Datasheet

LTC3858
17
3858fc
(5a) Using a Resistor to Sense Current (5b) Using the Inductor DCR to Sense Current
Figure 5. Current Sensing Methods
APPLICATIONS INFORMATION
Inductor DCR Sensing
For applications requiring the highest possible effi ciency
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 1m 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 effi ciency 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 fi lter 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
SENSE(EQUIV)
=
V
SENSE(MAX)
I
MAX
+
ΔI
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
coeffi cient 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
SENSE(EQUIV)
DCR
MAX
atT
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 =
L
DCR at 20°C
()
•C1
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
PLACE CAPACITOR NEAR
SENSE PINS
SENSE
+
SENSE
–
SGND
LTC3858
V
OUT
R
SENSE
3858 F05a
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
*PLACE C1 NEAR
SENSE PINS
INDUCTOR
DCRL
SENSE
+
SENSE
–
SGND
LTC3858
V
OUT
3858 F05b
R1
R2C1*
(R1
||
R2) • C1 =
L
DCR
R
SENSE(EQ)
= DCR
R2
R1 + R2