LTC3857-1_15 Datasheet, PDF (16/38 Pages) Linear Technology – Low IQ, Dual, 2-Phase Synchronous Step-Down Controller

LTC3857-1
APPLICATIONS INFORMATION
Low Value Resistor Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 4a. R_SENSE is chosen based on the required
output current.
The current comparator has a maximum threshold
V_SENSE(MAX). The current comparator threshold voltage
sets the peak of the inductor current, yielding a maximum
average output current, I_MAX, equal to the peak value less
half the peak-to-peak ripple current, ΔI_L. To calculate the
sense resistor value, use the equation:
R_SENSE
=
V_{SE N SE(M AX )}
I_MAX
+
Δ I_L
2
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 cri-
terion for buck regulators operating at greater than 50%
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 LTC3857-1 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 4b. 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 appli-
cations 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 Induc-
tor 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 threshold
voltage (V_SENSE(MAX)).
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_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.
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