Datasheet

LTC3865/LTC3865-1
16
3865fb
APPLICATIONS INFORMATION
the sense traces on the PCB. A typical fi lter consists of
two series 10Ω resistors connected to a parallel 1000pF
capacitor, resulting in a time constant of 20ns.
This same RC fi lter, with minor modifi cations, can be used
to extract the resistive component of the current sense
signal in the presence of parasitic inductance. For example,
Figure 3 illustrates the voltage waveform across a 2mΩ
sense resistor with a 2010 footprint for the 1.2V/15A
converter operating at 100% load. The waveform is the
superposition of a purely resistive component and a
purely inductive component. It was measured using two
scope probes and waveform math to obtain a differential
measurement. Based on additional measurements of the
inductor ripple current and the on-time and off-time of
the top switch, the value of the parasitic inductance was
determined to be 0.5nH using the equation:
ESL
V
I
tt
tt
ESL STEP
L
ON OFF
ON OFF
=
Δ+
()
•
If the RC time constant is chosen to be close to the parasitic
inductance divided by the sense resistor (L/R), the result-
ing 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 fi lter. Keep the RC time constant less than or equal
to the inductor time constant to maintain a high enough
ripple voltage on V
RSENSE
.
The above generally applies to high density/high current
applications where I
(MAX)
>10A and low values of inductors
are used. For applications where I
(MAX)
< 10A, set R
F
to
10 and C
F
to 1000pF. This will provide a good starting
point.
The fi lter 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 effi ciency
at high load currents, the LTC3865/LTC3865-1 are 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 resis-
tor would cost several points of effi ciency 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 fi lter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
500ns/DIV
V
SENSE
20mV/DIV
3865 F03
V
ESL(STEP)
500ns/DIV
V
SENSE
20mV/DIV
3865 F04
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