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LTC3704 数据表(PDF) 9 Page - Linear Technology |
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LTC3704 数据表(HTML) 9 Page - Linear Technology |
9 / 32 page LTC1871X 9 1871xf For more information www.linear.com/LTC1871X OPERATION The nominal operating frequency of the LTC1871X is pro- grammed using a resistor from the FREQ pin to ground and can be controlled over a 65kHz to 900kHz range. In addition, the internal oscillator can be synchronized to an external clock applied to the MODE/SYNC pin and can be locked to a frequency between 100% and 130% of its nominal value. When the MODE/SYNC pin is left open, it is pulled low by an internal 50k resistor and Burst Mode operation is enabled. If this pin is taken above 2V or an external clock is applied, Burst Mode operation is disabled and the IC operates in continuous mode. With no load (or an extremely light load), the controller will skip pulses in order to maintain regulation and prevent excessive output ripple. TheRUNpincontrolswhethertheICisenabledorisinalow current shutdown state. A micropower 1.248V reference and comparator C2 allow the user to program the supply voltage at which the IC turns on and off (comparator C2 has 100mV of hysteresis for noise immunity). With the RUN pin below 1.248V, the chip is off and the input supply current is typically only 10µA. An overvoltage comparator OV senses when the FB pin exceeds the reference voltage by 6.5% and provides a reset pulse to the main RS latch. Because this RS latch is reset-dominant, the power MOSFET is actively held off for the duration of an output overvoltage condition. The LTC1871X can be used either by sensing the voltage drop across the power MOSFET or by connecting the SENSE pin to a conventional shunt resistor in the source of the power MOSFET, as shown in Figure 2. Sensing the voltage across the power MOSFET maximizes converter efficiency and minimizes the component count, but limits theoutputvoltagetothemaximumratingforthispin(36V). By connecting the SENSE pin to a resistor in the source of the power MOSFET, the user is able to program output voltages significantly greater than 36V. Programming the Operating Mode For applications where maximizing the efficiency at very light loads (e.g., <100µA) is a high priority, the current in the output divider could be decreased to a few micro- amps and Burst Mode operation should be applied (i.e., the MODE/SYNC pin should be connected to ground). In applications where fixed frequency operation is more critical than low current efficiency, or where the lowest outputrippleisdesired,pulse-skipmodeoperationshould be used and the MODE/SYNC pin should be connected to the INTVCC pin. This allows discontinuous conduction mode (DCM) operation down to near the limit defined by the chip’s minimum on-time (about 175ns). Below this output current level, the converter will begin to skip cycles in order to maintain output regulation. Figures 3 and 4 show the light load switching waveforms for Burst Mode and pulse-skip mode operation for the converter in Figure 1. Burst Mode Operation Burst Mode operation is selected by leaving the MODE/ SYNC pin unconnected or by connecting it to ground. In normaloperation,therangeontheITHpincorrespondingto no load to full load is 0.30V to 1.2V. In Burst Mode opera- tion, if the error amplifier EA drives the ITH voltage below 0.525V, the buffered ITH input to the current comparator C1 will be clamped at 0.525V (which corresponds to 25% of maximum load current). The inductor current peak is then held at approximately 30mV divided by the power Figure 2. Using the SENSE Pin On the LTC1871 COUT VSW VSW 2a. SENSE Pin Connection for Maximum Efficiency (VSW < 36V) VOUT VIN GND L D + COUT RS 1871x F02 2b. SENSE Pin Connection for Precise Control of Peak Current or for VSW > 36V VOUT VIN GND L D + GATE GND VIN SENSE GATE GND VIN SENSE |
类似零件编号 - LTC3704 |
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类似说明 - LTC3704 |
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