LTC3613 Datasheet, PDF (16/36 Pages) Linear Technology – 24V, 15A Monolithic Step Down Regulator

LTC3613
APPLICATIONS INFORMATION
SW
LTC3613
SENSE⁺
SENSE^–
R_SENSE RESISTOR
AND
PARASITIC INDUCTANCE
R ESL
V_OUT
R_F
C_{F RF}
3613 F03
FILTER COMPONENTS
PLACED NEAR SENSE PINS
Figure 3. R_SENSE Current Sensing
R_SENSE Inductor Current Sensing
A typical R_SENSE inductor current sensing scheme is
shown in Figure 3. 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 the
V_RNG pin, and maximum inductor ripple current, ΔI_L(MAX),
the value of R_SENSE can be chosen as:
R_SENSE
=
V_SENSE(MAX)
I_OUT(MAX)
–
ΔI_L(MAX)
2
Conversely, given R_SENSE and I_OUT(MAX), V_SENSE(MAX)
and thus the V_RNG voltage could be determined from the
above equation. To assure that the maximum rated output
current can be supplied for different operating conditions
and component variations, sufficient design margin should
be built into these calculations.
Because of possible PCB noise in the current sensing loop,
the current ripple of ΔV_SENSE = ΔI_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 ΔV_SENSE voltage is recommended as a conservative
number to start with, either for R_SENSE or DCR sensing
applications.
For today’s highest current density solutions the value of
the sense resistor can be less than 1mΩ and the maxi-
mum sense voltage can be as low as 30mV. In addition,
inductor ripple currents greater than 50% with operation
up to 1MHz are becoming more common. Under these
conditions, the voltage drop across the sense resistor’s
parasitic inductance becomes more relevant. A small RC
filter placed near the IC has been traditionally used to re-
duce the effects of capacitive and inductive noise coupled
in the sense traces on the PCB. A typical filter consists of
two series 10Ω resistors connected to a parallel 1000pF
capacitor, resulting in a time constant of 20ns.
The filter components need to be placed close to the IC.
The positive and negative sense traces need to be routed
as a differential pair and Kelvin (4-wire) connected to the
sense resistor.
DCR Inductor Current Sensing
For applications requiring higher efficiency at high load
currents, the LTC3613 is capable of sensing the voltage
drop across the inductor DCR, as shown in Figure 4. The
DCR of the inductor represents the small amount of DC
winding resistance, which can be less than 1mΩ for to-
day’s low value, high current inductors. In a high current
application requiring such an inductor, conduction loss
through a sense resistor would cost several points of
efficiency compared to DCR sensing.
SW
LTC3613
SENSE⁺
SENSE^–
INDUCTOR
L DCR
L/DCR = (R1||R2) C1
R1
V_OUT
C_OUT
C1
R2
(OPT)
3613 F04
C1 NEAR SENSE PINS
Figure 4. DCR Current Sensing
3613fa
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