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LTC3731CG Datasheet(PDF) 11 Page - Linear Technology |
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LTC3731CG Datasheet(HTML) 11 Page - Linear Technology |
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11 / 32 page  11 LTC3731 3731fa OPERATIO (Refer to Functional Diagram) C) Continuous Current Operation Tying the FCB pin to ground will force continuous current operation. This is the least efficient operating mode, but may be desirable in certain applications. The output can source or sink current in this mode. When forcing con- tinuous operation and sinking current, this current will be forced back into the main power supply, potentially boosting the input supply to dangerous voltage levels— BEWARE! Frequency Synchronization The phase-locked loop allows the internal oscillator to be synchronized to an external source using the PLLIN pin. The output of the phase detector at the PLLFLTR pin is also the DC frequency control input of the oscillator which operates over a 250kHz to 600kHz range corresponding to a voltage input from 0V to 2.4V. When locked, the PLL aligns the turn on of the top MOSFET to the rising edge of the synchronizing signal. When no frequency information is supplied to the PLLIN pin, PLLFLTR goes low, forcing the oscillator to minimum frequency. A DC source can be applied to the PLLFLTR pin to externally set the desired operating frequency. An approximate 20µA discharge cur- rent will be present at the pin with no PLLIN input signal. Input capacitance ESR requirements and efficiency losses are reduced substantially in a multiphase architecture because the peak current drawn from the input capacitor is effectively divided by the number of phases used and power loss is proportional to the RMS current squared. A 3-stage, single output voltage implementation can reduce input path power loss by 90%. Differential Amplifier This amplifier provides true differential output voltage sensing. Sensing both VOUT+ and VOUT– benefits regula- tion in high current applications and/or applications hav- ing electrical interconnection losses. This sensing also isolates the physical power ground from the physical signal ground preventing the possibility of troublesome “ground loops” on the PC layout and prevents voltage errors caused by board-to-board interconnects, particu- larly helpful in VRM designs. Power Good The PGOOD pin is connected to the drain of an internal N-channel MOSFET. The MOSFET is turned on once an internal delay of about 100µs has elapsed and the output voltage has been away from its nominal value by greater than 10%. If the output returns to normal prior to the delay timeout, the timer is reset. There is no delay time for the rising of the PGOOD output once the output voltage is within the ±10% “window.” Phase Mode The PHASMD pin determines the phase shift between the rising edge of the TG1 output and the rising edge of the CLKOUT signal. Grounding the pin will result in 30 degrees phase shift and tying the pin to VCC will result in 60 degrees. These phase shift values enable extension to 6- and 12-phase systems. The PGOOD function above and the PHASMD function are tied to a common pin in the UH package. Undervoltage Shutdown Adjust The voltage applied to the UVADJ pin is compared to the internal 1.2V reference to have an externally program- mable undervoltage shutdown. The RUN/SS pin is inter- nally held low until the voltage applied to the UVADJ pin exceeds the 1.2V threshold. Short-Circuit Detection The RUN/SS capacitor is used initially to turn on and limit the inrush current from the input power source. Once the controllers have been given time, as determined by the capacitor on the RUN/SS pin, to charge up the output capacitors and provide full load current, the RUN/SS capacitor is then used as a short-circuit timeout circuit. If the output voltage falls to less than 70% of its nominal output voltage, the RUN/SS capacitor begins discharg- ing, assuming that the output is in a severe overcurrent and/or short-circuit condition. If the condition lasts for a long enough period, as determined by the size of the RUN/SS capacitor, the controller will be shut down until the RUN/SS pin voltage is recycled. This built-in latchoff |
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