Datasheet
LTC2978A
73
2978afa
For more information www.linear.com/LTC2978A
Figure 25. Sense Resistor Current Sensing Circuits
Figure 26. Inductor DCR Current Sensing Circuits
R
CM
R
CM
R
SNS
2978a F25
L
LOAD CURRENT
C
CM
C
CM
LTC2978A
V
SENSEP1
V
SENSEM1
R
CM2
R
CM2
R
CM1
R
CM1
DCR
2978a F26
L
SWX0
C
CM2
C
CM1
C
CM1
C
CM2
LTC2978A
V
SENSEP1
V
SENSEM1
APPLICATIONS INFORMATION
LTC2978 bit b[9] = 1 in order to enable high res mode.
The V
OUT_EN
pin will assert low in this mode and cannot
be used to control a DC/DC converter. The V
DACP
output
pin is also unavailable.
Measuring Current with a Sense Resistor
A circuit for measuring current with a sense resistor is
shown in Figure 25. 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 induc
-
tor. 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 er-
rors due to the current sense inputs’ internal resistance.
Measuring Current with Inductor DCR
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
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.