Datasheet

LTC3855
17
3855f
The above generally applies to high density/high current
applications where I
(MAX)
>10A and low values of induc-
tors are used. For applications where I
(MAX)
<10A, set R
F
to 10 Ohms 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 connected to the sense
resistor.
Inductor DCR Sensing
For applications requiring the highest possible efficiency at
high load currents, the LTC3855 is capable of sensing the
voltage drop across the inductor DCR, as shown in Figure
2b. The DCR of the inductor represents the small amount
of DC winding resistance of the copper, which can be less
than 1mΩ 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 sev-
eral points of efficiency compared to DCR sensing.
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 drop across
the inductor DCR multiplied by R2/(R1 + R2). R2 scales the
voltage across the sense terminals for applications where
the DCR is greater than the target sense resistor value.
To properly dimension the external filter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
applicaTions inForMaTion
Figure 2. Two Different Methods of Sensing Current
(2a) Using a Resistor to Sense Current (2b) Using the Inductor DCR to Sense Current
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PGND
FILTER COMPONENTS
PLACED NEAR SENSE PINS
SENSE
+
SENSE
–
SGND
LTC3855
V
OUT
3855 F02a
C
F
• 2
RF
≤ ESL/R
S
POLE-ZERO
CANCELLATION
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R
S
ESL
C
F
R
F
R
F
V
IN
V
IN
INTV
CC
ITEMP
BOOST
TG
SW
BG
PGND
*PLACE C1 NEAR SENSE
+
,
SENSE
–
PINS
**PLACE R1 NEXT TO
INDUCTOR
INDUCTOR
DCRL
SENSE
+
SENSE
–
SGND
LTC3855
OPTIONAL
TEMP COMP
NETWORK
V
OUT
3855 F02b
R1**
R2C1*
R
P
R
NTC
R
S
R1
||
R2 × C1 =
L
DCR
R
SENSE(EQ)
= DCR
R2
R1 + R2
Figure 3. Voltage Waveform Measured
Directly Across the Sense Resistor.
Figure 4. Voltage Waveform Measured After the
Sense Resistor Filter. C
F
= 1000pF, R
F
= 100Ω.
500ns/DIV
V
SENSE
20mV/DIV
3855 F03
V
ESL(STEP)
500ns/DIV
V
SENSE
20mV/DIV
3855 F04
the resulting waveform looks resistive again, as shown
in Figure 4. For applications using low maximum sense
voltages, check the sense resistor manufacturer’s data
sheet for information about parasitic inductance. In the
absence of data, measure the voltage drop directly across
the sense resistor to extract the magnitude of the ESL
step and use the equation above to determine the ESL.
However, do not over-filter. Keep the RC time constant less
than or equal to the inductor time constant to maintain a
high enough ripple voltage on V
RSENSE
.