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LTC4215 Datasheet(PDF) 11 Page - Linear Technology |
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LTC4215 Datasheet(HTML) 11 Page - Linear Technology |
11 / 32 page LTC2945 11 2945f secondary supply connected to the VDD pin as shown in Figure 2b. The SENSE pins can be biased independent of the part’s supply voltage. Alternatively, if a low voltage supply is present it can be connected to the INTVCC pin as shown in Figure 2c to minimize on-chip power dissipa- tion. When INTVCC is 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 exter- nal 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. 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 negative terminal of the input supply 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 to the following equation: VS(MAX) – 5.9V 35mA ≤R SHUNT ≤ VS(MIN) – 6.7V 1mA +I LOAD(MAX ) 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 regula- tor. The shunt resistor must also be rated to safely dissipate 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 1.9k and 5.9k according to the above equation, 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. APPLICATIONS INFORMATION out of the six total ADC data registers (ΔSENSE MSB/LSB, VIN MSB/LSB, and ADIN MSB/LSB), with the 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 snapshot 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 register) will be set to indicate that the conversion is in 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 circuitryconnectedtotheINTVCCpinfromhighVDDvoltages. The regulator operates with VDD voltages 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 outputisprotectedagainstaccidentalshorts.Bypasscapaci- tors 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 powerdissipation,andcareisrequiredtoensuretheoperating junctiontemperaturestaysbelow125°C.Forimprovedpower dissipation,usetheQFNpackageandsoldertheexposedpad 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 (1) |
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