LTC3829
16
3829fc
For more information www.linear.com/LTC3829
APPLICATIONS INFORMATION
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 =
ESL(STEP)
∆
ON
OFF
+
(1)
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 Equation 1 to determine the ESL. However, do not
overfilter. Keep the RC time constant, less than or equal
to the inductor time constant to maintain a high enough
ripple voltage of ∆V
SENSE
. The above generally applies to
high density/high current applications where I
MAX
> 10A
and low values of inductors are used. For applications
500ns/DIV
V
SENSE
20mV/DIV
3829 F03
V
ESL(STEP)
Figure 3. Voltage Waveform Measured
Directly Across the Sense Resistor
500ns/DIV
V
SENSE
20mV/DIV
3829 F04
Figure 4. Voltage Waveform Measured After
the Sense Resistor Filter. C
F
= 1000pF, R
F
= 100Ω
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 nega
-
tive 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 LTC3829 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.
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)
=
SENSE(MAX)
I
MAX
+
∆I
L
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 (25mV,
45mV 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.
