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LTC1736 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1736 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 32 page  10 LTC1735 1735fc OPERATIO (Refer to Functional Diagram) When the FCB pin is driven by an external oscillator, a low noise cycle-skipping mode is invoked and the internal oscillator is synchronized to the external clock by com- parator C. In this mode the 25% minimum inductor current clamp is removed, providing constant frequency discontinuous operation over the widest possible output current range. This constant frequency operation is not quite as efficient as Burst Mode operation, but provides a lower noise, constant frequency spectrum. Tying the FCB pin to ground enables forced continuous operation. This is the least efficient mode, but is desirable in certain applications. The output can source or sink current in this mode. When sinking current while in forced continuous operation, current will be forced back into the main power supply potentially boosting the input supply to dangerous voltage levels—BEWARE. 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 cur- rent, 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 condi- tion. If the condition lasts for a long enough period as determined by the size of 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 dis- charge 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. 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 to the FCB pin and can lock to a frequency between 90% and 130% of its nominal rate set by capaci- tor 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 ITH input to the current comparator is gradu- ally 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 has three low current modes controlled by the FCB pin. Burst Mode operation is selected when the FCB pin is above 0.8V (typically tied to INTVCC). In Burst Mode operation, if the error amplifier drives the ITH voltage below 0.86V, the buffered ITH input to the current com- parator 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 maximize efficiency. The load current will be supplied solely by the output capacitor until ITH rises above the 60mV hysteresis of the comparator and switch- ing is resumed. Burst Mode operation is disabled by comparator F when the FCB pin is brought below 0.8V. This forces continuous operation and can assist second- ary winding regulation. |
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Similar Description - LTC1736 |
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