ltc3858-2 Linear Technology Corporation, ltc3858-2 Datasheet - Page 17

• Manufacturer Part Number: ltc3858-2
• Description: Ltc3858-2 - Low Iq, Dual 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3850 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 4b. The DCR of the inductor represents the small
amount of DC resistance of the copper wire, which can be
less than 1mΩ for today’s low value, high current inductors.
In a high current application requiring such an inductor,
power loss through a sense resistor would cost several
points of efficiency compared to inductor 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 filter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
always the same and varies with temperature; consult
the manufacturers’ data sheets for detailed information.
APPLICATIONS INFORMATION
LTC3858-2
SENSE
SENSE
INTV
BOOST
SGND
(4a) Using a Resistor to Sense Current
SW
V
TG
BG
CC
IN
+
–
PLACE CAPACITOR NEAR
SENSE PINS
V
IN
R
SENSE
Figure 4. Current Sensing Methods
38582 F04a
V
OUT
Using the inductor ripple current value from the Inductor
Value Calculation section, the target sense resistor value is:
To ensure that the application will deliver full load current
over the full operating temperature range, choose the
minimum value for the Maximum Current Sense Thresh-
old Voltage (V
table (30mV, 50mV or 75mV depending on the state of
the I
Next, determine the DCR of the inductor. When provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of copper, which is approximately 0.4%/°C. A
conservative value for T
To scale the maximum inductor DCR to the desired sense
resistor (R
*PLACE C1 NEAR
SENSE PINS
R
R
LTC3858-2
LIM
SENSE(EQUIV)
D
=
SENSE
SENSE
INTV
BOOST
pin).
SGND
DCR
(4b) Using the Inductor DCR to Sense Current
SW
V
BG
TG
D
CC
IN
R
+
–
) value, use the divider ratio:
SENSE(EQUIV)
MAX
SENSE(MAX)
(R1 || R2) C1 =
=
C1*
at T
V
I
L(MAX)
SENSE(MAX)
MAX
R2
R1
L(MAX)
DCR
) in the Electrical Characteristics
L
+
ΔI
2
R
SENSE(EQ)
L
is 100°C.
V
LTC3858-2
INDUCTOR
IN
L
= DCR
DCR
R1 + R2
R2
17
38582 F04b
V
38582f
OUT