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LTC3727 Datasheet(PDF) 19 Page - Linear Technology |
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LTC3727 Datasheet(HTML) 19 Page - Linear Technology |
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19 / 32 page  19 LTC3727/LTC3727-1 3727f Regulating an output voltage of 1.8V, the maximum value of R1 should be 32K. Note that for an output voltage above 2.4V, R1 has no maximum value necessary to absorb the sense currents; however, R1 is still bounded by the VOSENSE feedback current. Soft-Start/Run Function The RUN/SS1 and RUN/SS2 pins are multipurpose pins that provide a soft-start function and a means to shut down the LTC3727. Soft-start reduces the input power source’s surge currents by gradually increasing the controller’s current limit (proportional to VITH). This pin can also be used for power supply sequencing. An internal 1.2 µA current source charges up the CSS capacitor. When the voltage on RUN/SS1 (RUN/SS2) reaches 1.5V, the particular controller is permitted to start operating. As the voltage on RUN/SS increases from 1.5V to 3.0V, the internal current limit is increased from 45mV/ RSENSE to 135mV/RSENSE. The output current limit ramps up slowly, taking an additional 1.25s/ µF to reach full current. The output current thus ramps up slowly, reduc- ing the starting surge current required from the input power supply. If RUN/SS has been pulled all the way to ground there is a delay before starting of approximately: t V A Cs F C DELAY SS SS = µ =µ () 15 12 125 . . ./ t VV A Cs F C IRAMP SS SS = − µ =µ () 31 5 12 125 . . ./ By pulling both RUN/SS pins below 1V, the LTC3727 is put into low current shutdown (IQ = 20µA). The RUN/SS pins can be driven directly from logic as shown in Fig- ure 7. Diode D1 in Figure 7 reduces the start delay but allows CSS to ramp up slowly providing the soft-start function. Each RUN/SS pin has an internal 6V zener clamp (See Functional Diagram). Fault Conditions: Overcurrent Latchoff (LTC3727 Only) The RUN/SS pins also provide the ability to latch off the controller(s) when an overcurrent condition is detected. The RUN/SS capacitor, CSS, is used initially to turn on and limit the inrush current. After the controller has been started and been given adequate time to charge up the output capacitor and provide full load current, the RUN/SS capacitor is used for a short-circuit timer. If the regulator’s output voltage falls to less than 70% of its nominal value after CSS reaches 4.1V, CSS begins discharging on the assumption that the output is in an overcurrent condition. If the condition lasts for a long enough period as deter- mined by the size of the CSS and the specified discharge current, the controller will be shut down until the RUN/SS pin voltage is recycled. If the overload occurs during start- up, the time can be approximated by: tLO1 ≈ [CSS(4.1 – 1.5 + 4.1 – 3.5)]/(1.2µA) = 2.7 • 106 (CSS) If the overload occurs after start-up the voltage on CSS will begin discharging from the zener clamp voltage: tLO2 ≈ [CSS (6 – 3.5)]/(1.2µA) = 2.1 • 106 (CSS) This built-in overcurrent latchoff can be overridden by providing a pull-up resistor to the RUN/SS pin as shown in Figure 7. This resistance shortens the soft-start period and prevents the discharge of the RUN/SS capacitor during an over current condition. Tying this pull-up resis- tor to VIN as in Figure 7a, defeats overcurrent latchoff. Diode-connecting this pull-up resistor to INTVCC, as in Figure 7b, eliminates any extra supply current during controller shutdown while eliminating the INTVCC loading from preventing controller start-up. This pull-up resistor is not needed in LTC3727-1 designs. Figure 7. RUN/SS Pin Interfacing APPLICATIO S I FOR ATIO 3.3V OR 5V RUN/SS VIN INTVCC RUN/SS D1 CSS RSS* CSS RSS* 3727 F07 (7a) (7b) *OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF (NOT NEEDED WITH THE LTC3727-1) |
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Similar Description - LTC3727 |
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