Datasheet
LTC2974
88
2974fc
For more information www.linear.com/LTC2974
applicaTions inForMaTion
Measuring Current with a Sense Resistor
A circuit for measuring current with a sense resistor is
shown in Figure 33. The balanced filter rejects both com
-
mon mode and differential mode noise from the output
of the DC/DC converter. The filter is placed directly across
the sense resistor in series with the DC/DC converter’s in
-
ductor. Note that the current sense inputs must be limited
to less than 6V with respect to ground. Select R
CM
and
C
CM
such that the filter’s corner frequency is < 1/10 the
DC/DC converter’s switching frequency. This will result in
a current sense waveform that offers a good compromise
between the voltage ripple and the delay through the filter.
A value 1kΩ for R
CM
is suggested in order to minimize gain
errors due to the current sense inputs’ internal resistance.
Measuring Current with Inductor DCR
Figure 34 shows the circuit for applications that require
DCR current sense. A second order R-C filter is required
in these applications in order to minimize the ripple volt
-
age seen at the current sense inputs. A value of 1kΩ
is suggested for R
CM1
and R
CM2
in order to minimize
gain errors due the current sense inputs’ internal resis-
tance. C
CM1
should be selected to provide cancellation
of the zero created by the DCR and inductance, i.e.
C
CM1
= L/(DCR• R
CM1
). C
CM2
should be selected to provide
a second stage corner frequency at < 1/10 of the DC/DC
converter’s switching frequency. In addition, C
CM2
needs to
be much smaller than C
CM1
in order to prevent significant
loading of the filter’s first stage.
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 •50kHz •1kΩ
= 3.18nF
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,
R-C averaging may be employed. Figure 35 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
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 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.
Figure 33.Sense Resistor Current Sensing Circuits
Figure 34. DCR Current Sensing Circuits
R
CM
R
CM
R
SNS
2974 F33
L
LOAD CURRENT
C
CM
C
CM
LTC2974
I
SENSEP
I
SENSEM
2974 F34
R
CM2
R
CM2
R
CM1
R
CM1
DCR
L
C
CM2
C
CM2
LTC2974
I
SENSEP
I
SENSEM
C
CM1
C
CM1
SWX0