LTC3858_15 Datasheet, PDF (17/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3858
APPLICATIONS INFORMATION
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 5b. 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.
Using the inductor ripple current value from the Inductor
Value Calculation section, the target sense resistor value
is:
R_{SE N SE(E Q U IV )}
=
V_{SE N SE(M AX )}
I_MAX
+
Δ I_L
2
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_SENSE(MAX)) in the Electrical Characteristics
table (30mV, 50mV or 75mV depending on the state of
the I_LIM pin).
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_L(MAX) is 100°C.
To scale the maximum inductor DCR to the desired sense
resistor (R_D) value, use the divider ratio:
R_D
=
R_{SE N SE(E Q U IV )}
DCR_MAX at T_L(MAX)
C1 is usually selected to be in the range of 0.1μF to 0.47μF.
This forces R1||R2 to around 2k, reducing error that might
have been caused by the SENSE⁺ pin’s ±1μA current.
The equivalent resistance R1||R2 is scaled to the room
temperature inductance and maximum DCR:
R1|| R2
=
(DCR
at
L
20°C)
•
C1
V_IN
INTV_CC
BOOST
TG
SW
LTC3858
BG
SENSE⁺
SENSE^–
SGND
V_IN
R_SENSE
V_OUT
PLACE CAPACITOR NEAR
SENSE PINS
3858 F05a
(5a) Using a Resistor to Sense Current
V_IN
V_IN
INTV_CC
BOOST
TG
SW
LTC3858
BG
INDUCTOR
L DCR
V_OUT
SENSE⁺
SENSE^–
SGND
*PLACE C1 NEAR
SENSE PINS
R1
C1* R2
(R1||R2) • C1 = ^L
DCR
R_SENSE(EQ) = DCR
R2
R1 + R2
3858 F05b
(5b) Using the Inductor DCR to Sense Current
Figure 5. Current Sensing Methods
3858fc
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