Datasheet

LTC3850/LTC3850-1
15
38501fc
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 2b. The DCR of the inductor represents the small
amount of DC winding resistance of the copper, which can be
less than 1mΩ for today’s low value, high current inductors.
In a high current application requiring such an inductor,
conduction loss through a sense resistor would cost sev-
eral points of efficiency compared to 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’ datasheets 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)
+
∆I
L
2
To ensure that the application will deliver full load cur-
rent over the full operating temperature range, choose
the minimum value for the Maximum Current Sense
Threshold (V
SENSE(MAX)
) in the Electrical Characteristics
table (20mV, 40mV, or 60mV, depending on the state of
the I
LIM
pin).
Next, determine the DCR of the inductor. Where provided,
use the manufacturer’s maximum value, usually given
at 20°C. Increase this value to account for the tempera-
ture coefficient of resistance, 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 value, use the divider ratio:
R
D
=
R
SENSE(EQUIV)
DCR
(MAX)
at T
L(MAX)
C1 is usually selected to be in the range of 0.047µF to
0.47µF. This forces R1|| R2 to around 2kΩ, reducing error
that might have been caused by the SENSE pins’ ±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
The sense resistor values are:
R1=
R1|| R2
R
D
; R2=
R1• R
D
1−R
D
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
P
LOSS
R1=
V
IN(MAX)
− V
OUT
( )
• V
OUT
R1
APPLICATIONS INFORMATION
Figure 3. Voltage Waveform Measured
Directly Across the Sense Resistor.
Figure 4. Voltage Waveform Measured After the
Sense Resistor Filter. C
F
= 1000pF, R
F
= 100Ω.
500ns/DIV
V
SENSE
20mV/DIV
38501 F03
V
ESL(STEP)
500ns/DIV
V
SENSE
20mV/DIV
38501 F04