LTC3811_15 Datasheet, PDF (26/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
This same RC filter, with minor modifications, can be used
to extract the resistive component of the current sense
signal in the presence of parasitic inductance. For example,
Figure 13 illustrates the voltage waveform across a 1.5mΩ
Panasonic metal strip resistor (ERJ-M1WTJ1M5U). The
waveform is the superposition of a purely resistive compo-
nent and a purely inductive component and was measured
with a single low impedance scope probe through a BNC
connected directly across the sense resistor terminals.
Based on additional measurements of the inductor ripple
current and the on- and off-times of the primary switch,
the value of the parasitic inductance was determined to
be 0.5nH using the equation:
V_ESL(STEP)
=
ESL
•
⎡
⎢
⎣
ΔI_L
t_ON
+
ΔI_L
t_OFF
⎤
⎥
⎦
If the R∙C time constant is chosen to be exactly the same
as the parasitic inductance divided by the sense resistor
(L/R), the resulting waveform looks resistive again, as
shown in Figure 14. For applications using low maximum
10mV/DIV
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 using a low impedance con-
nection to extract the magnitude of the ESL step and
use the equation above to determine the proper filter
time constant, keeping the two filter resistor values equal
and less than about 200Ω each. Finally, place these filter
components close to the IC and run the positive and
negative sense traces parallel to each other all the way
to the sense resistor.
Inductor DCR Sensing
For applications requiring the highest possible efficiency,
the LTC3811 is capable of sensing the voltage drop across
the inductor DCR, as shown in Figure 15. 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. If the external
RC 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. Check the manufacturer’s data sheet for specifications
regarding the inductor DCR in order to properly dimension
the external filter components. The DCR of the inductor
can also be measured using a good RLC meter.
250ns/DIV
3811 F13
Figure 13. Current Sense Waveform for the Circuit in Figure 33
10mV/DIV
250ns/DIV
3811 F14
Figure 14. Waveform at the SENSE⁺ and SENSE^– Pins
Using RC – L/R Time Constant Cancellation
26
V_IN
V_IN DRV_CC
LTC3811
BOOST
TG
SW
INDUCTOR
SW
L
DCR
V_OUT
BG
PGND
SENSE^{+ R1 = R2}
SENSE^–
R2
C1
SGND
R1
R1 • C1 = L/DCR
FILTER COMPONENTS
SHOULD BE PLACED NEAR
SENSE⁺, SENSE^– PINS
PLACE R1 NEAR
INDUCTOR TO
MINIMIZE SW NODE
NOISE COUPLING
3811 F15
Figure15: Current Mode Control Using the Inductor DCR
3811f