LTC3875
19
3875fb
For more information
www.linear.com/LTC3875
APPLICATIONS INFORMATION
output inductor and capacitors C1 and C2 are close to
the IC pins to prevent noise coupling to the sense signal.
For applications where the inductor DCR is large, the
LTC3875 could also be used like any typical current
mode controller with conventional DCR sensing by
disabling the SNSD
+
pin, shorting it to ground. An
R
SENSE
resistor or a DCR sensing RC filter can be used
to sense the output inductor signal and connects to the
SNSA
+
pin. If the RC filter is used, its time constant,
R • C, equals L/DCR of the output inductor. In these ap-
plications, the current limit, V
SENSE(MAX)
, will be five times
the value of V
SENSE(MAX)
with DC loop enabled, and the
operating voltage range of SNSA
+
and SNS
–
is from 0V to
5V. An output voltage of 5V can be generated.
Low Inductor DCR Sensing and Current Limit
Estimation
The LTC3875 is specifically designed for high load current
applications requiring the highest possible efficiency; it is
capable of sensing the signal of an inductor DCR in the sub
milliohm range (Figure 4b). The DCR is the inductor DC
winding resistance, which is often less than 1mΩ for high
current inductors. In high current and low output voltage
applications, conduction loss of a high DCR inductor or a
sense resistor will cause a significant reduction in power
efficiency. For a specific output requirement and induc-
tor, choose the current limit sensing level that provides
proper margin for maximum load current, and uses the
relationship of the sense pin filters to output inductor
characteristics as depicted below.
DCR
=
V
SENSE(MAX)
I
MAX
+
∆
I
L
2
L/DCR
=
R1•C1
=
5•R2•C2
where:
V
SENSE(MAX)
is the maximum sense voltage for a given
ILIM threshold.
I
MAX
is the maximum load current.
ΔI
L
is the inductor ripple current.
L/DCR is the output inductor characteristics.
C
OUT
TO SENSE FILTER,
NEXT TO THE CONTROLLER
INDUCTOR
3875 F04a
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
TCOMP/ITEMP
R
NTC
100k
INDUCTOR
DCR
L
SNSD
+
SNSA
+
SNS
–
SGND
LTC3875
V
OUT
3875 F04b
R1
C1
C2
PLACE C1, C2 NEXT TO IC
PLACE R1, R2 NEXT TO INDUCTOR
R1C1 = 5 • R2C2
R
S
22.6k
R
ITEMP
R
P
90.9k
R2
Figure 4a. Sense Lines Placement with Inductor DCR
Figure 4b. Inductor DCR Current Sensing
