Datasheet

LTC3891
14
3891fa
The Typical Application on the first page is a basic LTC3891
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
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.
Current Limit Programming
The I
LIM
pin is a tri-level logic input which sets the maximum
current limit of the controller. When I
LIM
is grounded, the
maximum current limit threshold voltage of the current
comparator is programmed to be 30mV. When I
LIM
is
floated, the maximum current limit threshold is 75mV.
When I
LIM
is tied to INTV
CC
, 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
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 (~700μA) 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.
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
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR OR R
SENSE
3891 F01
V
IN
V
IN
R
SENSE
INTV
CC
BOOST
TG
SW
BG
PLACE CAPACITOR NEAR
SENSE PINS
SENSE
+
SENSE
–
SGND
LTC3891
V
OUT
3891 F02a
*R1 AND C1 ARE OPTIONAL
C1*
R1*
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
*PLACE C1 NEAR
SENSE PINS
INDUCTOR
DCRL
SENSE
+
SENSE
–
SGND
LTC3891
V
OUT
3891 F02b
R1
R2C1*
(R1
||
R2) • C1 =
L
DCR
R
SENSE(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