LTC3861-1 Datasheet, PDF (20/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Applications Information
TPE, TPF series, the Kemet T520, T530 and A700 series,
NEC/Tokin NeoCapacitors and Panasonic SP series. Other
capacitor types include Nichicon PL series and Sprague
595D series. Consult the manufacturer for other specific
recommendations.
Current Sensing
To maximize efficiency, the LTC3861-1 is designed to sense
current through the inductor’s DCR, as shown in Figure 5
The DCR of the inductor represents the small amount
of DC winding resistance of the copper, which for most
inductors applicable to this application, is between 0.3mΩ
and 1mΩ. If the filter RC time constant is chosen to be
exactly equal to the L/DCR time constant of the inductor,
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.
Since the temperature coefficient of the inductor’s DCR is
3900ppm/°C, first order compensation of the filter time
constant is possible by using filter resistors with an equal
but opposite (negative) TC, assuming a low TC capacitor is
used. That is, as the inductor’s DCR rises with increasing
temperature, the L/DCR time constant drops. Since we
20
VINSNS
5V
LTC3861-1
V_CC
PWM
GND ISNSN ISNSP
C_F
R_F
R_F
V_LOGIC BOOST
V_CC
TG
LTC4449
IN
TS
BG
GND
FILTER COMPONENTS PLACED NEAR SENSE PINS
V_IN
12V
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
L
R_S ESL
C_F • 2R_F ≤ ESL/R_S
POLE-ZERO
CANCELLATION
V_OUT
38611 F05a
(5a) Using a Resistor to Sense Current
VINSNS
5V
LTC3861-1
V_CC
PWM
GND ISNSN ISNSP
C1*
V_LOGIC BOOST
V_CC
TG
LTC4449
IN
TS
BG
GND
V_IN
12V
INDUCTOR
L
DCR
R1*
V_OUT
R1
•
C1
=
L
DCR
*PLACE R1 NEAR INDUCTOR
PLACE C1 NEAR ISNSP, ISNSN PINS
(5b) Using the Inductor to Sense Current
38611 F05b
Figure 5. Two Different Methods of Sensing Current
38611f