LTC3858_15 Datasheet, PDF (16/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3858
APPLICATIONS INFORMATION
The Typical Application on the first page is a basic LTC3858
application circuit. LTC3858 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 tradeoff 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 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
floated, 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 28V (Abs Max), enabling the LTC3858
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 (~550μA) 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.
Filter components mutual to the sense lines should be
placed close to the LTC3858, 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.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
C_OUT
3858 F04
INDUCTOR OR R_SENSE
Figure 4. Sense Lines Placement with Inductor or Sense Resistor
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_{SE N SE(M AX )}
I_MAX
+
Δ I_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 criterion
for buck regulators operating at greater than 50% duty
factor. A curve is provided in the Typical Performance Char-
acteristics section to estimate this reduction in peak output
current depending upon the operating duty factor.
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