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LTM8050 Datenblatt(PDF) 15 Page - Linear Technology |
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LTM8050 Datenblatt(HTML) 15 Page - Linear Technology |
15 / 24 page LTM8050 15 8050fc For more information www.linear.com/LTM8050 APPLICATIONS INFORMATION Figure 1. Apply an RC Network to RUN/SS to Control the Soft-Start Behavior of the LTM8050 at Power-Up Minimum Input Voltage The LTM8050 is a step-down converter, so a minimum amount of headroom is required to keep the output in regulation. 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, the minimum input voltage to run a 3.3V output at light load is only about 3.6V, but, if RUN/SS is pulled up to VIN, it takes 5.5VIN to start. If the LTM8050 is enabled with the RUN/SS pin after VIN is applied, the minimum voltage to start at light loads is lower, about 4.3V. Similar curves detailing this behavior of the LTM8050 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 LTM8050, 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. (See Figure 1). By choos- ing 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. Output voltage soft-start waveforms for various values of RSS and CSS are given in the Typical Performance Characteristics section. RUN/SS RUN RUN 100k CSS 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 TheinternaloscillatoroftheLTM8050canbesynchronized by applying an external 250kHz to 2MHz clock to the SYNC pin. Do not leave this pin floating. When synchronizing the LTM8050, select an RTresistorvaluethatcorresponds 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 LTM8050 will not enter 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 LTM8050 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 LTM8050’s output. If the VIN pin is allowed to float and the SHDN pin is held high (either by a logic signal or because it is tied to VIN), then the LTM8050’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 LTM8050 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 LTM8050. The LTM8050 is neverthe- less a switching power supply, and care must be taken to Frequency Foldback The LTM8050 is equipped with frequency foldback which actstoreducethethermalandenergystressontheinternal power elements during a short circuit or output overload condition.IftheLTM8050detectsthattheoutputhasfallen out of regulation, the switching frequency is reduced as a function of how far the output is below the target voltage. |
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Ähnliche Beschreibung - LTM8050 |
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