Linear LTC3875 Datasheet page 19

APPLICATIONS INFORMATION
R
ITEMP
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 resistor or a DCR sensing RC filter can be used
SENSE
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
the value of V
SENSE(MAX)
operating voltage range of SNSA
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
Figure 4a. Sense Lines Placement with Inductor DCR
INTV
BOOST
LTC3875
TCOMP/ITEMP
PGND
R
S
22.6k
SNSD
SNS
R
R
NTC
P
SNSA
100k
90.9k
SGND
PLACE C1, C2 NEXT TO IC
PLACE R1, R2 NEXT TO INDUCTOR
R1C1 = 5 • R2C2
Figure 4b. Inductor DCR Current Sensing
, will be five times
SENSE(MAX)
with DC loop enabled, and the
+ –
and SNS is from 0V to
For more information
TO SENSE FILTER,
NEXT TO THE CONTROLLER
C
OUT
3875 F04a
INDUCTOR
V
IN
CC
INDUCTOR
TG
SW
BG
R1
+
C1
–
C2
+
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.
V
SENSE(MAX)
DCR =
I
MAX
L/DCR = R1•C1= 5 •R2 •C2
where:
V
SENSE(MAX)
ILIM threshold.
I is the maximum load current.
MAX
ΔI is the inductor ripple current.
L
L/DCR is the output inductor characteristics.
www.linear.com/LTC3875
LTC3875
V
IN
L DCR
V
OUT
R2
3875 F04b
∆I
L
+
2
is the maximum sense voltage for a given
3875fb
19