Datasheet

LTC3838
23
3838fa
R1
R2
(OPT)
DCRL
INDUCTOR
L/DCR = (R1||R2) C1
C1 NEAR SENSE PINS
SENSE
+
LTC3838
SENSE
–
C1
3838 F05
V
OUT
C
OUT
Figure 5. DCR Current Sensing
APPLICATIONS INFORMATION
< 10A, set R
F
to 10 and C
F
to 1000pF. This will provide
a good starting point.
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 LTC3838 is capable of sensing the voltage
drop across the inductor DCR, as shown in Figure 5. The
DCR of the inductor represents the small amount of DC
winding resistance, which can be less than 1m for today’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 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 5. 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 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.
Resistor R1 should be placed close to the switching node,
to prevent noise from coupling into sensitive small-signal
nodes. Capacitor C1 should be placed close to the IC pins.
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’
data sheets for detailed information.
From the DCR value, V
SENSE(MAX)
is easily calculated as:
V
SENSE(MAX)
=DCR
MAX(25°C)
•1+0.4% T
L(MAX)
–25°C
()
⎡
⎣
⎤
⎦
•I
OUT(MAX)
–
ΔI
L
2
⎛
⎝
⎜
⎞
⎠
⎟
If V
SENSE(MAX)
is within the maximum sense voltage (30mV
to 100mV) of the LTC3838 as programmed by the V
RNG
pin, 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.