Datasheet

LTC3868-1
16
38681fd
APPLICATIONS INFORMATION
Figure 4. Sense Lines Placement with Inductor or Sense Resistor
(5a) Using a Resistor to Sense Current
(5b) Using the Inductor DCR to Sense Current
Figure 5. Current Sensing Methods
The Typical Application on the fi rst page is a basic
LTC3868-1 application circuit. LTC3868-1 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 trade off 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
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 cur-
rent comparators. The common mode voltage range on
these pins is 0V to 16V (Absolute Maximum), enabling
the LTC3868-1 to regulate output voltages up to a nominal
14V (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 fl ows out
of the pin. When SENSE
–
is above INTV
CC
+ 0.5V, a higher
current (~550µA) fl ows 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 LTC3868-1, 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
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.
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R
SENSE
38681 F04
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PLACE CAPACITOR NEAR
SENSE PINS
SENSE
+
SENSE
–
SGND
LTC3868-1
V
OUT
38681 F05a
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
*PLACE C1 NEAR
SENSE PINS
INDUCTOR
DCRL
SENSE
+
SENSE
–
SGND
LTC3868-1
V
OUT
38681 F05b
R1
R2C1*
(R1
||
R2) • C1 =
L
DCR
R
SENSE(EQ)
= DCR
R2
R1 + R2