Datasheet

LTC3833
16
3833f
APPLICATIONS INFORMATION
Because of possible PCB noise in the current sensing loop,
the current 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 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 2MHz 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 LTC3833 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
today’s low value, high current inductors. In a high cur-
rent application requiring such an inductor, conduction
loss through a sense resistor would cost several points
of efficiency compared to DCR sensing.
The inductor DCR is sensed by connecting an RC filter
across the inductor. This filter typically consists of one
or two resistors (R1 and R2) and one capacitor (C1) as
shown in Figure 4. If the external R1||R2 • C1 time constant
is chosen to be exactly equal to the L/DCR time constant,
the voltage drop across the external capacitor is equal
to the voltage drop across the inductor DCR multiplied
by R2/(R1 + R2). Therefore, R2 may be used to scale
the voltage across the sense terminals when the DCR is
greater than the target sense resistance. With the ability
to program current limit through the V
RNG
pin, R2 may
be optional. C1 is usually selected to be in the range of
0.01μF to 0.47μF. This forces R1|| R2 to around 2k to 4k,
reducing error that might have been caused by the SENSE
pins’ input bias currents.
The first step in designing DCR current sensing is to
determine the DCR of the inductor. Where provided, use
the manufacturer’s maximum value, usually given at 25°C.
Increase this value to account for the temperature coef-
ficient of resistance, which is approximately 0.4%/°C. A
conservative value for inductor temperature T
L
is 100°C.
The DCR of the inductor can also be measured using a good
RLC meter, but the DCR tolerance is not always the same
and varies with temperature; consult the manufacturers’
datasheets for detailed information.
From the DCR value, V
SENSE(MAX)
is calculated as:
V
SENSE(MAX)
= DCR
MAX
at 25°C• 1+ 0.4% T
L(MAX)
– 25°C
( )




• I
OUT(MAX)
– ∆I
L
/2




If V
SENSE(MAX)
is within the maximum sense voltage of
the LTC3833 as programmed by the V
RNG
pin (30mV to
100mV), then the RC filter only needs R1. If V
SENSE(MAX)
is
higher, then R2 may be used to scale down the maximum
sense voltage so that it falls within range.
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
P
LOSS
R1
( )
=
V
IN(MAX)
– V
OUT
( )
• V
OUT
R1
R1
R2
(OPT)
DCRL
INDUCTOR
L/DCR = (R1||R2) C1
C1 NEAR SENSE PINS
SENSE
+
LTC3833
SENSE
–
C1
3833 F04
V
OUT
C
OUT
Figure 4. DCR Current Sensing