LTC3853_15 Datasheet, PDF (15/36 Pages) Linear Technology – Triple Output, Multiphase Synchronous Step-Down Controller

LTC3853
APPLICATIONS INFORMATION
For example, Figure 3 illustrates the voltage waveform
across a 2mΩ sense resistor with a 2010 footprint for the
1.2V/15A converter operating at 100% load. The waveform
is the superposition of a purely resistive component and a
purely inductive component. It was measured using two
scope probes and waveform math to obtain a differential
measurement. Based on additional measurements of the
inductor ripple current and the on-time and off-time of
the top switch, the value of the parasitic inductance was
determined to be 0.5nH using the equation:
ESL
=
V_ESL(STEP)
∆I_L
t_ON • t_OFF
t_ON + t_OFF
If the RC time constant is chosen to be close to the parasitic
inductance divided by the sense resistor (L/R), the result-
ing 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
V_SENSE
20mV/DIV
V_ESL(STEP)
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 induc-
tors 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.
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.
Inductor DCR Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3853 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.
500ns/DIV
3853 F03
Figure 3. Voltage Waveform Measured
Directly Across The Sense Resistor
V_SENSE
20mV/DIV
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.
500ns/DIV
3853 F04
Figure 4. Voltage Waveform Measured After the
Sense Resistor Filter. C_F = 1000pF, R_F = 100Ω
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
3853fc
For more information www.linear.com/LTC3853
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