Datasheet LTC3772B (Analog Devices) - 8

LTC3772B
U OPERATIO (Refer to the Functional Diagram) Main Control Loop (Normal Operation)
cause the external P-channel MOSFET to be turned on 100%; i.e., DC. The output voltage will then be determined The LTC3772B is a constant frequency current mode step- by the input voltage minus the voltage drop across the down switching regulator controller. During normal op- sense resistor, the MOSFET and the inductor. eration, the external P-channel MOSFET is turned on each cycle when the oscillator sets the RS latch and turned off
Undervoltage Lockout Protection
when the current comparator resets the latch. The peak inductor current at which the current comparator trips is To prevent operation of the external P-channel MOSFET controlled by the voltage on the ITH/RUN pin, which is the with insufficient gate drive, an undervoltage lockout cir- output of the error amplifier. The negative input to the error cuit is incorporated into the LTC3772B. When the input amplifier is the output feedback voltage VFB, which is supply voltage drops below approximately 2V, the generated by an external resistor divider connected be- P-channel MOSFET and all internal circuitry other than the tween VOUT and ground. When the load current increases, undervoltage block itself are turned off. Input supply it causes a slight decrease in VFB relative to the 0.8V current in undervoltage is approximately 1µA. reference, which in turn causes the ITH/RUN voltage to increase until the average inductor current matches the
Short-Circuit Protection
new load current. If the output is shorted to ground, the frequency of the The main control loop is shut down by pulling the ITH/RUN oscillator is folded back from 550kHz to approximately pin to ground. Releasing the ITH/RUN pin allows an 200kHz while maintaining the same minimum on time. internal 1µA current source to charge up the external This lower frequency allows the inductor current to safely compensation network. When the ITH/RUN pin voltage discharge, thereby preventing current runaway. After the reaches approximately 0.6V, the main control loop is short is removed, the oscillator frequency will gradually enabled and the ITH/RUN voltage is pulled up by a clamp increase back to 550kHz as VFB rises through 0.3V on its to its zero current level of approximately one diode way back to 0.8V. voltage drop (0.7V). As the external compensation net- work continues to charge up, the corresponding peak
Overvoltage Protection
inductor current level follows, allowing normal operation. If VFB exceeds its regulation point of 0.8V by more than The maximum peak inductor current attainable is set by a 10% for any reason, such as an output short-circuit to a clamp on the ITH/RUN pin at 1.2V above the zero current higher voltage, the overvoltage comparator will hold the level (approximately 1.9V). external P-channel MOSFET off. This comparator has a typical hysteresis of 40mV.
Dropout Operation
When the input supply voltage decreases towards the
Peak Current Sense Voltage Selection and Slope
output voltage, the rate of change of inductor current
Compensation (IPRG Pins)
during the on cycle decreases. This reduction means that When a controller is operating below 20% duty cycle, the at some input-output differential, the external P-channel maximum sense voltage allowed across the external MOSFET will remain on for more than one oscillator cycle P-channel MOSFET is 138mV, 70mV or 208mV for the (start dropping off-cycles) since the inductor current has three respective states of the IPRG pin. not ramped up to the threshold set by the error amplifier. Further reduction in input supply voltage will eventually 3772bfa 8