Datasheet LTC3805 (Analog Devices) - 8

LTC3805
operation
The LTC3805 is a programmable-frequency current mode does the voltage on the ISENSE pin. The LTC3805’s cur- controller for flyback, boost and SEPIC DC/DC converters. rent comparator trips when the voltage on the ISENSE pin The LTC3805 is designed so that none of its pins need exceeds a voltage proportional to the voltage on the ITH pin. to come in contact with the input or output voltages of This resets the SR latch and turns off the external power the power supply circuit of which it is a part, allowing the MOSFET. In this way, the peak current levels through the conversion of voltages well beyond the LTC3805’s absolute external MOSFET and the flyback transformer’s primary maximum ratings. and secondary windings are controlled by the voltage on the ITH pin. If the current comparator does not trip, the
Main Control Loop
LTC3805 automatical y limits the duty cycle to 80%, resets Please refer to the Block Diagram of this data sheet and the SR latch, and turns off the external MOSFET. the Typical Application shown on the front page. An ex- The path from the FB pin, through the error amplifier, ternal resistive voltage divider presents a fraction of the current comparator and the SR latch implements the output voltage to the FB pin. The divider is designed so closed-loop current mode control required to regulate the that when the output is at the desired voltage, the FB pin output voltage against changes in input voltage or output voltage equals the 800mV internal reference voltage. If current. For example, if the load current increases, the the load current increases, the output voltage decreases output voltage decreases slightly, and sensing this, the slightly, causing the FB pin voltage to fall below the 800mV error amplifier sources current from the ITH pin, raising reference. The error amplifier responds by feeding current the current comparator threshold, thus increasing the peak into the ITH pin causing its voltage to rise. Conversely, if currents through the transformer primary and secondary. the load current decreases, the FB voltage rises above the This delivers more current to the load and restores the 800mV reference and the error amplifier sinks current away output voltage to the desired level. from the ITH pin causing its voltage to fall. The ITH pin serves as the compensation point for the control The voltage at the ITH pin controls the pulse-width modula- loop. Typically, an external series RC network is connected tor formed by the oscillator, current comparator and SR from ITH to ground and is chosen for optimal response to latch. Specifically, the voltage at the ITH pin sets the current load and line transients. The impedance of this RC network comparator’s trip threshold. The current comparator’s converts the output current of the error amplifier to the ISENSE input monitors the voltage across an external cur- I rent sense resistor in series with the source of the external TH voltage which sets the current comparator threshold and commands considerable influence over the dynamics MOSFET. At the start of a cycle, the LTC3805’s oscillator of the voltage regulation loop. sets the SR latch and turns on the external power MOSFET. The current through the external power MOSFET rises as 3805fg 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration 0rder Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts