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LTM8033 数据表(PDF) 17 Page - Linear Technology |
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LTM8033 数据表(HTML) 17 Page - Linear Technology |
17 / 26 page LTM8065 17 8065fa For more information www.linear.com/LTM8065 APPLICATIONS INFORMATION Frequency Foldback The LTM8065 is equipped with frequency foldback which actstoreducethethermalandenergystressontheinternal power elements during a short circuit or output overload condition. If the LTM8065 detects that the output has fallenoutofregulation,theswitchingfrequencyisreduced as a function of how far the output is below the target voltage. This in turn limits the amount of energy that can be delivered 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. When a clock is applied to the SYNC pin, the SYNC pin is floated or held high, the frequency foldback is disabled, and the switching frequency will slow down only during overcurrent conditions. Synchronization To select low ripple Burst Mode operation, tie the SYNC pin below about 0.4V (this can be ground or a logic low output). To synchronize the LTM8065 oscillator to an external frequency, connect a square wave (with about 20% to 80% duty cycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.4V and peaks above 1.5V. The LTM8065 will not enter Burst Mode operation at low output loads while synchronized to an external clock, but insteadwillpulseskiptomaintainregulation.TheLTM8065 maybesynchronizedovera200kHzto3MHzrange.TheRT resistor should be chosen to set the switching frequency equal to or below the lowest synchronization input. For example, if the synchronization signal will be 500kHz and higher, the RT should be selected for 500kHz. For some applications it is desirable for the LTM8065 to operate in pulse-skipping mode, offering two major dif- ferences from Burst Mode operation. The first is that the clock stays awake at all times and all switching cycles are aligned to the clock. The second is that full switching frequency is reached at lower output load than in Burst Modeoperation.Thesetwodifferencescomeattheexpense of increased quiescent current. To enable pulse-skipping mode, the SYNC pin is floated. The LTM8065 features spread spectrum operation to further reduce EMI/EMC emissions. To enable spread spectrum operation, apply between 2.9V and 4.2V to the SYNC pin. In this mode, triangular frequency modulation is used to vary the switching frequency between the value programmed by RT to about 20% higher than that value. Themodulationfrequencyisabout3kHz.Forexample,when the LTM8065 is programmed to 2MHz, the frequency will vary from 2MHz to 2.4MHz at a 3kHz rate. When spread spectrum operation is selected, Burst Mode operation is disabled, and the part will run in pulse-skipping mode. TheLTM8065doesnotoperateinforcedcontinuousmode regardless of SYNC signal. Negative Output The LTM8065 is capable of generating a negative output voltage by connecting its VOUT to system GND and the LTM8065 GND to the negative voltage rail. An example of this is shown in the Typical Applications section. The most versatile way to generate a negative output is to use a dedicated regulator that was designed to generate a negative voltage, but using a buck regulator like the LTM8065 to generate a negative voltage is a simple and cost effective solution, as long as certain restrictions are kept in mind. Figure 1showsatypicalnegativeoutputvoltageapplication. Note that LTM8065 VOUT is tied to system GND and input power is applied from VIN to LTM8065 VOUT. As a result, the LTM8065 is not behaving as a true buck regulator, and the maximum output current depends upon the input voltage. In the example shown in the Typical Applications section, there is an attending graph that shows how much current the LTM8065 can deliver for given input voltages. Note that this configuration requires that any load current transient will directly impress the transient voltage onto NEGATIVE OUTPUT VOLTAGE LTM8065 VOUT VIN 8065 F01 GND VIN Figure 1. The LTM8065 Can Be Used to Generate a Negative Voltage |
类似零件编号 - LTM8033 |
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类似说明 - LTM8033 |
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