Datasheet

LTC3880/LTC3880-1
40
3880fc
For more information www.linear.com/LTC3880
APPLICATIONS INFORMATION
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
(1)
If the RC time constant is chosen to be close to the para
-
sitic inductance divided by the sense resistor (L/R), the
resultant waveform looks resistive, as shown in Figure
20.
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 the signal. Keep the RC time constant less
than or equal to the inductor time constant to maintain a
sufficient ripple voltage on V
RSENSE
for optimal operation
of the current loop controller.
INDUCTOR DCR CURRENT SENSING
For applications requiring the highest possible efficiency
at high load currents, the LTC3880 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 18a. 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 a few points of efficiency compared to DCR sensing.
If the external (R1+R3)||R2 • C1 time constant is chosen to
be exactly equal to the 2 • L/DCR time constant, assuming
R1=R3, the voltage drop across the external capacitor is
equal to the drop across the inductor DCR multiplied by
R2/(R1+R2+R3). R2 scales the voltage across the sense
terminals for applications where the DCR is greater than
the target sense resistor value. The DCR value is entered
as the IOUT_CAL_GAIN in mΩ unless R2 is required. If R2
is used, IOUT_CAL_GAIN = DCR • R2/(R1+R2+R3). If there
is no need to attenuate the signal, R2 can be removed. 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 manufactur
-
ers’ data sheets for detailed information. The LTC3880 will
account for temperature variation if the correct parameter
is
entered
into the MFR_IOUT_CAL_GAIN_TC register.
Typically the resistance has a 3900ppm/°C coefficient.
C2 can be optimized for a flat frequency response, assum
-
ing R1 = R3 by the following equation
C2 = [2R1 • R2 • C1–L/DCR • (2R1+R2)]/R1
2
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, be sure to pick
the optimum I
LIMIT
value accounting for errors in the DCR
versus the MFR_IOUT_CAL_GAIN parameter entered.
Figure 19. Voltage Measured Directly Across R
SENSE
Figure 20. Voltage Measured After the R
SENSE
Filter
500ns/DIV
V
SENSE
20mV/DIV
3880 F19
V
ESL(STEP)
500ns/DIV
V
SENSE
20mV/DIV
3880 F20