LTC3839 Datasheet, PDF (19/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
recommended lower limits of V_SENSE(MAX) (by each indi-
vidual channel, calculated as half of the 2-channel-sum) for
statistical tolerancing design of a 2-phase application are:
• 24mV at V_RNG = 0.6V or SGND (30mV typical);
• 42mV at V_RNG = 1V or INTV_CC (50mV typical);
• 85mV at V_RNG = 2V (100mV typical).
Either worst-case or statistical limits can be chosen to
establish absolute minimums for current limit of the
LTC3839. Linear guarantees the worst-case minimum
and maximum for each channel, but does not guarantee
any statistical distributions of or relationship between the
two channels.
R_SENSE Inductor Current Sensing
The LTC3839 can be configured to sense the inductor
currents through either low value series current sensing
resistors (R_SENSE) or inductor DC resistance (DCR). 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 ac-
curate current limits for the controller.
A typical R_SENSE inductor current sensing scheme is shown
in Figure 3a. The filter components (R_F, C_F) need to be
placed close to the IC. The positive and negative sense
traces need to be routed as a differential pair close to-
gether and Kelvin (4-wire) connected underneath the sense
resistor, as shown in Figure 3b. Sensing current elsewhere
can effectively add parasitic inductance to the current sense
element, degrading the information at the sense terminals
and making the programmed current limit unpredictable.
R_SENSE is chosen based on the required maximum output
current. Given the maximum current, I_OUT(MAX), maximum
sense voltage, V_SENSE(MAX), set by V_RNG, and maximum
inductor ripple current ∆I_L(MAX), the value of R_SENSE can
be chosen as:
R_SENSE
=
V_{SENSE(MAX )}
I_{OUT(MAX )}
–
ΔI_{L(MAX )}
2
LTC3839
SENSE⁺
SENSE^–
R_F
C_{F RF}
R_SENSE RESISTOR
AND
PARASITIC INDUCTANCE
R ESL
C_F
t
3_F ≤ ESL/R_S
POLE-ZERO
CANCELLATION
V_OUT
3839 F03a
FILTER COMPONENTS
PLACED NEAR SENSE PINS
Figure 3a. R_SENSE Current Sensing
TO SENSE FILTER,
NEXT TO THE CONTROLLER
R_SENSE ^{3839 F03b}
C_OUT
Figure 3b. Sense Lines Placement with Sense Resistor
Conversely, given R_SENSE and I_OUT(MAX), V_SENSE(MAX) and
thus V_RNG voltage can be determined from the above equa-
tion. To ensure the maximum output current, sufficient
margin should be built in the calculations to account for
variations of LTC3839 under different operating conditions
and tolerances of external components.
Because of possible PCB noise in the current sensing
loop, the current sensing voltage ripple ∆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, 10mV of ∆V_SENSE is recommended as
a conservative number to start with, either for R_SENSE or
Inductor DCR sensing applications.
For today’s highest current density solutions the value
of the sense resistor can be less than 1mΩ and the
peak sense voltage can be as low as 20mV. In addition,
inductor ripple currents greater than 50% with operation
up to 2MHz are becoming more common. Under these
conditions, the voltage drop across the sense resistor’s
parasitic inductance becomes more relevant. A small RC
3839fa
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