LTC4261 datasheet(13/32 Pages) LINER

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LTC2945

2945f

APPLICATIONS INFORMATION

GND

VNEG

–4V TO –80V

RSNS

VOUT

VDD

INTVCC

SENSE–

GND

2945 F03b

SENSE+

LTC2945

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

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.

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