LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 16

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LTC3858-1

Manufacturer Part Number
LTC3858-1
Description
Dual 2-Phase Synchronous Step-Down Controller
Manufacturer
Linear Technology Corporation
Datasheet
www.DataSheet4U.com
LTC3858-1
Low Value Resistors Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 5a. R
output current.
The current comparator has a maximum threshold
V
old voltage sets the peak of the inductor current, yielding
a maximum average output current, I
peak value less half the peak-to-peak ripple current, ΔI
To calculate the sense resistor value, use the equation:
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to the
internal compensation required to meet stability criterion
for buck regulators operating at greater than 50% duty
factor. A curve is provided in the Typical Performance Char-
acteristics section to estimate this reduction in peak output
current depending upon the operating duty factor.
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 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 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
APPLICATIONS INFORMATION
16
SENSE(MAX)
R
SENSE
=
of 50mV (typ). The current comparator thresh-
V
I
SENSE MAX
MAX
SENSE
+
(
Δ
is chosen based on the required
2
I
L
)
MAX
, equal to the
L
.
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:
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.
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
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
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
The equivalent resistance R1||R2 is scaled to the room
temperature inductance and maximum DCR:
The sense resistor values are:
R
R
R
R
SENSE EQUIV
D
1
1
||
=
=
R
R
DCR
2
1
(
R
=
||
R
D
SENSE EQUIV
R
(
MAX
SENSE(MAX)
DCR at
2
;  
)
=
at T
R
(
   
2
V
I
L MAX
SENSE MAX
MAX
=
L
(
20
R R
L(MAX)
1
) in the Electrical Characteristics
)
°
1
+
C C
)
(
R
)
Δ
D
D
2
I
L
is 100°C.
1
)
+
pin’s ±1μA current.
38581f

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