LTC3899_15 Datasheet, PDF (19/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Applications Information
The Typical Application on the first page is a basic LTC3899
application circuit. LTC3899 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 cur-
rent comparators.
Buck Controllers (SENSE1⁺/SENSE1^–, SENSE2⁺/SENSE2^–):
The common mode voltage range on these pins is 0V to
65V (absolute maximum), enabling the LTC3899 to regu-
late buck 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.
Boost Controller (SENSE3⁺/SENSE3^–): The common
mode input range for these pins is 2.2V to 60V, 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 into 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 LTC3899, and the sense lines should
run close together to a Kelvin connection underneath the
current sense element (shown in Figure 1). 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 2b), R1 should be placed close to the switch-
ing node, to prevent noise from coupling into sensitive
small-signal nodes.
TO SENSE FILTER
NEXT TO THE CONTROLLER
INDUCTOR OR R_SENSE
CURRENT FLOW
3899 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.
The current comparators have a maximum threshold
V_SENSE(MAX) of 75mV. 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 the buck controllers in very low dropout con-
ditions, 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%
For more information www.linear.com/LTC3899
3899f
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