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LTC3832 数据表(PDF) 17 Page - Linear Technology |
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LTC3832 数据表(HTML) 17 Page - Linear Technology |
17 / 32 page 17 LTC1735 1735fc Topside MOSFET Driver Supply (CB, DB) An external bootstrap capacitor CB connected to the BOOST pin supplies the gate drive voltage for the topside MOSFET. Capacitor CBintheFunctionalDiagramischarged though external diode DB from INTVCC when the SW pin is low. Note that the voltage across CB is about a diode drop below INTVCC. When the topside MOSFET is to be turned on, the driver places the CB voltage across the gate-source of the MOSFET. This enhances the MOSFET and turns on the topside switch. The switch node voltage SW rises to VIN and the BOOST pin rises to VIN + INTVCC. The value of the boost capacitor CB needs to be 100 times greater than the total input capacitance of the topside MOSFET. In most applications 0.1 µF to 0.33µF is ad- equate. The reverse breakdown on DB must be greater than VIN(MAX). When adjusting the gate drive level, the final arbiter is the total input current for the regulator. If you make a change and the input current decreases, then you improved the efficiency. If there is no change in input current, then there is no change in efficiency. SENSE +/SENSE – Pins The common mode input range of the current comparator is from 0V to 1.1(INTVCC). Continuous linear operation in step-down applications is guaranteed throughout this range allowing output voltages anywhere from 0.8V to 7V. A differential NPN input stage is used and is biased with internal resistors from an internal 2.4V source as shown in the Functional Diagram. This causes current to either be sourced or sunk by the sense pins depending on the output voltage. If the output voltage is below 2.4V current will flow out of both sense pins to the main output. This forces a minimum load current that can be fulfilled by the APPLICATIO S I FOR ATIO Figure 4. Setting the LTC1735 Output Voltage VOSENSE SGND VOUT R2 1735 F04 LTC1735 R1 47pF VOUT resistive divider. The maximum current flowing out of the sense pins is: ISENSE+ + ISENSE– = (2.4V – VOUT)/24k Since VOSENSE is servoed to the 0.8V reference voltage, we can choose R1 in Figure 4 to have a maximum value to absorb this current: Rk V VV MAX OUT 124 08 24 () . .– = Regulating an output voltage of 1.8V, the maximum value of R1 should be 32k. Note that at output voltages above 2.4V no maximum value of R1 is necessary to absorb the sense pin currents; however, R1 is still bounded by the VOSENSE feedback current. Soft-Start/Run Function The RUN/SS pin is a multipurpose pin that provides a soft- start function and a means to shut down the LTC1735. Soft-start reduces surge currents from VIN by gradually increasing the controller’s current limit ITH(MAX). This pin can also be used for power supply sequencing. Pulling the RUN/SS pin below 1.5V puts the LTC1735 into shutdown. This pin can be driven directly from logic as shown in Figure 5. The VIN quiescent current is a function of RUN/SS voltage (refer to Typical Performance Charac- teristics graphs on page 6). Releasing the RUN/SS pin allows an internal 1.2 µA current source to charge up the external soft-start capacitor CSS. If RUN/SS has been pulled all the way to ground there is a delay before starting of approximately: t V A Cs F C DELAY SS SS = µ =µ () 15 12 125 . . ./ When the voltage on RUN/SS reaches 1.5V the LTC1735 begins operating with a current limit at approximately 25mV/RSENSE. As the voltage on the RUN/SS pin increases from 1.5V to 3.0V, the internal current limit is increased from 25mV/RSENSE to 75mV/RSENSE. The output current limit ramps up slowly, taking an additional 1.25s/ µF to reach full current. The output current thus ramps up slowly, reducing the starting surge current required from the input power supply. |
类似零件编号 - LTC3832 |
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类似说明 - LTC3832 |
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