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

LTC3857
Applications Information
placed close to the switching node, to prevent noise from
coupling into sensitive small-signal nodes.
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) determined by the I_LIM setting. The current
comparator threshold voltage sets the peak of the induc-
tor current, yielding a maximum average output current,
I_MAX, equal to the peak value less half the peak-to-peak
ripple current, ∆I_L. To calculate the sense resistor value,
use the equation:
R_SENSE
=
V_SENSE(MAX)
I_MAX
+
∆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 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 efficiency
at high load currents, the LTC3857 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 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.
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 current
over the full operating temperature range, choose the
minimum value for the maximum current sense threshold
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
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_SENSE(EQUIV)
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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