Datasheet

LTC2977
87
2977fa
For more information www.linear.com/LTC2977
Sensing Negative Voltages
Figure 29 shows the LTC2977 sensing a negative power
supply (V
EE
). The R1/R2 resistor divider translates the
negative supply voltage to the LTC2977s 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
Connecting the DC1613 USB to I
2
C/SMBus/PMBus
Controller to the LTC2977 in System
The DC1613 USB to I
2
C/SMBus/PMBus Controller can be
interfaced to LTC2977s on the user’s board for program-
ming, telemetr
y
and system debug. The controller, when
used in conjunction with LTpowerPlay software, provides
a powerful way to debug an entire power system. Failures
are quickly diagnosed using telemetry, fault status registers
and the fault log. The final configuration can be quickly
developed and stored to the LTC2977’s EEPROM.
Figures 30 and 31 illustrate application schematics for
powering, programming and communicating with one
or more LTC2977’s via the DC1613 I
2
C/SMBus/PMBus
controller regardless of whether or not system power is
present.
Figure 30 shows the recommended schematic to use when
the LTC2977 is powered by the system intermediate bus
through its V
PWR
pin.
Figure 31 shows the recommended schematic to use when
the LTC2977 is powered by the system 3.3V through its
V
DD33
and V
PWR
pins. The LTC4412 ideal OR’ing circuit
allows either the controller or system to power the LTC2977.
Because of the controller’s limited current sourcing capabil
-
ity, only the LTC2977s, their associated pull up resistors
and the I
2
C/SMBus pull-up resistors should be powered
from the ORed 3.3V supply. In addition, any device sharing
I
2
C/SMBus bus connections with the LTC2977 should not
APPLICATIONS INFORMATION
Figure 29. Sensing Negative Voltages
LTC2977
4.5V < V
IBUS
< 15V
GND
ONLY ONE OF EIGHT CHANNELS SHOWN,
SOME DETAILS OMITTED FOR CLARITY
USE POWER_GOOD_ON = 0.5V FOR V
EE
POWER_GOOD = –11.414V
0.1µF
1.23V TYP
2977 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