Datasheet LTC1779 (Analog Devices) - 5

LTC1779
FuncTional DiagraM
SENSE– VIN 4 5 + OVERTEMP ICMP DETECT – 2Ω VIN RS1 SLOPE SWITCHING R 1× 24× COMP LOGIC AND OSC Q BLANKING S CIRCUIT SW 6 – FREQ BURST FOLDBACK + + OVP CMP + 0.3V SHORT-CIRCUIT SLEEP DETECT VREF – – + 0.15V 60mV VIN EAMP VREF + 0.8V 0.5µA VFB + 1 ITH/RUN – 3 V V IN IN 0.3V – 0.325V + SHDN VOLTAGE VREF SHDN REFERENCE 0.8V CMP – UV GND 2 UNDERVOLTAGE LOCKOUT 1.2V 1779FD
operaTion (Refer to Functional Diagram) Main Control Loop
VFB relative to the 0.8V reference, which in turn causes the The LTC1779 is a constant frequency current mode ITH/RUN voltage to increase until the average inductor switching regulator. During normal operation, the internal current matches the new load current. P-channel power MOSFET is turned on each cycle when The main control loop is shut down by pulling the ITH/RUN the oscillator sets the RS latch (RS1) and turned off when pin low. Releasing ITH/RUN allows an internal 0.5µA the current comparator (ICMP) resets the latch. The peak current source to charge up the external compensation inductor current at which ICMP resets the RS latch is network. When the ITH/RUN pin reaches 325mV, the controlled by the voltage on the ITH/RUN pin, which is the main control loop is enabled with the ITH/RUN voltage output of the error amplifier EAMP. An external resistive then pulled up to its zero current level of approximately divider connected between VOUT and ground allows the 0.7V. As the external compensation network continues EAMP to receive an output feedback voltage VFB. When to charge up, the corresponding output current trip level the load current increases, it causes a slight decrease in follows, allowing normal operation. 1779fa For more information www.linear.com/LTC1779 5