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LTC3890-3 Datasheet, PDF (15/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller
LTC3890-3
Applications Information
The Typical Application on the first page is a basic
LTC3890‑3 application circuit. LTC3890-3 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.
SENSE+ and SENSE– Pins
The SENSE+ and SENSE– pins are the inputs to the current
comparators. The common mode voltage range on these
pins is 0V to 28V (abs max), enabling the LTC3890-3 to
regulate output voltages up to a nominal 24V (allowing
margin for tolerances and transients). This common mode
range is independent of the state of the VFB pin.
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) flows into the pin. Between INTVCC –
0.5V and INTVCC + 0.5V, the current transitions from the
smaller current to the higher current.
Filter components mutual to the sense lines should be
placed close to the LTC3890-3, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 3). 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 4b), sense resistor R1 should be
placed close to the switching node, to prevent noise from
coupling into sensitive small-signal nodes.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
COUT
38903 F03
INDUCTOR OR RSENSE
Figure 3. Sense Lines Placement with Inductor or Sense Resistor
VIN
VIN
INTVCC
BOOST
TG
SW
LTC3890-3
BG
RSENSE
VOUT
SENSE+
SENSE–
R1*
C1* PLACE CAPACITOR NEAR
SENSE PINS
SGND
*R1 AND C1 ARE OPTIONAL.
38903 F04a
(4a) Using a Resistor to Sense Current
VIN
VIN
INTVCC
BOOST
TG
SW
LTC3890-3
BG
INDUCTOR
L DCR
VOUT
SENSE+
SENSE–
SGND
*PLACE C1 NEAR
SENSE PINS
R1
C1* R2
(R1||R2) • C1 = L
DCR
RSENSE(EQ) = DCR
R2
R1 + R2
38903 F04b
(4b) Using the Inductor DCR to Sense Current
Figure 4. Current Sensing Methods
For more information www.linear.com/3890-3
38903f
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