Datasheet
LTC2978A
74
2978afa
For more information www.linear.com/LTC2978A
Single Phase Design Example
As a design example for a DCR current sense application,
assume L = 2.2μH, DCR = 10mΩ, and F
SW
= 500kHz.
Let R
CM1
= 1kΩ and solve for C
CM1
:
C
CM1
≥
2.2µH
10mΩ • 1kΩ
= 220nF
Let R
CM2
= 1kΩ. In order to get a second pole at
F
SW
/10 = 50kHz:
C
CM2
≅
1
2π• 5 0kHz • 1kΩ
= 3.18n
F
Let C
CM2
= 3.3nF. Note that since C
CM2
is much less than
C
CM1
the loading effects of the second stage filter on the
matched first stage are not significant. Consequently, the
delay time constant through the filter for the current sense
waveform will be approximately 3μs.
Measuring Multiphase Currents
For current sense applications with more than one phase,
RC averaging may be employed. Figure 27 shows an
example of this approach for a 3-phase system with DCR
current sensing. The current sense waveforms are averaged
together prior to being applied to the second stage of the
filter consisting of R
CM2
and C
CM2
. Because the R
CM1
resistors for the three phases are in parallel, the value of
R
CM1
must be multiplied by the number of phases. Also
note that since the DCRs are effectively in parallel, the
value for IOUT_CAL_GAIN will be equal to the inductor’s
DCR divided by the number of phases. Care should to be
taken in the layout of the multiphase inductors to keep the
PCB trace resistance from the DC side of each inductor to
the summing node balanced in order to provide the most
accurate results.
Multiphase Design Example
Using the same values for inductance and DCR from
the previous design example, the value for R
CM1
will be
3kΩ for a three phase DC/DC converter if C
CM1
is left at
220nF. Similarly, the value for IOUT_CAL_GAIN will be
DCR/3=3.33mΩ.
APPLICATIONS INFORMATION
Figure 27. Multiphase DCR Current Sensing Circuits
R
CM2
R
CM1
R
CM1
R
CM1
R
CM2
R
CM1
/3
DCR
DCR
DCR
L
LL
2978a F27
TO LOAD
SWX3SWX2
SWX1
LTC2978A
V
SENSEP1
V
SENSEM1
C
CM2
C
CM1
C
CM1
C
CM2