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LTM8031 Datasheet(PDF) 11 Page - Linear Technology |
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LTM8031 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 22 page  LTM8031 11 8031fb applicaTions inForMaTion BIAS Pin Considerations The BIAS pin is used to provide drive power for the internal power switching stage and operate internal circuitry. For proper operation, it must be powered by at least 2.8V. If the output voltage is programmed to be 2.8V or higher, simply tie BIAS to AUX. If VOUT is less than 2.8V, BIAS can be tied to VIN or some other voltage source. In all cases, ensure that the maximum voltage at the BIAS pin is both less than 25V and the sum of VIN and BIAS is less than 56V. If BIAS power is applied from a remote or noisy voltage source, it may be necessary to apply a decoupling capacitor locally to the LTM8031. Load Sharing TwoormoreLTM8031smaybeparalleledtoproducehigher currents. This may, however, alter the EMI performance of theLTM8031s.Todothis,tietheVIN,ADJ,VOUTandSHARE pins of all the paralleled LTM8031s together. To ensure that paralleledmodulesstartuptogether,theRUN/SSpinsmay be tied together, as well. Synchronize the LTM8031s to an external clock to eliminate beat frequencies, if required. If the RUN/SS pins are not tied together, make sure that the same valued soft-start capacitors are used for each module. An example of two LTM8031 modules configured for load sharing is given in the Typical Applications sec- tion. For 2A applications also see the LTM8032, 2A EMC DC/DC µModule regulator Burst Mode Operation To enhance efficiency at light loads, the LTM8031 auto- matically switches to Burst Mode operation which keeps the output capacitor charged to the proper voltage while minimizingtheinputquiescentcurrent.DuringBurstMode operation, the LTM8031 delivers single cycle bursts of current to the output capacitor followed by sleep periods wheretheoutputpowerisdeliveredtotheloadbytheoutput capacitor. In addition, VIN and BIAS quiescent currents are reduced to typically 25µA and 60µA respectively during the sleep time. As the load current decreases towards a no-loadcondition,thepercentageoftimethattheLTM8031 operates in sleep mode increases and the average input current is greatly reduced, resulting in higher efficiency. Burst Mode operation is enabled by tying SYNC to GND. To disable Burst Mode operation, tie SYNC to a stable voltage above 0.7V. Do not leave the SYNC pin floating. Minimum Input Voltage The LTM8031 is a step-down converter, so a minimum amountofheadroomisrequiredtokeeptheoutputinregu- lation. In addition, the input voltage required to turn on is higherthanthatrequiredtorun,anddependsuponwhether the RUN/SS is used. As shown in the Typical Performance Characteristics section, it takes only about 3.6VIN for the LTM8031 to run a 3.3V output at light load. If RUN/SS is pulled up to VIN, it takes 5.7VIN to start. If the LTM8031 is enabled via the RUN/SS pin, the minimum voltage to start at light loads is lower, about 4.4V. Similar curves for 2.5VOUT, 5VOUT and 8VOUT operation are also provided in the Typical Performance Characteristics section. Soft-Start The RUN/SS pin can be used to soft-start the LTM8031, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC network to create a voltage ramp at this pin. Figure 1 shows the start- up and shutdown waveforms with the soft-start circuit. By choosinganappropriateRCtimeconstant,thepeakstart-up current can be reduced to the current that is required to regulate the output, with no overshoot. Choose the value of the resistor so that it can supply at least 20µA when the RUN/SS pin reaches 2.5V. |
Similar Part No. - LTM8031_12 |
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Similar Description - LTM8031_12 |
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