Datasheet LTC1735 (Analog Devices) - 10

LTC1735
U OPERATIO (Refer to Functional Diagram)
The main control loop is shut down by pulling Pin 2 When the FCB pin is driven by an external oscillator, a low (RUN/SS) low. Releasing RUN/SS allows an internal 1.2µA noise cycle-skipping mode is invoked and the internal current source to charge soft-start capacitor C oscillator is synchronized to the external clock by com- SS. When C parator C. In this mode the 25% minimum inductor SS reaches 1.5V, the main control loop is enabled with the I current clamp is removed, providing constant frequency TH voltage clamped at approximately 30% of its maximum value. As C discontinuous operation over the widest possible output SS continues to charge, ITH is gradually re- leased allowing normal operation to resume. If V current range. This constant frequency operation is not OUT has not reached 70% of its final value when C quite as efficient as Burst Mode operation, but provides a SS has charged to 4.1V, latchoff can be invoked as described in the lower noise, constant frequency spectrum. Applications Information section. Tying the FCB pin to ground enables forced continuous The internal oscillator can be synchronized to an external operation. This is the least efficient mode, but is desirable clock applied to the FCB pin and can lock to a frequency in certain applications. The output can source or sink between 90% and 130% of its nominal rate set by capaci- current in this mode. When sinking current while in forced tor C continuous operation, current will be forced back into the OSC. main power supply potentially boosting the input supply to An overvoltage comparator, OV, guards against transient dangerous voltage levels—BEWARE. overshoots (>7.5%) as well as other more serious conditions that may overvoltage the output. In this case,
Foldback Current, Short-Circuit Detection and
the top MOSFET is turned off and the bottom MOSFET is
Short-Circuit Latchoff
turned on until the overvoltage condition is cleared. The RUN/SS capacitor, CSS, is used initially to limit the Foldback current limiting for an output shorted to ground inrush current of the switching regulator. After the con- is provided by amplifier A. As VOSENSE drops below 0.6V, troller has been started and been given adequate time to the buffered ITH input to the current comparator is gradu- charge up the output capacitors and provide full load cur- ally pulled down to a 0.86V clamp. This reduces peak rent, CSS is used as a short-circuit time-out circuit. If the inductor current to about 1/4 of its maximum value. output voltage falls to less than 70% of its nominal output voltage, CSS begins discharging on the assumption that
Low Current Operation
the output is in an overcurrent and/or short-circuit condi- The LTC1735 has three low current modes controlled by tion. If the condition lasts for a long enough period as the FCB pin. Burst Mode operation is selected when the determined by the size of CSS, the controller will be shut FCB pin is above 0.8V (typically tied to INTVCC). In Burst down until the RUN/SS pin voltage is recycled. This built- Mode operation, if the error amplifier drives the ITH voltage in latchoff can be overridden by providing a current >5µA below 0.86V, the buffered ITH input to the current com- at a compliance of 5V to the RUN/SS pin. This current parator will be clamped at 0.86V. The inductor current shortens the soft-start period but also prevents net dis- peak is then held at approximately 20mV/RSENSE (about charge of CSS during an overcurrent and/or short-circuit 1/4 of maximum output current). If ITH then drops below condition. Foldback current limiting is activated when the 0.5V, the Burst Mode comparator B will turn off both output voltage falls below 70% of its nominal level whether MOSFETs to maximize efficiency. The load current will be or not the short-circuit latchoff circuit is enabled. 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. 1735fc 10