Datasheet

LTC2974
89
2974fc
For more information www.linear.com/LTC2974
LTC2974
I
SENSEP
I
SENSEM
2974 F35
C
CM2
C
CM2
R
CM2
C
CM2
C
CM1
R
CM2
R
CM1
R
CM1
R
CM1
R
CM1/3
DCR
DCR
L
L
DCR
L
SWX2 SWX3
TO LOAD
SWX1
Figure 35. Multiphase DCR Current Sensing Circuits
applicaTions inForMaTion
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.
Anti-aliasing Filter Considerations
Noisy environments require an anti-aliasing filter on
the input to the LTC2974’s ADC. The R-C circuit shown
in Figure 36 is adequate for most situations. Keep
R40 = R50 ≤ 200Ω to minimize ADC gain errors, and select
a value for capacitors C10 and C20 that does not add too
much additional response time
to the OV/UV supervisor,
e.g. τ = 10μs (R = 100Ω, C = 0.10μF).
Sensing Negative Voltages
Figure 37 shows the LTC2974 sensing a negative power
supply (V
EE
). The R1/R2 resistor divider translates the
negative supply voltage to the LTC2974’s V
SENSEM1
input
while the V
SENSEP1
input is tied to the REFP pin which has
a typical output voltage of 1.23V. Read_vout is determined
from the following equation:
V
EE
= V
REFP
– READ_ VOUT
( )
•
R2
R1
+ 1






–
1µA • R2
(14)
Where READ_VOUT returns V
SENSEP
– V
SENSEM
The voltage divider should be configured in order to present
about 0.5V to the voltage sense inputs when the negative
supply reaches its POWER_GOOD_ON threshold so that
Figure 36. Anti-Aliasing Filter on V
SENSE
Lines
V
PWR
V
DD33
V
DD33
V
DD25
V
DD25
V
IN_SNS
V
DAC0
V
SENSEP0
V
SENSEM0
V
OUT_EN0
LTC2974*
0.1µF
0.1µF
4.5V < V
IBUS
< 15V
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF FOUR CHANNELS SHOWN
V
IN
V
OUT
R20
R30
R10
2974 F36
RUN/SS
SGND
V
FB
GND
DC/DC
CONVERTER
LOAD
C10
C20
R50
R40
GND