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LTC3854EMSE 数据表(PDF) 14 Page - Linear Technology |
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LTC3854EMSE 数据表(HTML) 14 Page - Linear Technology |
14 / 28 page LTC3854 3854fa applicaTions inForMaTion capacitor CB needs to be at least 100 times that of the total input capacitance of the topside MOSFET. The reverse breakdown of the external Schottky diode must be greater than VIN(MAX). When adjusting the gate-drive level, the final arbiter is the total input current for the regulator. If a change is made and the input current decreases, then the efficiency has improved. If there is no change in input current, then there is no change in efficiency. Undervoltage Lockout The LTC3854 has two functions that help protect the controller in case of undervoltage conditions. A precision UVLO comparator constantly monitors the INTVCCvoltage to ensure that an adequate gate-drive voltage is present. Switching action is disabled when INTVCC is below 3.5V. To prevent oscillation caused by a disturbance on INTVCC, the UVLO comparator has 350mV of hysteresis. Another way to detect an undervoltage condition is to monitor the VIN supply. The RUN/SS pin has a precision turn-on reference of 1.2V, enabling a resistor divider to VIN to turn on the IC when VIN is above the desired value. It is recommended that the resistor divider be used if the input voltage will be quickly cycled on and off. CIN Selection In forced continuous mode, the source current of the top N-channel MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. IRMS = IOUT 2 The maximum RMS capacitor current is: IRMS = IMAX VIN VOUT ( )• V IN − VOUT ) ( ) 1/2 This formula has a maximum at VIN = 2•VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturers’ ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor or to choose a capacitor rated at a higher temperature than required.Severalcapacitorsmayalsobeparalleledtomeet size or height requirements in the design. Always consult the manufacturer if there is any question. COUT Selection The selection of COUT is primarily determined by the ef- fective series resistance (ESR) to minimize voltage ripple. The output ripple ( ∆VOUT) in continuous mode is: ∆VOUT = ∆IL ESR+ 1 8 • fSW •COUT Where fSW = 400kHz, COUT = output capacitance and ∆IL = ripple current in the inductor. The output ripple is highest at maximum input voltage since ∆IL increases with input voltage. Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. With ∆IL= 0.3IOUT(MAX) and allowing 2/3 of the ripple due to ESR, the output ripple will be less than 50mV at max VIN assuming: COUT Required ESR < 2.2 RSENSE COUT > 1 8fSW RSENSE The first condition relates to the ripple current into the ESR of the output capacitance while the second term guar- antees that the output capacitance does not significantly discharge during the operating frequency period due to ripple current. The choice of smaller output capacitance increases the ripple voltage due to the discharging term but can be compensated with capacitors of very low ESR to maintain the ripple voltage at or below 50mV. The ITHpin OPTI-LOOP compensation components can be optimized to provide stable, high performance transient response regardless of the output capacitors selected. The selec- tion of output capacitors for applications with large load current transients is primarily determined by the voltage tolerance specifications of the load. The resistive compo- nent of the capacitor, ESR, multiplied by the load current change plus any output voltage ripple must be within the voltage tolerance of the load. |
类似零件编号 - LTC3854EMSE |
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类似说明 - LTC3854EMSE |
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