LTC2978
APPLICATIONS INFORMATION
Measuring Current with Inductor DCR
2. Calculate the maximum required output voltage for
V
DACP±
:
Figure 26 shows the circuit for applications that require
DCR current sense. A second order RC filter is required
in these applications in order to minimize the ripple volt-
age seen at the current sense inputs. A value of 1kΩ
ΔUP%
VDAC ≥ 1+
• V
(11)
P0
REF
ΔDOWN
%
is suggested for R
and R
in order to minimize
CM1
CM2
Note: Not all DC/DC’s converters follow these trim equa-
tionsespeciallynewerbricks.ConsultLTCFieldApplication
Engineering.
gain errors due the current sense inputs’ internal resis-
tance. C should be selected to provide cancellation
CM1
of the zero created by the DCR and inductance, i.e.
ꢀ=ꢀL/(DCR•ꢀR ).C shouldbeselectedtoprovide
C
CM1
CM1
CM2
Measuring Current
a second stage corner frequency at < 1/1± of the DC/DC
converter’sswitchingfrequency.Inaddition,C needsto
Odd numbered ADC channels may be used to measure
supply current. Set the ADC to high resolution mode to
configure for current measuring and improve sensitivity.
Note that no OV or UV faults or warnings are reported in
thismode,buttelemetryisavailablefromtheREAD_VOUT
command using the 11-bit signed mantissa plus ꢀ-bit
signed exponent L11 data format. Set the MFR_CON-
FIG_LTC2978bitb[9]=1inordertoenablehighresmode.
CM2
be much smaller than C
loading of the filter’s first stage.
in order to prevent significant
CM1
R
CM
R
CM
V
V
SENSEP1
C
C
CM
LTC2978
CM
The V
pin will assert low in this mode and cannot
OUT_EN
SENSEM1
L
R
SNS
be used to control a DC/DC converter. The V
pin is also unavailable.
output
2978 F25
DACP
LOAD CURRENT
Measuring Current with a Sense Resistor
Figure 25. Sense Resistor Current Sensing Circuits
A circuit for measuring current with a sense resistor is
shown in Figure 2ꢀ. The balanced filter rejects both com-
mon mode and differential mode noise from the output of
theDC/DCconverter. Thefilterisplaceddirectlyacrossthe
sense resistor in series with the DC/DC converter’s induc-
tor. Note that the current sense inputs must be limited to
R
R
CM2
CM2
V
V
SENSEP1
C
C
CM2
LTC2978
C
CM1
C
CM1
CM2
SENSEM1
less than 6V with respect to ground. Select R and C
CM
CM
2978 F26
such that the filter’s corner frequency is < 1/1± the DC/DC
converter’sswitchingfrequency.Thiswillresultinacurrent
sense waveform that offers a good compromise between
the voltage ripple and the delay through the filter. A value
R
R
CM1
CM1
L
DCR
SWX0
Figure 26. Sense Resistor Current Sensing Circuits
1kΩ for R is suggested in order to minimize gain er-
CM
rors due to the current sense inputs’ internal resistance.
2978fc
71
Linear Systems [ Linear Systems ]
