ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 16

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

LTC3858-2
The Typical Application on the first page is a basic
LTC3858-2 application circuit. LTC3858-2 can be config-
ured 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
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
comparators. The common mode voltage range on these
pins is 0V to 28V (Abs Max), enabling the LTC3858-2 to
regulate output voltages up to a nominal 24V (allowing
plenty of 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
0.5V and INTV
smaller current to the higher current.
APPLICATIONS INFORMATION
16
SENSE
LIM
CC
+
LIM
– 0.5V, a small current of less than 1μA flows out
(if R
pin is a tri-level logic input which sets the maximum
and SENSE
+
is tied to INTV
+
SENSE
and SENSE
pin is high impedance over the full common
CC
+ 0.5V, the current transitions from the
is used) and inductor value. Next, the
–
Pins
–
–
pins are the inputs to the current
is above INTV
CC
, the maximum current limit
–
pin changes depending on
LIM
CC
is grounded, the
+ 0.5V, a higher
–
is less than
LIM
CC
is
–
Filter components mutual to the sense lines should be
placed close to the LTC3858-2, 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 inductor DCR
sensing is used (Figure 4b), 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
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, ΔI
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%
duty factor. A curve is provided in the Typical Performance
Characteristics section to estimate this reduction in peak
output current depending upon the operating duty factor.
Figure 3. Sense Lines Placement with Inductor or Sense Resistor
MAX
SENSE(MAX)
R
SENSE
, equal to the peak value less half the peak-to-peak
=
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,
ΔI
is chosen based on the required
2
L
SENSE
LIM
3858 F03
setting. The current
C
OUT
38582f