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LTC3780EUHPBF Datasheet(PDF) 21 Page - Linear Technology |
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LTC3780EUHPBF Datasheet(HTML) 21 Page - Linear Technology |
21 / 30 page LTC3780 21 3780ff For more information www.linear.com/LTC3780 applicaTions inForMaTion EXTVCC Connection The LTC3780 contains an internal P-channel MOSFET switch connected between the EXTVCC and INTVCC pins. When the voltage applied to EXTVCC rises above 5.7V, the internal regulator is turned off and a switch connects the EXTVCC pin to the INTVCC pin thereby supplying internal power. The switch remains closed as long as the voltage applied to EXTVCC remains above 5.5V. This allows the MOSFET driver and control power to be derived from the output when (5.7V < VOUT < 7V) and from the internal regulator when the output is out of regulation (start-up, short-circuit). If more current is required through the EXTVCC switch than is specified, an external Schottky diode can be interposed between the EXTVCC and INTVCC pins. Ensure that EXTVCC ≤ VIN. The following list summarizes the three possible connec- tions for EXTVCC: 1. EXTVCC left open (or grounded). This will cause INTVCC to be powered from the internal 6V regulator at the cost of a small efficiency penalty. 2. EXTVCC connected directly to VOUT (5.7V < VOUT < 7V). This is the normal connection for a 6V regulator and provides the highest efficiency. 3. EXTVCC connected to an external supply. If an external supply is available in the 5.5V to 7V range, it may be used to power EXTVCC provided it is compatible with the MOSFET gate drive requirements. Output Voltage The LTC3780 output voltage is set by an external feedback resistivedividercarefullyplacedacrosstheoutputcapacitor. Theresultantfeedbacksignaliscomparedwiththeinternal precision 0.800V voltage reference by the error amplifier. The output voltage is given by the equation: VV R R OUT =+ 08 1 2 1 .• Topside MOSFET Driver Supply (CA, DA, CB, DB) Referring to Figure 11, the external bootstrap capacitors CA and CB connected to the BOOST1 and BOOST2 pins supply the gate drive voltage for the topside MOSFET switches A and D. When the top MOSFET switch A turns on, the switch node SW2 rises to VIN and the BOOST2 pin rises to approximately VIN + INTVCC. When the bottom MOSFET switch B turns on, the switch node SW2 drops to low and the boost capacitor CB is charged through DB from INTVCC. When the top MOSFET switch D turns on, the switch node SW1 rises to VOUT and the BOOST1 pin rises to approximately VOUT + INTVCC. When the bottom MOSFET switch C turns on, the switch node SW1 drops to low and the boost capacitor CA is charged through DA from INTVCC. The boost capacitors CA and CB need to store about 100 times the gate charge required by the top MOSFET switch A and D. In most applications a 0.1µF to 0.47µF, X5R or X7R dielectric capacitor is adequate. Run Function The RUN pin provides simple ON/OFF control for the LTC3780. Driving the RUN pin above 1.5V permits the controller to start operating. Pulling RUN below 1.5V puts theLTC3780intolowcurrentshutdown.Donotapplymore than 6V to the RUN pin. Soft-Start Function Soft-start reduces the input power sources’ surge cur- rents by gradually increasing the controller’s current limit (proportional to an internally buffered and clamped equivalent of VITH). An internal 1.2µA current source charges up the CSS ca- pacitor. As the voltage on SS increases from 0V to 2.4V, the internal current limit rises from 0V/RSENSE to 150mV/ RSENSE. The output current limit ramps up slowly, taking 1.5s/µFtoreachfullcurrent.Theoutputcurrentthusramps up slowly, eliminating the starting surge current required from the input power supply. TIRMP = 2.4V 1.2µA • CSS = 1.5s/µF ( )•CSS Do not apply more than 6V to the SS pin. Current foldback is disabled during soft-start until the voltage on CSS reaches 2V. Make sure CSS is large enough when there is loading during start-up. |
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