LTC2978
APPLICATIONS INFORMATION
Anti-aliasing Filter Considerations
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:
Noisy environments require an anti-aliasing filter on the
input to the LTC2978’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 doesn’t add too much
additional response time to the OV/UV supervisor, e.g. τ
≅ 10µs (R = 100Ω, C = 0.10µF).
Sensing Negative Voltages
R2
R1
+1 – 1µA•R2
VEE = VREFP –(READ_VOUT) •
Figure 29 shows the LTC2978 sensing a negative power
supply (V ). The R1/R2 resistor divider translates the
EE
negativesupplyvoltagetotheLTC2978sVSENSEM1input
Where READ_VOUT returns VSENSEP – VSENSEM
V
IN
V
V
IN
4.5V < V
< 15V
IBUS
V
V
IN_SNS
PWR
OUT
0.1µF
0.1µF
0.1µF
V
DACP0
R30
DC/DC
CONVERTER
V
V
V
V
SENSEP0
DD33
DD33
DD25
R40
R50
C10
C20
R20
R10
LTC2978*
GND
V
LOAD
FB
V
SENSEM0
V
SGND
DACM0
V
RUN/SS
OUT_EN0
GND
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF EIGHT CHANNELS SHOWN
2978 F28
Figure 28. Antialiasing Filter on VSENSE Lines
4.5V < V
< 15V
IBUS
V
V
IN_SNS
PWR
LTC2978
1.23V TYP
0.1µF
REFP
REFM
SDA
V
SENSEP1
SCL
PMBus
INTERFACE
1µA AT 0.5V
0.1µF
R1 = 4.99k
R2 = 120k
ALERTB
CONTROL
V
SENSEM1
WDI/RESETB
FAULTB
V
= –12V
EE
SHARE_CLK
ASEL0
POWER_GOOD_ON = 0.5V FOR V POWER_GOOD = –11.414V
EE
PWRGD
WHERE V POWER_GOOD =
EE
ASEL1
ONLY ONE OF EIGHT CHANNELS SHOWN,
SOME DETAILS OMITTED FOR CLARITY
WDI/RESETB
WP GND
2978 F29
Figure 29. Sensing Negative Voltages
2978fc
73
Linear Systems [ Linear Systems ]
