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LTM8065 Folha de dados(PDF) 16 Page - Linear Technology |
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LTM8065 Folha de dados(HTML) 16 Page - Linear Technology |
16 / 26 page LTM8065 16 8065fa For more information www.linear.com/LTM8065 APPLICATIONS INFORMATION mum voltage at the BIAS pin is less than 19V. If BIAS power is applied from a remote or noisy voltage source, it may be necessary to apply a decoupling capacitor locally to the pin. A 1µF ceramic capacitor works well. The BIAS pin may also be left open at the cost of a small degradation in efficiency. Maximum Load ThemaximumpracticalcontinuousloadthattheLTM8065 can drive, while rated at 2.5A, actually depends upon both the internal current limit and the internal temperature. The internal current limit is designed to prevent damage to the LTM8065 in the case of overload or short-circuit. The internal temperature of the LTM8065 depends upon operating conditions such as the ambient temperature, the power delivered, and the heat sinking capability of the system. For example, if the LTM8065 is configured to regulate at 1.2V, it may continuously deliver 3.5A from 12VIN if the ambient temperature is controlled to less than 55°C. This is quite a bit higher than the 2.5A continuous rating. Please see the “Derating, VOUT = 1.2V” curve in theTypicalPerformanceCharacteristicssection.Similarly, if the output voltage is 18V and the ambient temperature is 100°C, the LTM8065 will deliver at most 0.15A from 36VIN, which is less than the 2.5A continuous rating. Load Sharing The LTM8065 is not designed to load share. Burst Mode Operation To enhance efficiency at light loads, the LTM8065 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 LTM8065 delivers single cycle bursts ofcurrenttotheoutputcapacitorfollowedbysleepperiods where most of the internal circuitry is powered off and energy is delivered to the load by the output capacitor. During the sleep time, VIN and BIAS quiescent currents are greatly reduced, so, as the load current decreases towards a no load condition, the percentage of time that the LTM8065 operates in sleep mode increases and the average input current is greatly reduced, resulting in higher light load efficiency. Burst Mode operation is enabled by tying SYNC to GND. Minimum Input Voltage The LTM8065 is a step-down converter, so a minimum amount of headroom is required to keep the output in regulation. Keep the input above 3.4V to ensure proper operation. Voltage transients or ripple valleys that cause the input to fall below 3.4V may turn off the LTM8065. Output Voltage Tracking and Soft-Start The LTM8065 allows the user to adjust its output voltage ramp rate by means of the TR/SS pin. An internal 2μA pulls up the TR/SS pin to about 2.4V. Putting an external capaci- tor on TR/SS enables soft starting the output to reduce current surges on the input supply. During the soft-start ramp the output voltage will proportionally track the TR/ SS pin voltage. For output tracking applications, TR/SS can be externally driven by another voltage source. From 0V to 0.97V, the TR/SS voltage will override the internal 0.97V reference input to the error amplifier, thus regulat- ing the FB pin voltage to that of the TR/SS pin. When TR/ SS is above 0.97V, tracking is disabled and the feedback voltage will regulate to the internal reference voltage. The TR/SS pin may be left floating if the function is not needed. An active pull-down circuit is connected to the TR/SS pin which will discharge the external soft-start capacitor in the case of fault conditions and restart the ramp when the faults are cleared. Fault conditions that clear the soft-start capacitor are the RUN pin transitioning low, VIN voltage falling too low, or thermal shutdown. Pre-Biased Output As discussed in the Output Voltage Tracking and Soft- Start section, the LTM8065 regulates the output to the FB voltage determined by the TR/SS pin whenever TR/ SS is less than 0.97V. If the LTM8065 output is higher than the target output voltage, the LTM8065 will attempt to regulate the output to the target voltage by returning a small amount of energy back to the input supply. If there is nothing loading the input supply, its voltage may rise. Take care that it does not rise so high that the input voltage exceeds the absolute maximum rating of the LTM8065. |
Nº de peça semelhante - LTM8065 |
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Descrição semelhante - LTM8065 |
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