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LTC1709EG-8 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1709EG-8 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  11 LTC1709-8/LTC1709-9 OPERATIO (Refer to Functional Diagram) 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. The AMPMD pin allows selection of internal, precision feedback resistors for high common mode rejection differencing applica- tions, or direct access to the actual amplifier inputs without these internal feedback resistors for other applica- tions. The AMPMD pin is grounded to connect the internal precision resistors in a unity-gain differencing application, or tied to the INTVCC pin to bypass the internal resistors and make the amplifier inputs directly available. The amplifier is a unity-gain stable, 2MHz gain bandwidth, >120dB open-loop gain design. The amplifier has an output slew rate of 5V/ µs and is capable of driving capaci- tive loads with an output RMS current typically up to 35mA. The amplifier is not capable of sinking current and therefore must be resistively loaded to do so. Power Good (PGOOD) The PGOOD pin is connected to the drain of an internal MOSFET. The MOSFET turns on when the output voltage is not within ±7.5% of its nominal output level as deter- mined by the feedback divider. When the output is within ±7.5% of its nominal value, the MOSFET is turned off within 10 µs and the PGOOD pin should be pulled up by an external resistor to a source of up to 7V. Short-Circuit Detection The RUN/SS capacitor is used initially to 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 discharging 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 can be overidden by providing a current >5 µA at a compliance of 5V to the RUN/SS pin. This current shortens the soft-start period but also prevents net discharge of the RUN/SS capacitor during a severe overcurrent and/or short-circuit condi- tion. Foldback current limiting is activated when the output voltage falls below 70% of its nominal level whether or not the short-circuit latchoff circuit is enabled. APPLICATIO S I FOR ATIO The basic LTC1709 application circuit is shown in Fig- ure 1 on the first page. External component selection begins with the selection of the inductor(s) based on ripple current requirements and continues with the RSENSE1, 2 resistor selection using the calculated peak inductor current and/or maximum current limit. Next, the power MOSFETs and D1 and D2 are selected. The oper- ating frequency and the inductor are chosen based mainly on the amount of ripple current. Finally, CIN is selected for its ability to handle the input ripple current (that PolyPhase TM operation minimizes) and COUT is chosen with low enough ESR to meet the output ripple voltage and load step specifications (also minimized with PolyPhase). Current mode architecture provides inherent current sharing between output stages. The circuit shown in Figure 1 can be configured for operation up to an input voltage of 28V (limited by the external MOSFETs). RSENSE Selection For Output Current RSENSE1, 2 are chosen based on the required peak output current. The LTC1709 current comparator has a maxi- mum threshold of 75mV/RSENSE and an input common mode range of SGND to 1.1(INTVCC). The current com- parator threshold sets the peak inductor current, yielding a maximum average output current IMAX equal to the peak value less half the peak-to-peak ripple current, ∆IL. PolyPhase is a trademark of Linear Technology Corporation. |
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