ltc3869gn-2 Linear Technology Corporation, ltc3869gn-2 Datasheet - Page 14

• Manufacturer Part Number: ltc3869gn-2
• Description: Ltc3869/ltc3869-2 - Dual, 2-phase Synchronous Step-down Dc/dc Controllers
• Manufacturer: Linear Technology Corporation

LTC3869/LTC3869-2
APPLICATIONS INFORMATION
For previous generation current mode controllers, the
maximum sense voltage was high enough (e.g., 75mV for
the LTC1628 / LTC3728 family) that the voltage drop across
the parasitic inductance of the sense resistor represented
a relatively small error. For today’s highest current density
solutions, however, the value of the sense resistor can
be less than 1mΩ and the peak sense voltage can be as
low as 20mV. In addition, inductor ripple currents greater
than 50% with operation up to 1MHz are becoming more
common. Under these conditions the voltage drop across
the sense resistor’s parasitic inductance is no longer neg-
ligible. A typical sensing circuit using a discrete resistor is
shown in Figure 2a. In previous generations of controllers,
a small RC filter placed near the IC was commonly used to
reduce the effects of capacitive and inductive noise coupled
inthe sense traces on the PCB. A typical filter consists of
two series 10Ω resistors connected to a parallel 1000pF
capacitor, resulting in a time constant of 20ns.
14
3869 F02a
LTC3869
SGND
SENSE
SENSE
INTV
BOOST
PGND
V
SW
(2a) Using a Resistor to Sense Current
BG
TG
CC
PLACED NEAR SENSE PINS
IN
+
–
FILTER COMPONENTS
C
F
R
R
F
F
Figure 2. Two Different Methods of Sensing Current
V
SENSE RESISTOR
C
PLUS PARASITIC
CANCELLATION
IN
F
INDUCTANCE
R
• 2
POLE-ZERO
S
RF
≤ ESL/R
ESL
S
V
OUT
**PLACE R1 NEXT TO
*PLACE C1 NEAR SENSE
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 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:
SENSE
INDUCTOR
ESL =
SGND
–
LTC3869
PINS
SENSE
SENSE
INTV
BOOST
(2b) Using the Inductor DCR to Sense Current
PGND
V
SW
V
BG
TG
ESL(STEP)
CC
IN
+
–
ΔI
+
L
,
R1 || R2 × C1 =
C1*
t
t
ON
ON
R2
+ t
• t
R1**
DCR
OFF
OFF
L
R
SENSE(EQ)
INDUCTOR
L
= DCR
V
IN
DCR
R1 + R2
R2
V
OUT
3869 F02b
3869f