LTC2974 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2974' PDF : 32 Pages View PDF
LTC2945
APPLICATIONS INFORMATION
GND
VDD
INTVCC
C2
LTC2945
GND
VNEG
–4V TO –80V
SENSE– SENSE+
2945 F03b
RSNS
VOUT
Figure 3d. LTC2945 Derives Power from the Supply
Being Monitored in Low-Side Current Sense Topology
If the supply input is nominally below 80V and transient
is limited to below 100V, the shunt resistor is not required
and VDD can be connected to GND of the supply as shown
in Figure 3d.
Supply Undervoltage Lockout
During power-up, the internal I2C logic and the ADC are
enabled when either VDD or INTVCC rises above its under-
voltage lockout threshold. During power-down, the ADC is
disabled when VDD and INTVCC fall below their respective
undervoltage lockout thresholds. The internal I2C logic is
reset when VDD and INTVCC fall below their respective I2C
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
I2C bus remains active, and although the ADR1 and ADR0
pins are disabled, the device will retain the most recently
programmed I2C bus address. All on-board registers re-
tain their contents and can be accessed through the I2C
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 IQ mode is not required and the
regulator is used to power I2C 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 I2C 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 volt-
age data for multiplication can be selected between VDD,
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 con-
nected to the supply and VDD 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 VDD
• 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.
2945f
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