Datasheet LTC1772 (Analog Devices) - 5

LTC1772
U OPERATIO (Refer to Functional Diagram) Main Control Loop
oscillator cycle will turn the external MOSFET on and the switching cycle repeats. The LTC1772 is a constant frequency current mode switch- ing regulator. During normal operation, the external
Dropout Operation
P-channel power MOSFET is turned on each cycle when the oscillator sets the RS latch (RS1) and turned off when When the input supply voltage decreases towards the the current comparator (ICMP) resets the latch. The peak output voltage, the rate of change of inductor current inductor current at which ICMP resets the RS latch is during the ON cycle decreases. This reduction means that controlled by the voltage on the I the external P-channel MOSFET will remain on for more TH/RUN pin, which is the output of the error amplifier EAMP. An external resistive than one oscillator cycle since the inductor current has not divider connected between V ramped up to the threshold set by EAMP. Further reduc- OUT and ground allows the EAMP to receive an output feedback voltage V tion in input supply voltage will eventually cause the FB. When the load current increases, it causes a slight decrease in V P-channel MOSFET to be turned on 100%, i.e., DC. The FB relative to the 0.8V reference, which in turn causes the output voltage will then be determined by the input voltage I minus the voltage drop across the MOSFET, the sense TH/RUN voltage to increase until the average inductor current matches the new load current. resistor and the inductor. The main control loop is shut down by pulling the ITH/RUN
Undervoltage Lockout
pin low. Releasing ITH/RUN allows an internal 0.5µA To prevent operation of the P-channel MOSFET below safe current source to charge up the external compensation input voltage levels, an undervoltage lockout is incorpo- network. When the ITH/RUN pin reaches 0.35V, the main rated into the LTC1772. When the input supply voltage control loop is enabled with the ITH/RUN voltage then drops below approximately 2.0V, the P-channel MOSFET pulled up to its zero current level of approximately 0.7V. and all circuitry is turned off except the undervoltage block, As the external compensation network continues to charge 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 > 7.5% by turning off the external P-channel power oscillator will be reduced to about 120kHz. This lower MOSFET and keeping it off until the fault is removed. frequency allows the inductor current to safely discharge, thereby preventing current runaway. The oscillator’s fre-
Burst Mode Operation
quency will gradually increase to its designed rate when The LTC1772 enters Burst Mode operation at low load the feedback voltage again approaches 0.8V. currents. In this mode, the peak current of the inductor is set as if VITH/RUN = 1V (at low duty cycles) even though
Overvoltage Protection
the voltage at the ITH/RUN pin is at a lower value. If the As a further protection, the overvoltage comparator in the inductor’s average current is greater than the load require- LTC1772 will turn the external MOSFET off when the ment, the voltage at the ITH/RUN pin will drop. When the feedback voltage has risen 7.5% above the reference ITH/RUN voltage goes below 0.85V, the sleep signal goes voltage of 0.8V. This comparator has a typical hysteresis high, turning off the external MOSFET. The sleep signal of 20mV. goes low when the ITH/RUN voltage goes above 0.925V and the LTC1772 resumes normal operation. The next 1772fb 5