Datasheet

LTC3613
16
3613fa
APPLICATIONS INFORMATION
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 1m 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 10 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 1m 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.
R
F
R ESL
R
SENSE
RESISTOR
AND
PARASITIC INDUCTANCE
FILTER COMPONENTS
PLACED NEAR SENSE PINS
R
F
SENSE
+
SW
LTC3613
SENSE
–
C
F
3613 F03
V
OUT
Figure 3. R
SENSE
Current Sensing
R1
R2
(OPT)
DCRL
INDUCTOR
L/DCR = (R1||R2) C1
C1 NEAR SENSE PINS
SENSE
+
SW
LTC3613
SENSE
–
C1
3613 F04
V
OUT
C
OUT
Figure 4. DCR Current Sensing