIN – VOUT
R1C1
VOUT
VIN f
Power MOSFET Selection
Two external power MOSFETs must be selected for the
LTC3833 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 5.3V.
Consequently, logic-level threshold MOSFETs must be
used in most applications. 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 usu-
ally provided on the MOSFET manufacturers’ data sheet.
CMILLER is equal to the increase in gate charge along the
horizontal axis where the curve is approximately flat, di-
vided by the specified change in VDS. This result is then
multiplied by the ratio of the application 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 DTOP
(
) = VOUT
VIN
Synchronous Switch Duty Cycle DBOT
(
) =1– VOUT
VIN
The MOSFET power dissipations at maximum output
current are given by:
PTOP =DTOP IOUT(MAX)2 RDS(ON)(MAX) 1+δ
( )+VIN2
IOUT(MAX)
2
CMILLER
RTG(HI)
VINTVCC – VMILLER
+
RTG(LO)
VMILLER
f
PBOT = DBOT IOUT(MAX)2 RDS(ON)(MAX) (1 + δ)
whereDTOPandDBOTarethedutycyclesofthetopMOSFET
and bottom MOSFET respectively,
δ is the temperature de-
pendency of RDS(ON), RTG(HI) is the TG pull-up resistance,
and RTG(LO) is the TG pull-down resistance. VMILLER is the
Miller effect VGS voltage and is taken graphically from the
MOSFET’s data sheet.
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 (TJ – TA) can be used as an approximation
for low voltage MOSFETs (TJ is estimated junction tem-
perature of the MOSFET and TA is ambient temperature).
CIN and COUT Selection
In continuous mode, the source current of the top N-chan-
nel MOSFET is a square wave of duty cycle VOUT/VIN. To
prevent large voltage transients, a low ESR input capacitor
sized for the maximum RMS current must be used. The
maximum RMS capacitor current is given by:
IRMS IOUT(MAX)
VOUT
VIN
VIN
VOUT
– 1
This formula has a maximum at VIN = 2VOUT, where IRMS
= IOUT(MAX)/2. This simple worst-case condition is com-
monly used for design because even significant deviations
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