LTC3869-2_15 Datasheet, PDF (16/42 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down DC/DC Controllers

LTC3869/LTC3869-2
Applications Information
The filter components need to be placed close to the IC.
The positive and negative sense traces need to be routed
as a differential pair and Kelvin connected to the sense
resistor.
V_SENSE
20mV/DIV
V_ESL(STEP)
500ns/DIV
3869 F03
Figure 3. Voltage Waveform Measured
Directly Across the Sense Resistor
Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3869 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 several points of efficiency compared to DCR sensing.
V_SENSE
20mV/DIV
500ns/DIV
3869 F04
Figure 4. Voltage Waveform Measured After the
Sense Resistor Filter. C_F = 1000pF, R_F = 100Ω
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.
If the RC time constant is chosen to be close to the
parasitic inductance divided by the sense resistor (L/R),
the resulting waveform looks resistive again, as shown
in Figure 4. For applications using low maximum sense
voltages, check the sense resistor manufacturer’s data
sheet for information about parasitic inductance. In the
absence of data, measure the voltage drop directly across
the sense resistor to extract the magnitude of the ESL
step and use the equation above to determine the ESL.
However, do not over-filter. Keep the RC time constant
less than or equal to the inductor time constant to maintain
a high enough ripple voltage on V_RSENSE.
The above generally applies to high density/high current
applications where I_MAX >10A and low values of inductors
are used. For applications where I_MAX <10A, set R_F to 10Ω
and C_F to 1000pF. This will provide a good starting point.
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
(V_SENSE(MAX)) in the Electrical Characteristics table (23mV,
43mV, or 68mV, 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 temperature
coefficient of resistance, which is approximately 0.4%/°C.
A conservative value for T_L(MAX) is 100°C.
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For more information www.linear.com/LTC3869