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LTC3780EG-PBF Datasheet(PDF) 15 Page - Linear Technology |
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LTC3780EG-PBF Datasheet(HTML) 15 Page - Linear Technology |
15 / 30 page LTC3780 15 3780ff For more information www.linear.com/LTC3780 operaTion on. switch A and synchronous switch B are alternately turned on to maintain the output voltage independent of direction of inductor current. Every ten cycles, synchro- nous switch D is forced off for about 300ns to allow CB to recharge. This is the least efficient operating mode at light load, but may be desirable in certain applications. In this mode, the output can source or sink current. WhentheFCBpinvoltageisbelowVINTVCC–1V,butgreater than 0.8V, the controller enters Burst Mode operation in boost operation or enters skip-cycle mode in buck opera- tion. During boost operation, Burst Mode operation sets a minimum output current level before inhibiting the switch C and turns off synchronous switch D when the inductor current goes negative. This combination of requirements will, at low currents, force the ITH pin below a voltage threshold that will temporarily inhibit turn-on of power switches C and D until the output voltage drops. There is 100mV of hysteresis in the burst comparator tied to the ITH pin. This hysteresis produces output signals to the MOSFETs C and D that turn them on for several cycles, followed by a variable “sleep” interval depending upon the loadcurrent.Themaximumoutputvoltagerippleislimited to 3% of the nominal DC output voltage as determined by a resistive feedback divider. During buck operation at no load, switch A is turned on for its minimum on-time. This will not occur every clock cycle when the output load current drops below 1% of the maximum designed load. The body diode of synchronous switch B or the Schottky diode, which is in parallel with switch B, is used to dis- charge the inductor current; switch B only turns on every ten clock cycles to allow CB to recharge. As load current is applied, switch A turns on every cycle, and its on-time begins to increase. At higher current, switch B turns on briefly after each turn-off of switch A. switches C and D remain off at light load, except to refresh CA (Figure 11) every 10 clock cycles. In Burst Mode operation/skip-cycle mode, the output is prevented from sinking current. When the FCB pin voltage is tied to the INTVCC pin, the controllerentersconstantfrequencydiscontinuouscurrent mode (DCM). For boost operation, synchronous switch D is held off whenever the ITH pin is below a threshold volt- age. In every cycle, switch C is used to charge inductor current. After the output voltage is high enough, the controller will enter continuous current buck mode for one cycle to discharge inductor current. In the following cycle, the controller will resume DCM boost operation. For buckoperation,constantfrequencydiscontinuouscurrent mode sets a minimum negative inductor current level. synchronous switch B is turned off whenever inductor current is lower than this level. At very light loads, this constant frequency operation is not as efficient as Burst Mode operation or skip-cycle, but does provide lower noise, constant frequency operation. FREQUENCY SYNCHRONIzATION AND FREQUENCY SETUP The phase-locked loop allows the internal oscillator to be synchronized to an external source via the PLLIN pin. The phase detector output at the PLLFLTR pin is also the DC frequency control input of the oscillator. The frequency ranges from 200kHz to 400kHz, corresponding to a DC voltage input from 0V to 2.4V at PLLFLTR. When locked, the PLL aligns the turn on of the top MOSFET to the ris- ing edge of the synchronizing signal. When PLLIN is left open, the PLLFLTR pin goes low, forcing the oscillator to its minimum frequency. INTVCC/EXTVCC Power Power for all power MOSFET drivers and most inter- nal circuitry is derived from the INTVCC pin. When the EXTVCC pin is left open, an internal 6V low dropout linear regulator supplies INTVCC power. If EXTVCC is taken above 5.7V, the 6V regulator is turned off and an internal switch is turned on, connecting EXTVCC to INTVCC. This allows the INTVCC power to be derived from a high efficiency external source. POWER GOOD (PGOOD) PIN ThePGOODpinisconnectedtoanopendrainofaninternal MOSFET. The MOSFET turns on and pulls the pin low when the output is not within ±7.5% of the nominal output level as determined by the resistive feedback divider. When the output meets the ±7.5% requirement, the MOSFET is turned off and the pin is allowed to be pulled up by an external resistor to a source of up to 7V. |
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