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LTM8025 Datasheet(PDF) 13 Page - Linear Technology |
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LTM8025 Datasheet(HTML) 13 Page - Linear Technology |
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13 / 22 page  LTM8025 13 8025fc For more information www.linear.com/LTM8025 applicaTions inForMaTion Figure 1. To Soft-Start the LTM8025, Add a Resistor and Capacitor to the RUN/SS Pin whether the RUN/SS is used. As shown in the Typical Performance Characteristics section, the minimum input voltage to run a 3.3V output at light load is only about 3.6V, but, if the RUN/SS is pulled up to VIN, it takes 5.5VIN to start. If the LTM8025 is enabled with the RUN/SS pin, the minimum voltage to start at light loads is lower, about 4.3V. Similar curves detailing this behavior of the LTM8025 for other outputs are also included in the Typical Performance Characteristics section. Soft-Start The RUN/SS pin can be used to soft-start the LTM8025, reducing the maximum input current during start-up. The RUN/SS pin is driven through an external RC filter to create a voltage ramp at this pin, as shown in Figure 1. By choosing an appropriate RC time constant, the peak start-up current can be reduced to the current that is re- quired 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. Frequency Foldback 8025 F01 RUN/SS RUN GND 15k 0.22µF The LTM8025 is equipped with frequency foldback which actstoreducethethermalandenergystressontheinternal power elements during a short circuit or output overload condition.IftheLTM8025detectsthattheoutputhasfallen out of regulation, the switching frequency is reduced as a function of how far the output is below the target voltage. Thisinturnlimitstheamountofenergythatcanbedelivered to the load under fault. During the start-up time, frequency foldback is also active to limit the energy delivered to the potentially large output capacitance of the load. Synchronization TheinternaloscillatoroftheLTM8025canbesynchronized by applying an external 250kHz to 2MHz clock to the SYNC pin. Do not leave this pin floating. When synchronizing the LTM8025, select an RT resistor value that corresponds to an operating frequency 20% lower than the intended synchronization frequency (see the Frequency Selection section). Inadditiontosynchronization,theSYNCpincontrolsBurst Mode behavior. If the SYNC pin is driven by an external clock, or pulled up above 0.7V, the LTM8025 will not en- ter Burst Mode operation, but will instead skip pulses to maintain regulation instead. Shorted Input Protection Care needs to be taken in systems where the output will be held high when the input to the LTM8025 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode ORed with the LTM8025’s output. If the VIN pin is allowed to float and the SHDN pin is held high (either by a logicsignalorbecauseitistiedtoVIN),thentheLTM8025’s internal circuitry will pull its quiescent current through its internal power switch. This is fine if your system can tolerate a few milliamps in this state. If you ground the RUN/SS pin, the input current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LTM8025 can pull large currents from the output through the VIN pin. Figure 2 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. PCB Layout Most of the headaches associated with PCB layout have been alleviated or even eliminated by the high level of integration of the LTM8025. The LTM8025 is neverthe- less a switching power supply, and care must be taken to minimize EMI and ensure proper operation. Even with the high level of integration, you may fail to achieve specified operation with a haphazard or poor layout. See Figure 3 for a suggested layout. Ensure that the grounding and heatsinking are acceptable. |
Similar Part No. - LTM8025_15 |
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Similar Description - LTM8025_15 |
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