LTC2450 datasheet(12/34 Pages) AD

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LTC2945

Rev. C

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Figure 2b. The SENSE pins can be biased independent of

the part’s supply voltage. Alternatively, if a low voltage sup-

ply is present it can be connected to the INTVCC pin as

shown in Figure 2c to minimize on-chip power dissipation.

When INTVCCis powered from a secondary supply, connect

VDD to INTVCC.

For supply voltages above 80V, the shunt regulator at

INTVCC can be used in both high and low side configura-

tions to provide power to the LTC2945 through an external

shunt resistor, RSHUNT. Figure 3a shows a high side power

monitor with an input monitoring range beyond 80V in a

high side shunt regulator configuration. The device ground

is separated from ground through RSHUNT and clamped at

6.3V below the input supply. Note that due to the different

ground levels, the I2C signals from the part need to be level

shifted for communication with other ground referenced

components. The bus voltage can be measured with the

ADIN pin as shown in Figure 3a. To mitigate the effect of

VBE mismatch in the PNP mirror, select R1 (=R2) to drop 1V

at the operating voltage. For details on the power calcula-

tion, refer to the Power Calculation and Configuration sec-

tion. Figure 3b shows a high side rail-to-rail power monitor

which derives power from a greater than 80V secondary

supply. The voltage at INTVCC is clamped at 6.3V above

ground in a low side shunt regulator configuration to power

the part. In low side power monitors, the device ground

and the current sense inputs are connected to the nega-

tive terminal of the input supply and the ADIN pin can be

used to measure the bus voltage with an external resistive

divider as shown in Figure 3c. The low side shunt regulator

configuration allows operation with input supplies above

80V by clamping the voltage at INTVCC. RSHUNT should be

sized according Equation 1.

VS(MAX) – 5.9V

35mA

≤ RSHUNT ≤

VS(MIN) – 6.7V

1mA

+ ILOAD(MAX)

(1)

where VS(MAX) and VS(MIN) are the operating maximum

and minimum of the supply. ILOAD(MAX) is the maximum

external current load that is connected to the shunt regu-

lator. The shunt resistor must also be rated to safely dis-

sipate the worst-case power. As an example, consider the

–48V Telecom System where the supply operates from

–36V to –72V and the shunt regulator is used to supply

an external load up to 4mA. RSHUNT needs to be between

APPLICATIONS INFORMATION

eight MSBs in the first register and the four LSBs in the

second (see Table 2). The lowest 4 bits in the LSB registers

are set to 0. These data registers are updated immediately

following the corresponding ADC conversion, giving an

effective refresh rate of 7.5Hz in continuous scan mode.

The data converter also features a snapshot mode which

makes a measurement of a single selected voltage (either

ΔSENSE, VDD or VSENSE+, or VADIN). To make a snap-

shot measurement, set CONTROL register bit A7 and

write the two-bit code of the desired ADC channel to A6

and A5 (Table 3) using a Write Byte command. When the

Write Byte command is completed, the ADC converts the

selected voltage and the Busy Bit (A3 in the CONTROL

progress. After completing the conversion, the ADC will

halt and the Busy Bit will reset to indicate that the data is

ready. To make another snapshot measurement, rewrite

the CONTROL register.

Flexible Power Supply to LTC2945

The LTC2945 can be externally configured to flexibly

derive power from a wide range of supplies. The LTC2945

includes an onboard linear regulator to power the low-

voltage internal circuitry connected to the INTVCC pin from

high VDD voltages. The regulator operates with VDD volt-

ages from 4V to 80V, and produces a 5V output capable of

supplying 10mA at the INTVCC pin when VDD is greater than

7V. The regulator is disabled when die temperature rises

above 150°C, and the output is protected against accidental

shorts. Bypass capacitors between 0.1μF and 1μF at both

the VDD and INTVCC pins are recommended for optimal

transient performance. Note that operation with high VDD

voltages can cause significant power dissipation, and care

is required to ensure the operating junction temperature

stays below 125°C. For improved power dissipation, use

the QFN package and solder the exposed pad to a large

copper region for improved thermal resistance.

Figure 2a shows the LTC2945 being used to monitor an

input supply that ranges from 4V to 80V. No secondary

supply is needed since VDD can be connected directly to

the input supply. If the LTC2945 is used to monitor an input

supply of 0V to 80V, it can derive power from a wide range

secondary supply connected to the VDD pin as shown in

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