Datasheet

LTC2945
14
2945fa
For more information www.linear.com/LTC2945
applicaTions inForMaTion
GND
C2
V
NEG
–4V TO –80V
R
SNS
V
OUT
V
DD
INTV
CC
SENSE
–
GND
2945 F03b
SENSE
+
LTC2945
Figure 3d. LTC2945 Derives Power from the Supply
Being Monitored in Low-Side Current Sense Topology
power dissipation, a larger resistance is advantageous.
The worst-case power dissipated in an R
SHUNT
of 5.4k is
calculated to be 0.8W. So, three 0.5W rated 1.8k resistors
in series would suffice for this example.
If the supply input is nominally below 80V and transient
is limited to below 100V, the shunt resistor is not required
and V
DD
can be connected to GND of the supply as shown
in Figure 3d.
Supply Undervoltage Lockout
During power-up, the internal I
2
C logic and the ADC are
enabled when either V
DD
or INTV
CC
rises above its under-
voltage lockout threshold. During power-down, the ADC is
disabled when V
DD
and INTV
CC
fall below their respective
undervoltage lockout thresholds. The internal I
2
C logic is
reset when V
DD
and INTV
CC
fall below their respective I
2
C
reset thresholds.
Shutdown Mode
The LTC2945 includes a low quiescent current shutdown
mode, controlled by bit A1 in the CONTROL register
(Table 3). Setting A1 puts the part in shutdown mode,
powering down the ADC and internal reference. The internal
I
2
C bus remains active, and although the ADR1 and ADR0
pins are disabled, the device will retain the most recently
programmed I
2
C bus address. All on-board registers re-
tain their contents and can be accessed through the I
2
C
interface. To re-enable ADC conversions, reset bit A1 in
the CONTROL register. The analog circuitry will power up
and all registers will retain their contents.
The onboard linear regulator is disabled in shutdown mode
to conserve power. If low I
Q
mode is not required and the
regulator is used to power I
2
C bus-related circuitry such as
opto-couplers or pull-ups, ensure bit A1 in the CONTROL
register is masked off during software development. In such
applications, the user is advised that accidentally disabling
the regulator would prevent I
2
C communication from the
master and cause the LTC2945 to disengage from the system.
The LTC2945 would then have to be reset by cycling its power
to come out of shutdown. It is recommended that external
regulators be used in such applications if powering down
the LTC2945 is desirable. Quiescent current drops below
80µA in shutdown mode with the internal regulator disabled.
Power Calculation and Configuration
The LTC2945 calculates power by multiplying the measured
current with the measured voltage. In continuous mode, the
differential voltage between SENSE
+
and SENSE
–
is measured
to obtain load current data. The supply voltage data for mul-
tiplication can be selected between V
DD
, SENSE
+
, or ADIN.
SENSE
+
is selected by default as it is normally connected
to the supply voltage. In negative supply voltage systems
such as shown in Figure 3d, the device ground (GND pin
of LTC2945) and SENSE
–
are connected to the supply and
V
DD
measures the supply voltage at GND with respect to the
device ground. For negative supply voltages of more than
80V, use external resistors to divide down the voltage to suit
the ADIN measurement range. In the CONTROL register,
• writebitsA2=1,A0=1toselectSENSE
+
(Default)
• writebitsA2=0,A0=1toselectV
DD
• writebitsA2=1,A0=0toselectADIN
More details on the CONTROL register can be found in
Table 3.
Once the ADC conversions are complete, a 24-bit power
value is generated by digitally multiplying the 12-bit load
current data with the 12-bit supply voltage data. 1LSB of
power is 1LSB of voltage multiplied by 1LSB of ΔSENSE
(current). The result is held in the three adjacent POWER
registers (Table 2). The POWER registers initialize with
undefined data and subsequently refresh at a frequency
of 7.5Hz in continuous scan mode. In snapshot mode, the
POWER registers are not refreshed.