參數(shù)資料
型號(hào)： LTC3867IUF#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 0.1 A SWITCHING CONTROLLER, 1200 kHz SWITCHING FREQ-MAX, PQCC24
封裝： 4 X 4 MM, LEAD FREE, PLASTIC, MO-220WGGD, QFN-24
文件頁(yè)數(shù)： 14/36頁(yè)
文件大小： 380K
代理商： LTC3867IUF#PBF
LTC3867
21
3867f
APPLICATIONS INFORMATION
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency, fOSC, directly determine
the inductor’s peak-to-peak ripple current:
IRIPPLE =
VOUT
VIN
VIN – VOUT
fOSC L
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of IOUT(MAX). Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
L
≥
VIN – VOUT
fOSC IRIPPLE
VOUT
VIN
Inductor Core Selection
Once the inductance value is determined, the type of in-
ductor must be selected. 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
go down. Unfortunately, increased inductance requires
moreturnsofwireandthereforecopperlosseswillincrease.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current 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
At least two external power MOSFETs need to be selected:
One N-channel MOSFET for the top (main) switch and one
or more N-channel MOSFET(s) for the bottom (synchro-
nous) switch. The number, type and on-resistance of all
MOSFETsselectedtakeintoaccountthevoltagestep-down
ratio as well as the actual position (main or synchronous)
in which the MOSFET will be used. A much smaller and
much lower input capacitance MOSFET should be used
for the top MOSFET in applications that have an output
voltage that is less than 1/3 of the input voltage. In applica-
tions where VIN >> VOUT, the top MOSFETs’ on-resistance
is normally less important for overall efficiency than its
input capacitance at operating frequencies above 300kHz.
MOSFET manufacturers have designed special purpose
devices that provide reasonably low on-resistance with
significantlyreducedinputcapacitanceforthemainswitch
application in switching regulators.
The peak-to-peak MOSFET gate drive levels are set by the
voltage, VINTVCC, 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
output current. MOSFET input capacitance is a combina-
tion of several components but can be taken from the
typical gate charge curve included on most data sheets
(Figure 9). The curve is generated by forcing a constant
input current into the gate of a common source, current
source loaded stage and then plotting the gate voltage
versus time.
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
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