LTC3869-2_15 Datasheet, PDF (14/42 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down DC/DC Controllers

LTC3869/LTC3869-2
Applications Information
The Typical Application on the first page is a basic LTC3869
application circuit. LTC3869 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 resis-
tors 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 require-
ment, and begins with the selection of R_SENSE (if R_SENSE is
used) and inductor value. Next, the power MOSFETs 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 maxi-
mum current limit of the controller. When I_LIM is either
grounded, floated or tied to INTV_CC, the typical value for
the maximum current sense threshold will be 30mV, 50mV
or 75mV, respectively.
Filter components mutual to the sense lines should be
placed close to the LTC3869, 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), sense resistor R1 should be placed
close to the switching node, to prevent noise from coupling
into sensitive small-signal nodes. The capacitor C1 should
be placed close to the IC pins.
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R_SENSE
C_OUT
3869 F01
Figure 1. Sense Lines Placement with Sense Resistor
Low Value Resistors Current Sensing
Which setting should be used? For the best current limit
accuracy, use the 75mV setting. The 30mV setting will
allow for the use of very low DCR inductors or sense
resistors, but at the expense of current limit accuracy.
The 50mV setting is a good balance between the two. For
single output dual phase applications, use the 50mV or
75mV setting for optimal current sharing.
SENSE⁺ and SENSE^– Pins
The SENSE⁺ and SENSE^– pins are the inputs to the current
comparators. The common mode input voltage range of
the current comparators is 0V to 12.5V. Both SENSE pins
are high impedance inputs with small base currents of less
than 1µA. When the SENSE pins ramp up from 0V to 1.4V,
the small base currents flow out of the SENSE pins. When
the SENSE pins ramp down from 12.5V to 1.1V, the small
base currents flow into the SENSE pins. The high imped-
ance inputs to the current comparators allow accurate
DCR sensing. However, care must be taken not to float
these pins during normal operation. The LTC3869GN-2
defaults to 50mV current limit value.
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 comparator has a maximum threshold
V_SENSE(MAX) determined by the I_LIM setting. The input
common mode range of the current comparator is 0V to
12.5V. 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, ∆I_L. To calculate the sense resistor
value, use the equation:
R_SENSE
=
V_{SENSE(MAX )}
I_MAX
+
∆ I_L
2
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of ∆V_SENSE = ∆I_L • R_SENSE
also needs to be checked in the design to get a good
signal-to-noise ratio. In general, for a reasonably good
PCB layout, a 10mV ∆V_SENSE voltage is recommended as
a conservative number to start with, either for R_SENSE or
DCR sensing applications, for duty cycles less than 40%.
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