LTC3868 Linear Technology, LTC3868 Datasheet - Page 17

• Manufacturer Part Number: LTC3868
• Description: Low IQ Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology

duty factor. A curve is provided in the Typical Performance
Characteristics section to estimate this reduction in
peak output current depending upon the operating duty
factor.
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.
applicaTions inForMaTion
LTC3868
SENSE
SENSE
INTV
BOOST
SGND
SW
(5a) Using a Resistor to Sense Current
V
TG
BG
CC
IN
+
–
PLACE CAPACITOR NEAR
SENSE PINS
V
IN
R
SENSE
Figure 5. Current Sensing Methods
3868 F05a
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 threshold
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 resistance, which is approximately
0.4%/°C. A conservative value for T
To scale the maximum inductor DCR to the desired sense
resistor (R
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
*PLACE C1 NEAR
SENSE PINS
R
R
LTC3868
SENSE(EQUIV)
LIM
D
=
SENSE
SENSE
INTV
BOOST
pin).
SGND
DCR
(5b) Using the Inductor DCR to Sense Current
SW
V
BG
TG
CC
IN
SENSE(MAX)
D
R
+
–
) value, use the divider ratio:
SENSE EQUIV
MAX
(R1 || R2)
C1*
=
at T
(
•
V
C1 =
I
L MAX
SENSE(MAX)
MAX
) in the Electrical Characteristics
R2
(
R1
DCR
)
L
+
)
∆I
R
2
SENSE(EQ)
L
+
www.DataSheet4U.com
pin’s ±1µA current.
V
INDUCTOR
IN
L
L(MAX)
= DCR
LTC3868
DCR
R1 + R2
is 100°C.
R2

3868 F05b
V
OUT
3868fb