LTC3892_15 Datasheet, PDF (16/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3892/LTC3892-1
Applications Information
The Typical Application on the first page is a basic LTC3892/
LTC3892-1 application circuit. LTC3892/LTC3892-1 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 be-
tween 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 cur-
rent comparators. The common mode voltage range on
these pins is 0V to 65V (absolute maximum), enabling
the LTC3892/LTC3892-1 to regulate output voltages up
to a nominal 60V (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 (≈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.
Filter components mutual to the sense lines should be
placed close to the LTC3892/LTC3892-1, and the sense
lines should run close together to a Kelvin connection
underneath the current sense element (shown in Figure 1).
Sensing current 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 2b), 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
INDUCTOR OR R_SENSE
C_OUT
CURRENT FLOW
38921 F03
Figure 1. 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 2a. R_SENSE is chosen based on the required
output current.
Each controller’s current comparator has a maximum
threshold V_SENSE(MAX). For the LTC3892-1, V_SENSE(MAX)
is fixed at 75mV, while for the LTC3892 V_SENSE(MAX) is
either 50mV, 75mV or 100mV, as determined by the state
of the ILIM pin. The current comparator threshold voltage
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, ∆I_L. To calculate the
sense resistor value, use the equation:
R_SENSE
=
V_{SENSE(MAX )}
I_MAX
+
∆I_L
2
When using a controller in very low dropout conditions,
the maximum output current level will be reduced due to
the internal compensation required to meet stability criteria
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
inductor current depending upon the operating duty factor.
Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3892/LTC3892-1 is capable
of sensing the voltage drop across the inductor DCR, as
shown in Figure 2b. The DCR of the inductor represents
the small amount of DC winding resistance of the copper,
which can be less than 1mΩ for today’s low value, high
current inductors. In a high current application requiring
such an inductor, power loss through a sense resistor
16
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38921f