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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 / 20 page  LTM8031 11 8031fa APPLICATIONS INFORMATION 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 Two or more LTM8031s may be paralleled to produce higher currents. This may, however, alter the EMI performance of the LTM8031s. To do this, tie the VIN, ADJ, VOUT and SHARE pins of all the paralleled LTM8031s together. To ensure that paralleled modules start up together, the RUN/SS pins may 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 minimizing the input quiescent current. During Burst Mode operation, the LTM8031 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capacitor. In addition, VIN and BIAS quiescent currents are reduced to typically 20μA and 50μA respectively during the sleep time. As the load current decreases towards a no-load condition, the percentage of time that the LTM8031 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 amount of headroom is required to keep the output in regu- lation. In addition, the input voltage required to turn on is higher than that required to run, and depends upon whether 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 choosing an appropriate RC time constant, the peak start-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. 8031 F01 IL 0.5A/DIV VRUN/SS 2V/DIV VOUT 2V/DIV RUN/SS GND 0.22μF RUN 15k 2ms/DIV Figure 1. To Soft-Start the LTM8031, Add a Resistor and Capacitor to the RUN/SS Pin Synchronization The internal oscillator of the LTM8031 can be synchro- nized by applying an external 250kHz to 2MHz clock to the SYNC pin. Do not leave this pin floating. The resistor tied from the RT pin to ground should be chosen such that the LTM8031 oscillates 20% lower than the intended synchronization frequency (see the Frequency Selection section). The LTM8031 will not enter Burst Mode operation while synchronized to an external clock, but will instead skip pulses to maintain regulation. |
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Similar Description - LTM8031 |
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