Datasheet

LTC3868
16
3868fd
APPLICATIONS INFORMATION
Figure 4. Sense Lines Placement with Inductor or Sense Resistor
The Typical Application on the fi rst page is a basic LTC3868
application circuit. LTC3868 can be confi gured 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 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.
Current Limit Programming
The I
LIM
pin is a tri-level logic input which sets the maximum
current limit of the converter. When I
LIM
is grounded, the
maximum current limit threshold voltage of the current
comparator is programmed to be 30mV. When I
LIM
is
fl oated, the maximum current limit threshold is 50mV.
When I
LIM
is tied to INTV
CC
, the maximum current limit
threshold is set to 75mV.
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 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, 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
3868 F04
Low Value Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 5a. R
SENSE
is chosen based on the required
output current.
The current comparator has a maximum threshold
V
SENSE(MAX)
determined by the I
LIM
setting. The current
comparator threshold voltage sets the peak of the induc-
tor current, yielding a maximum average output current,
I
MAX
, equal to the peak value less half the peak-to-peak
ripple current, ∆I
L
. To calculate the sense resistor value,
use the equation:
R
SENSE
=
V
SENSE(MAX)
I
MAX
+
ΔI
L
2
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
the internal compensation required to meet stability cri-
terion for buck regulators operating at greater than 50%