LTC3883 Datasheet, PDF (39/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
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, t_ON, and off-time,
t_OFF, 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.
V_SENSE
20mV/DIV
V_ESL(STEP)
500ns/DIV
3883 F19
Figure 19. Voltage Measured Directly Across R_SENSE
V_SENSE
20mV/DIV
500ns/DIV
3883 F20
Figure 20. Voltage Measured After the R_SENSE Filter
Inductor DCR Current Sensing
For applications requiring the highest possible efficiency
at high load currents, the LTC3883 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 L/DCR time constant, the voltage
drop across the external capacitor,C1, 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. 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 an accurate RLC meter, but the
DCR tolerance is not always the same and varies with
temperature. Consult the manufacturers’ data sheets
for detailed information. The LTC3883 will account for
temperature variation if the correct parameter is entered
into the MFR_IOUT_CAL_GAIN_TC command. Typically
the resistance has a 3900ppm/°C coefficient.
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.
3883f
39