LTC3613
APPLICATIONS INFORMATION
R
RESISTOR
SENSE
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 507 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.
AND
PARASITIC INDUCTANCE
R
ESL
V
SW
LTC3613
OUT
R
R
F
F
+
SENSE
SENSE
C
F
–
3613 F03
FILTER COMPONENTS
PLACED NEAR SENSE PINS
Figure 3. RSENSE Current Sensing
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.
R
Inductor Current Sensing
SENSE
A typical R
showninFigure3. R
inductor current sensing scheme is
SENSE
SENSE
ischosenbasedontherequired
maximum output current. Given the maximum current,
,maximumsensevoltage,V ,setbythe
I
DCR Inductor Current Sensing
OUT(MAX)
SENSE(MAX)
V
pin, and maximum inductor ripple current, ΔI
,
RNG
L(MAX)
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.
the value of R
can be chosen as:
SENSE
VSENSE(MAX)
RSENSE
=
ΔIL(MAX)
IOUT(MAX)
–
2
Conversely, given R
and thus the V
above equation. To assure that the maximum rated output
current can be supplied for different operating conditions
andcomponentvariations,sufficientdesignmarginshould
be built into these calculations.
and I
, V
OUT(MAX) SENSE(MAX)
SENSE
voltage could be determined from the
RNG
INDUCTOR
L
DCR
SW
LTC3613
V
OUT
C
OUT
L/DCR = (R1||R2) C1
R1
BecauseofpossiblePCBnoiseinthecurrentsensingloop,
+
the current ripple of ΔV
= ΔI • R
also needs
SENSE
SENSE
SENSE
L
SENSE
R2
(OPT)
to be checked in the design to get a good signal-to-noise
C1
–
3613 F04
ratio. In general, for a reasonably good PCB layout, a
10mVΔV
number to start with, either for R
applications.
voltageisrecommendedasaconservative
SENSE
C1 NEAR SENSE PINS
or DCR sensing
SENSE
Figure 4. DCR Current Sensing
3613fa
16
Linear Systems [ Linear Systems ]
