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LTC1735IGN-1 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1735IGN-1 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  9 LTC1735-1 OPERATIO (Refer to Functional Diagram) The main control loop is shut down by pulling Pin 2 (RUN/ SS) low. Releasing RUN/SS allows an internal 1.2 µA current source to charge soft-start capacitor CSS. When CSS reaches 1.5V, the main control loop is enabled with the ITH voltage clamped at approximately 30% of its maximum value. As CSS continues to charge, ITH is gradually re- leased allowing normal operation to resume. If VOUT has not reached 70% of its final value when CSS has charged to 4.1V, latchoff can be invoked as described in the Applications Information section. The internal oscillator can be synchronized to an external clock applied though a series resistor to the PGOOD pin and can lock to a frequency between 90% and 130% of its nominal rate set by capacitor COSC. An overvoltage comparator OV guards against transient overshoots (> 7.5%) as well as other more serious conditions that may overvoltage the output. In this case, the top MOSFET is turned off and the bottom MOSFET is turned on until the overvoltage condition is cleared. Foldback current limiting for an output shorted to ground is provided by amplifier A. As VOSENSE drops below 0.6V, the buffered ITHinputtothecurrentcomparatorisgradually pulled down to a 0.86V clamp. This reduces peak inductor current to about 1/4 of its maximum value. Low Current Operation The LTC1735-1 has three low current modes controlled by the PGOOD pin. Burst Mode operation is selected when the PGOOD pin is above 0.8V (typically tied through a resistor to INTVCC). During Burst Mode operation, if the error amplifier drives the ITH voltage below 0.86V, the buffered ITH input to the current comparator will be clamped at 0.86V. The inductor current peak is then held at approximately 20mV/RSENSE (about 1/4 of maximum output current). If ITH then drops below 0.5V, the Burst Mode comparator B will turn off both MOSFETs to maxi- mize efficiency. The load current will be supplied solely by the output capacitor until ITH rises above the 60mV hysteresis of the comparator and switching is resumed. Burst Mode operation is disabled by comparator F when the PGOOD pin is brought below 0.8V. This forces continuous operation and assists in controlling voltage regulation. If the output voltage is not within 7.5% of its nominal value the PGOOD open-drain output will be pulled low and Burst Mode operation will be disabled. Foldback Current, Short-Circuit Detection and Short-Circuit Latchoff The RUN/SS capacitor, CSS, is used initially to limit the inrush current of the switching regulator. After the con- troller has been started and been given adequate time to charge up the output capacitors and provide full load current, CSS is used as a short-circuit time-out circuit. If the output voltage falls to less than 70% of its nominal output voltage, CSS begins discharging on the assumption that the output is in an overcurrent and/or short-circuit condition. If the condition lasts for a long enough period as determined by the size of the CSS, the controller will be shut down until the RUN/SS pin voltage is recycled. This built-in latchoff can be overridden 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 CSS during an overcurrent and/or short- circuit condition. 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. INTVCC/EXTVCC Power Power for the top and bottom MOSFET drivers and most of the internal circuitry of the LTC1735-1 is derived from the INTVCC pin. When the EXTVCC pin is left open, an internal 5.2V low dropout regulator supplies the INTVCC power from VIN. If EXTVCC is raised above 4.7V, the internal regulator is turned off and an internal switch connects EXTVCC to INTVCC. This allows a high efficiency source, such as the primary or a secondary output of the converter itself, to provide the INTVCC power. Voltages up to 7V can be applied to EXTVCC for additional gate drive capability. To provide clean start-up and to protect the MOSFETs, undervoltage lockout is used to keep both MOSFETs off until the input voltage is above 3.5V. |
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Similar Description - LTC1735IGN-1 |
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