參數(shù)資料
型號(hào)： LTC3869EUFD#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PQCC28
封裝： 5 X 4 MM, LEAD FREE, PLASTIC, MO-220WXXX-X, QFN-28
文件頁(yè)數(shù)： 5/40頁(yè)
文件大?。?/td> 2705K
代理商： LTC3869EUFD#PBF
LTC3869/LTC3869-2
13
3869f
APPLICATIONS INFORMATION
TheTypicalApplicationonthefirstpageisabasicLTC3869
applicationcircuit.LTC3869canbeconfiguredtouseeither
DCR (inductor resistance) sensing or low value resistor
sensing. The choice between the two current sensing
schemes is largely a design trade-off between cost, power
consumption, and accuracy. DCR sensing is becoming
popular because it saves expensive current sensing resis-
tors and is more power efficient, especially in high current
applications. However, current sensing resistors provide
the most accurate current limits for the controller. Other
externalcomponentselectionisdrivenbytheloadrequire-
ment, and begins with the selection of RSENSE (if RSENSE is
used)andinductorvalue.Next,thepowerMOSFETsarese-
lected. Finally, input and output capacitors are selected.
Current Limit Programming
The ILIM pin is a tri-level logic input which sets the maxi-
mum current limit of the controller. When ILIM is either
grounded, floated or tied to INTVCC, the typical value for
the maximum current sense threshold will be 30mV, 50mV
or 75mV, respectively.
Which setting should be used? For the best current limit
accuracy, use the 75mV setting. The 30mV setting will
allow for the use of very low DCR inductors or sense
resistors, but at the expense of current limit accuracy.
The 50mV setting is a good balance between the two. For
single output dual phase applications, use the 50mV or
75mV setting for optimal current sharing.
SENSE+ and SENSE– Pins
The SENSE+ and SENSE– pins are the inputs to the current
comparators. The common mode input voltage range of
the current comparators is 0V to 12.5V. Both SENSE pins
are high impedance inputs with small base currents of
less than 1A. When the SENSE pins ramp up from 0V to
1.4V, the small base currents flow out of the SENSE pins.
When the SENSE pins ramp down from 12.5V to 1.1V, the
small base currents flow into the SENSE pins. The high
impedance inputs to the current comparators allow ac-
curate DCR sensing. However, care must be taken not to
floatthesepinsduringnormaloperation.TheLTC3869GN-2
defaults to 50mV current limit value.
Filter components mutual to the sense lines should be
placed close to the LTC3869, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 1). Sensing cur-
rent elsewhere can effectively add parasitic inductance
and capacitance to the current sense element, degrading
the information at the sense terminals and making the
programmed current limit unpredictable. If DCR sensing
is used (Figure 2b), sense resistor R1 should be placed
closetotheswitchingnode,topreventnoisefromcoupling
into sensitive small-signal nodes. The capacitor C1 should
be placed close to the IC pins.
Figure 1. Sense Lines Placement with Sense Resistor
COUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
RSENSE
3869 F01
Low Value Resistors Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. RSENSE is chosen based on the required
output current.
The current comparator has a maximum threshold
VSENSE(MAX) determined by the ILIM setting. The input
common mode range of the current comparator is 0V to
12.5V. The current comparator threshold sets the peak of
the inductor current, yielding a maximum average output
current IMAX equal to the peak value less half the peak-to-
peak ripple current, IL. To calculate the sense resistor
value, use the equation:
RSENSE =
VSENSE(MAX)
IMAX +
ΔIL
2
Because of possible PCB noise in the current sensing loop,
theACcurrentsensingrippleofVSENSE=ILRSENSEalso
needs to be checked in the design to get a good signal-to-
noise ratio. In general, for a reasonably good PCB layout, a
10mV VSENSE voltage is recommended as a conservative
number to start with, either for RSENSE or DCR sensing
applications, for duty cycles less than 40%.
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