LTC3867_15 Datasheet, PDF (15/36 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3867
APPLICATIONS INFORMATION
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3867
BG
PGND
SENSE⁺
SENSE^–
SGND
R_F
C_F
R_F
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R_S ESL
C_F • 2 • R_F ≤ ESL/R_S
POLE-ZERO
CANCELLATION
V_OUT
FILTER COMPONENTS
PLACED NEAR SENSE PINS
3867 F03a
(3a) Using a Resistor to Sense Current
V_IN
V_IN
INTV_CC
OPTIONAL
TEMP COMP
NETWORK
R_S
R_NTC RP
BOOST
TG
SW
LTC3867
ITEMP
BG
PGND
SENSE⁺
SENSE^–
SGND
R1**
C1* R2
INDUCTOR
L DCR
V_OUT
*PLACE C1 NEAR SENSE⁺, R1||R2 × C1 =
SENSE^– PINS
L
DCR
R_SENSE(EQ) = DCR
R2
R1 + R2
**PLACE R1 NEXT TO INDUCTOR
(3b) Using the Inductor DCR to Sense Current
3867 F03b
Figure 3. Two Different Methods of Sensing Current
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 negligible.
In previous generations of controllers, a small RC filter
placed near the IC was commonly used to reduce the ef-
fects of capacitive and inductive noise coupled in the 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. This same RC filter,
with minor modifications, can be used to extract the resis-
tive component of the current sense signal in the presence
of parasitic inductance. For example, Figure 4 illustrates
the voltage waveform across a 2mΩ sense resistor with
a 2010 footprint for a 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
3867f
15