LTC3857-1_15 Datasheet, PDF (15/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857-1
APPLICATIONS INFORMATION
The Typical Application on the first page is a basic
LTC3857-1 application circuit. LTC3857-1 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
R_SENSE (if R_SENSE 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 LTC3857-1 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
INTV_CC – 0.5V, a small current of less than 1μA flows out
of the pin. When SENSE^– is above INTV_CC + 0.5V, a higher
current (~550μA) flows into the pin. Between INTV_CC –
0.5V and INTV_CC + 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 LTC3857-1, 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), 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
C_OUT
38571 F03
INDUCTOR OR R_SENSE
Figure 3. Sense Lines Placement with Inductor or Sense Resistor
V_IN
INTV_CC
BOOST
TG
SW
LTC3857-1
BG
V_IN
R_SENSE
V_OUT
SENSE⁺
SENSE^–
SGND
PLACE CAPACITOR NEAR
SENSE PINS
38571 F04a
(4a) Using a Resistor to Sense Current
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3857-1
BG
INDUCTOR
L DCR
V_OUT
SENSE⁺
SENSE^–
SGND
*PLACE C1 NEAR
SENSE PINS
R1
C1* R2
(R1||R2) • C1 = ^L
DCR
R_SENSE(EQ) = DCR
R2
R1 + R2
38571 F04b
(4b) Using the Inductor DCR to Sense Current
Figure 4. Current Sensing Methods
38571fc
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