LTC2977 Datasheet, PDF (85/92 Pages) Linear Technology – 8-Channel PMBus Power System Manager Featuring Accurate Output Voltage Measurement

LTC2977
APPLICATIONS INFORMATION
C_CM1
R_CM2
C_CM1
R_CM2
SWX0
R_CM1
L
R_CM1
DCR
C_CM2
C_CM2
V_SENSEP1
LTC2977
V_SENSEM1
2977 F26
Figure 26. Inductor DCR Current Sensing Circuits
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
≅
2π •
1
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,
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.
SWX1
R_CM1
R_CM1
R_CM2
L
DCR
R_CM1
C_CM1
R_CM1/3
R_CM2
C_CM2
V_SENSEP1
LTC2977
V_SENSEM1
2977 F27
DCR
DCR
L
TO LOAD
L
C_CM1
C_CM2
SWX2
SWX3
Figure 27. Multiphase DCR Current Sensing Circuits
For more information www.linear.com/LTC2977
2977f
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