Datasheet

LTC3859A
20
3859af
Figure 3. Sense Lines Placement with Inductor or Sense Resistor
applicaTions inForMaTion
The Typical Application on the first page is a basic LTC3859A
application circuit. LTC3859A 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.
SENSE
+
and SENSE
–
Pins
The SENSE
+
and SENSE
–
pins are the inputs to the current
comparators.
Buck Controllers (SENSE1
+
/SENSE1
–
,SENSE2
+
/SENSE2
–
):
The common mode voltage range on these pins is 0V
to 28V (absolute maximum), enabling the LTC3859A to
regulate buck output voltages up to a nominal 24V (al-
lowing 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.
Boost Controller (SENSE3
+
/SENSE3
–
): The common
mode input range for these pins is 2.5V to 38V, allowing
the boost converter to operate from inputs over this full
range. The SENSE3
+
pin also provides power to the cur-
rent comparator and draws about 170µA during normal
operation (when not shut down or asleep in Burst Mode
operation). There is a small bias current of less than 1µA
that flows out of the SENSE3
–
pin. This high impedance
on the SENSE3
–
pin allows the current comparator to be
used in inductor DCR sensing.
Filter components mutual to the sense lines should be
placed close to the LTC3859A, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 3). 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 DCR sensing
is used (Figure 4b), sense 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 4a. R
SENSE
is chosen based on the required
output current.
The current comparators have a maximum threshold
V
SENSE(MAX)
of 50mV. The current comparator threshold
sets the peak of the inductor current, yielding a maximum
average output current, I
MAX
, equal to the peak value less
half the peak-to-peak ripple current, DI
L
. To calculate the
sense resistor value, use the equation:
R
SENSE
=
V
SENSE(MAX)
I
MAX
+
DI
L
2
When using the buck controllers in very low dropout
conditions, the maximum output current level will be
reduced due to the internal compensation required to
meet stability criterion for buck regulators operating at
greater than 50% duty factor. A curve is provided in the
Typical Performance Characteristics section to estimate
this reduction in peak output current level depending upon
the operating duty factor.
3859A F03
TO SENSE FILTER
NEXT TO THE CONTROLLER
INDUCTOR OR R
SENSE
CURRENT FLOW