LTC2945CMS datasheet(14/34 Pages) AD

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LTC2945

Rev. C

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APPLICATIONS INFORMATION

GND

VNEG

–4V TO –80V

RSNS

VOUT

VDD

INTVCC

SENSE–

GND

2945 F04

SENSE+

LTC2945

Figure 4. LTC2945 Derives Power from the Supply

Being Monitored in Low-Side Current Sense Topology

1.9k and 5.9k according to Equation 1, and for reduced

power dissipation, a larger resistance is advantageous.

The worst-case power dissipated in an RSHUNT 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 VDD can be connected to GND of the supply as shown

in Figure 4.

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

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 mea-

sured to obtain load current data. The supply voltage data

for multiplication can be selected between VDD, SENSE+, or

ADIN. SENSE+ is selected by default as it is normally con-

nected to the supply voltage. In negative supply voltage sys-

tems such as shown in Figure 4, the device ground (GND

pin of LTC2945) and SENSE– are connected 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.

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 inter-

nal 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 regis-

ters retain 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.

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