Datasheet

LTC2978A
75
2978afa
For more information www.linear.com/LTC2978A
Figure 28. Antialiasing Filter on V
SENSE
Lines
Figure 29. Sensing Negative Voltages
V
PWR
V
DD33
V
DD33
V
DD25
V
IN_SNS
V
DACP0
V
SENSEP0
V
SENSEM0
V
DACM0
V
OUT_EN0
LTC2978A*
0.1µF
0.1µF
V
IN
4.5V < V
IBUS
< 15V
GND
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF EIGHT CHANNELS SHOWN
V
IN
V
OUT
R20
R30
R10
2978a F28
RUN/SS
SGND
V
FB
GND
DC/DC
CONVERTER
LOAD
C10
C20
R50
R40
0.1µF
LTC2978A
4.5V < V
IBUS
< 15V
GND
ONLY ONE OF EIGHT CHANNELS SHOWN,
SOME DETAILS OMITTED FOR CLARITY
POWER_GOOD_ON = 0.5V FOR V
EE
POWER_GOOD = –11.414V
WHERE V
EE
POWER_GOOD =
0.1µF
1.23V TYP
2978a F29
V
PWR
SDA
SCL
ALERTB
CONTROL
FAULTB
WDI/RESETB
SHARE_CLK
ASEL0
ASEL1
WP
V
IN_SNS
REFM
V
SENSEP1
WDI/RESETB
V
SENSEM1
REFP
PWRGD
0.1µF
R1 = 4.99k
1µA AT 0.5V
R2 = 120k
V
EE
= –12V
PMBus
INTERFACE
APPLICATIONS INFORMATION
Anti-aliasing Filter Considerations
Noisy environments require an anti-aliasing filter on the
input to the LTC2978A’s ADC. The R-C circuit shown in
Figure 28 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 29 shows the LTC2978A sensing a negative power
supply (V
EE
). The R1/R2 resistor divider translates the
negative supply voltage to the LTC2978As VSENSEM1 input
while the VSENSEP1 input is tied to the REFP pin which
has a typical output voltage of 1.23V. 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 the current flowing
out of the VSENSEMn pin is minimized to ~1µA. The
relationship
between the POWER_GOOD_ON register
value and the corresponding negative supply value can
be expressed as:
V
EE
= V
REFP
V
SENSEP SENSEM
V
– (READ_VOUT)
Where READ_VOUT returns
•
R2
R1
+1






–
–
1µA •
R2