LTC2978
APPLICATIONS INFORMATION
Single Phase Design Example
Measuring Multiphase Currents
As a design example for a DCR current sense application,
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 aver-
aged together prior to being applied to the second stage of
assume L = 2.2μH, DCR = 10mΩ, and F = 500kHz.
SW
Let R
= 1kΩ and solve for C
:
CM1
CM1
2.2µH
10mΩ • 1kΩ
C
≥
= 220nF
CM1
the filter consisting of R
and C . Because the R
CM2 CM2 CM1
resistors for the three phases are in parallel, the value of
must be multiplied by the number of phases. Also
R
Let R
SW
= 1kΩ. In order to get a second pole at
CM1
CM2
/10 = 50kHz:
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.
F
1
C
≅
= 3.18nF
CM2
2π• 50kHz • 1kΩ
Let C
CM1
= 3.3nF. Note that since C
is much less than
CM2
CM2
C
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.
Multiphase Design Example
Using the same values for inductance and DCR from
the previous design example, the value for R
will be
CM1
3kΩ for a three phase DC/DC converter if C
is left at
CM1
220nF. Similarly, the value for IOUT_CAL_GAIN will be
DCR/3 = 3.33mΩ.
SWX1
R
R
CM1
CM1
R
CM2
R
C
CM1
C
CM2
CM1
L
I
I
SENSEP
LTC2978
DCR
SENSEM
2978 F27
R
/3
CM1
R
CM2
DCR
DCR
C
CM1
C
CM2
L
L
TO LOAD
SWX2
SWX3
Figure 27. Multiphase DCR Current Sensing Circuits
2978fc
72
Linear Systems [ Linear Systems ]
