Datasheet LTC1874 (Analog Devices) - 6

LTC1874
U OPERATIO (Refer to Functional Diagram)
The LTC1874 is a dual, constant frequency current mode high, turning off the external MOSFET. The sleep signal switching regulator. The two switching regulators func- goes low when the ITH/RUN voltage goes above 0.925V tion identically but independent of each other. The follow- and the controller resumes normal operation. The next ing description of operation is written for a single oscillator cycle will turn the external MOSFET on and the switching regulator. switching cycle repeats.
Main Control Loop Dropout Operation
During normal operation, the external P-channel power When the input supply voltage decreases towards the MOSFET is turned on by the oscillator and turned off when output voltage, the rate of change of inductor current the current comparator (ICMP) resets the RS latch. The during the ON cycle decreases. This reduction means that peak inductor current at which ICMP resets the RS latch is the external P-channel MOSFET will remain on for more controlled by the voltage on the ITH/RUN pin, which is the than one oscillator cycle since the inductor current has not output of the error amplifier EAMP. An external resistive ramped up to the threshold set by EAMP. Further reduc- divider connected between VOUT and ground allows the tion in input supply voltage will eventually cause the EAMP to receive an output feedback voltage VFB. When the P-channel MOSFET to be turned on 100%, i.e., DC. The load current increases, it causes a slight decrease in VFB output voltage will then be determined by the input voltage relative to the 0.8V reference, which in turn causes the minus the voltage drop across the MOSFET, the sense ITH/RUN voltage to increase until the average inductor resistor and the inductor. current matches the new load current.
Undervoltage Lockout
The main control loop is shut down by pulling the ITH/RUN pin low. Releasing I To prevent operation of the P-channel MOSFET below safe TH/RUN allows an internal 0.5µA current source to charge up the external compensation input voltage levels, an undervoltage lockout is incorpo- network. When the I rated into the controller. When the input supply voltage TH/RUN pin reaches 0.35V, the main control loop is enabled with the I drops below approximately 2.0V, the P-channel MOSFET TH/RUN voltage then pulled up to its zero current level of approximately 0.7V. and all circuitry is turned off except the undervoltage As the external compensation network continues to charge block, which draws only several microamperes. up, the corresponding output current trip level follows, allowing normal operation.
Short-Circuit Protection
Comparator OVP guards against transient overshoots When the output is shorted to ground, the frequency of the greater than 7.5% by turning off the external P-channel oscillator will be reduced to about 120kHz. This lower power MOSFET and keeping it off until the fault is frequency allows the inductor current to safely discharge, removed. thereby preventing current runaway. The oscillator’s fre- quency will gradually increase to its designed rate when
Burst Mode Operation
the feedback voltage again approaches 0.8V. The controller enters Burst Mode operation at low load
Overvoltage Protection
currents. In this mode, the peak current of the inductor is set as if V As a further protection, the overvoltage comparator in the ITH/RUN = 1V (at low duty cycles) even though the voltage at the I controller will turn the external MOSFET off when the TH/RUN pin is at a lower value. If the inductor’s average current is greater than the load require- feedback voltage has risen 7.5% above the reference ment, the voltage at the I voltage of 0.8V. This comparator has a typical hysteresis TH/RUN pin will drop. When the I of 20mV. TH/RUN voltage goes below 0.85V, the sleep signal goes 6