LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 15

• Manufacturer Part Number: LTC3858-1
• Description: Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology Corporation

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The Typical Application on the fi rst page is a basic
LTC3858-1 application circuit. LTC3858-1 can be confi gured
to use either DCR (inductor resistance) sensing or low
value resistor sensing. The choice between the two cur-
rent sensing schemes is largely a design tradeoff between
cost, power consumption and accuracy. DCR sensing is
becoming popular because it saves expensive current
sensing resistors and is more power effi cient, 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
power MOSFETs and Schottky diodes are selected. Finally,
input and output capacitors are selected.
SENSE
The SENSE
comparators. The common mode voltage range on these
pins is 0V to 28V (Abs Max), enabling the LTC3858-1 to
regulate output voltages up to a nominal 24V (allowing
margin for tolerances and transients).
The SENSE
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
the common mode voltage. When SENSE
INTV
of the pin. When SENSE
current (~550μA) fl ows into the pin. Between INTV
and INTV
current to the higher current.
Filter components mutual to the sense lines should be
placed close to the LTC3858-1, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 4). 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
APPLICATIONS INFORMATION
SENSE
CC
+
– 0.5V, a small current of less than 1μA fl ows out
(if R
and SENSE
CC
+
+
+ 0.5V, the current transitions from the smaller
SENSE
and SENSE
pin is high impedance over the full common
is used) and inductor value. Next, the
–
Pins
–
–
pins are the inputs to the current
is above INTV
–
pin changes depending on
CC
+ 0.5V, a higher
–
is less than
CC
– 0.5V
programmed current limit unpredictable. If inductor DCR
sensing is used (Figure 5b), resistor R1 should be placed
close to the switching node, to prevent noise from coupling
into sensitive small-signal nodes.
Figure 4. Sense Lines Placement with Inductor or Sense Resistor
*PLACE C1 NEAR
SENSE PINS
LTC3858-1
LTC3858-1
SENSE
SENSE
INTV
BOOST
SGND
(5b) Using the Inductor DCR to Sense Current
SENSE
SENSE
INTV
BOOST
SW
V
BG
TG
SGND
CC
IN
+
–
SW
(5a) Using a Resistor to Sense Current
V
BG
TG
CC
IN
Figure 5. Current Sensing Methods
+
–
(R1 || R2) • C1 =
NEXT TO THE CONTROLLER
C1*
INDUCTOR OR R
PLACE CAPACITOR NEAR
SENSE PINS
TO SENSE FILTER,
R2
R1
DCR
L
R
SENSE
SENSE(EQ)
38581 F04
V
INDUCTOR
LTC3858-1
IN
L
= DCR
V
C
IN
OUT
DCR
R1 + R2
R2
38581 F05a
15
V
38581 F05b
OUT
V
38581f
OUT