LTC3868 Linear Technology, LTC3868 Datasheet - Page 16

• Manufacturer Part Number: LTC3868
• Description: Low IQ Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology

LTC3868
The Typical Application on the first page is a basic LTC3868
application circuit. LTC3868 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
R
power MOSFETs and Schottky diodes are selected. Finally,
input and output capacitors are selected.
Current Limit Programming
The I
current limit of the converter. When I
maximum current limit threshold voltage of the current
comparator is programmed to be 30mV. When I
floated, the maximum current limit threshold is 50mV.
When I
threshold is set to 75mV.
SENSE
The SENSE
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
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) flows into the pin. Between INTV
applicaTions inForMaTion
controller. Other external component selection is driven
by the load requirement, and begins with the selection of

SENSE
LIM
CC
+
(if R
– 0.5V, a small current of less than 1µA flows out
LIM
pin is a tri-level logic input which sets the maximum
and SENSE
is tied to INTV
+
+
pin is high impedance over the full common
SENSE
and SENSE
is used) and inductor value. Next, the
–
Pins
–
–
is above INTV
pins are the inputs to the cur-
CC
, the maximum current limit
–
pin changes depending on
LIM
CC
is grounded, the
+ 0.5V, a higher
–
is less than
CC
– 0.5V
LIM
is
and INTV
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.
Low Value Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 5a. R
output current.
The current comparator has a maximum threshold
V
comparator threshold voltage sets the peak of the induc-
tor current, yielding a maximum average output current,
I
ripple current, ∆I
use the equation:
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%
Figure 4. Sense Lines Placement with Inductor or Sense Resistor
MAX
SENSE(MAX)
R
SENSE
, equal to the peak value less half the peak-to-peak
CC
=
+ 0.5V, the current transitions from the smaller
determined by the I
V
I
SENSE(MAX)
MAX
SENSE
L
NEXT TO THE CONTROLLER
. To calculate the sense resistor value,
INDUCTOR OR R
+
TO SENSE FILTER,
is chosen based on the required
∆I
2
L
SENSE
LIM
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3868 F04
setting. The current
C
OUT
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