Datasheet

LTC2974
90
2974fc
For more information www.linear.com/LTC2974
applicaTions inForMaTion
the current flowing out of the V
SENSEMn
pin is minimized
to ~1µA. The relationship between the POWER_GOOD_ON
register value and the corresponding negative supply value
can be determined using equation 14.
Connecting the DC1613 USB to I
2
C/SMBus/PMBus
Controller to the LTC2974 in System
The DC1613 USB to I
2
C/SMBus/PMBus Controller can
be interfaced to the LTC2974s on the user’s board for
programming, telemetry and system debug. The control
-
ler, 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 LTC2974’s EEPROM.
Figure 38 and Figure 39 illustrate application schematics
for powering, programming and communicating with one
or more LTC2974’s via the DC1613 I
2
C/SMBus/PMBus
controller regardless of whether or not system power is
present.
Figure 38 shows the recommended schematic to use when
the LTC2974 is powered by the system intermediate bus
through its V
PWR
pin.
Figure 39 shows the recommended schematic to use when
the LTC2974 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 LTC2974.
Because of the controller’s limited current sourcing capabil
-
ity, only the LTC2974s, 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 LTC2974 should not
have body diodes between the SDA/SCL pins and its V
DD
node because this will interfere with bus communication
in the absence of system power.
The DC1613 controller’s I
2
C/SMBus connections are opto-
isolated from the PC’s USB port. The 3.3V supply from the
controller and the LTC2974’s V
DD33
pin can be paralleled
because the LTC LDOs that generate these voltages can be
backdriven and draw <10μA. The controller’s 3.3V current
limit is 100mA.
Figure 37. Sensing Negative Voltages
LTC2974
4.5V < V
IBUS
< 15V
GND
ONLY ONE OF FOUR CHANNELS SHOWN,
SOME DETAILS OMITTED FOR CLARITY
POWER_GOOD_ON = 0.5V FOR V
EE
POWER_GOOD = –11.414V
WHERE V
EE
POWER_GOOD =
V
REFP
– POWER_GOOD_ON (R2/R1 + 1) – 1µA • R2
0.1µF
1.23V TYP
2974 F37
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