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参数资料
| 型号： | LTC3891EFE#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 900 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封装： | 4.40 MM, LEAD FREE, PLASTIC, TSSOP-20 |
| 文件页数： | 6/32页 |
| 文件大小： | 1931K |
| 代理商： | LTC3891EFE#TRPBF |
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LTC3891
14
3891f
TheTypicalApplicationonthefirstpageisabasicLTC3891
application circuit. LTC3891 can be configured to use
either 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 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 selection of
RSENSE (if RSENSE is used) and inductor value. Next, the
power MOSFETs and Schottky diodes are selected. Finally,
input and output capacitors are selected.
Current Limit Programming
TheILIMpinisatri-levellogicinputwhichsetsthemaximum
current limit of the controller. When ILIM is grounded, the
maximum current limit threshold voltage of the current
comparator is programmed to be 30mV. When ILIM is
floated, the maximum current limit threshold is 75mV.
When ILIM is tied to INTVCC, the maximum current limit
threshold is set to 50mV.
SENSE+ and SENSE– Pins
The SENSE+ and SENSE– pins are the inputs to the cur-
rent comparators. The common mode voltage range on
these pins is 0V to 28V (abs max), enabling the LTC3891
to regulate output voltages up to a nominal 24V (allowing
margin for tolerances and transients).
The SENSE+ pin is high impedance over the full common
mode range, drawing at most ±1μA. This high impedance
allows the current comparators to be used in inductor
DCR sensing.
The impedance of the SENSE– pin changes depending on
the common mode voltage. When SENSE– is less than
INTVCC – 0.5V, a small current of less than 1μA flows out
of the pin. When SENSE– is above INTVCC + 0.5V, a higher
current(~700μA)flowsintothepin.BetweenINTVCC–0.5V
and INTVCC+0.5V,thecurrenttransitionsfromthesmaller
current to the higher current.
APPLICATIONS INFORMATION
Filter components mutual to the sense lines should be
placed close to the LTC3891, 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 inductor DCR
sensing is used (Figure 2b), sense resistor R1 should be
placed close to the switching node, to prevent noise from
coupling into sensitive small-signal nodes.
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
COUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR RSENSE
3891 F01
VIN
RSENSE
INTVCC
BOOST
TG
SW
BG
PLACE CAPACITOR NEAR
SENSE PINS
SENSE+
SENSE–
SGND
LTC3891
VOUT
3891 F02a
*R1 AND C1 ARE OPTIONAL
C1*
R1*
VIN
INTVCC
BOOST
TG
SW
BG
*PLACE C1 NEAR
SENSE PINS
INDUCTOR
DCR
L
SENSE+
SENSE–
SGND
LTC3891
VOUT
3891 F02b
R1
R2
C1*
(R1||R2) C1 = L
DCR
RSENSE(EQ) = DCR
R2
R1 + R2
Figure 2. Current Sensing Methods
(2b) Using the Inductor DCR to Sense Current
(2a) Using a Resistor to Sense Current
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