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)： 6/36頁(yè)
文件大?。?/td> 380K
代理商： LTC3867IUF#PBF
LTC3867
14
3867f
APPLICATIONS INFORMATION
The Typical Application on the first page of this data sheet
is a basic LTC3867 application circuit. The LTC3867 can be
configured to use either DCR (inductor resistance) sens-
ing 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 resistors and is more power efficient,
especially in high current applications. However, current
sensing resistors provide the most accurate current limits
for the controller. Other external component selection is
driven by the load requirement, and begins with the se-
lection of RSENSE (if RSENSE is used) and inductor value.
Next, the power MOSFETs are selected. Finally, input and
output capacitors are selected.
Current Limit Programming
The ILIM pin is a 5-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. Set ILIM between 1.1V and
1.9V (typically 1.5V) for a 40mV maximum current sense
threshold. For the 60mV setting, set ILIM between 3.3V
and 4.1V, typically 3.7V. These numbers are relative to a
5.3V INTVCC. Setting ILIM using a resistor divider off of
INTVCC will allow the maximum current sense threshold
setting to not change when the 5.3V LDO is in dropout
at start-up. Please note that the ILIM pin has an internal
500k pull-down to SGND and a 500k pull-up to INTVCC.
Which setting should be used? For the best current limit
accuracy,usethe75mVsetting.The30mVsettingwillallow
for the use of very low DCR inductors or sense resistors,
but at the expense of current limit accuracy.
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 14V. All SENSE pins are
high impedance inputs with small currents of less than
1A. The high impedance inputs to the current compara-
tors allow accurate DCR sensing. The SENSE– pin should
be connected to VOUT directly when DCR sensing is used.
Care must be taken not to float these pins during normal
operation. Filter components mutual to the sense lines
should be placed close to the LTC3867, and the sense
lines should run close together to a Kelvin connection
underneaththecurrentsenseelement(showninFigure 2).
Sensing current 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 3b), resistor R1 should be
placed close to the switching node, to prevent noise from
coupling into sensitive small-signal nodes. The capacitor
C1 should be placed close to the IC pins.
Figure 2. Sense Lines Placement with Sense Resistor
Low Value Resistors Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 3a. 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 14V. 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 +
IL
2
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of
VSENSE = IL RSENSE
also 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
DCRsensingapplications.Forpreviousgenerationcurrent
mode controllers, the maximum sense voltage was high
enough (e.g., 75mV for the LTC1628/LTC3728 family)
that the voltage drop across the parasitic inductance of
COUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
RSENSE
3867 F02
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