Datasheet LTC3803 (Analog Devices) - 7

LTC3803
OPERATION
The LTC3803 is a constant frequency current mode control- voltage regulation loop is closed. For example, whenever ler for fl yback and DC/DC boost converter applications in the load current increases, output voltage will decrease a tiny ThinSOT package. The LTC3803 is designed so that slightly, and sensing this, the error amplifi er raises the none of its pins need to come in contact with the input or ITH/RUN voltage by sourcing current into the ITH/RUN pin, output voltages of the power supply circuit of which it is raising the current comparator threshold, thus increasing a part, allowing the conversion of voltages well beyond the peak currents through the transformer primary and the LTC3803’s absolute maximum ratings. secondary. This delivers more current to the load, bringing the output voltage back up.
Main Control Loop
The ITH/RUN pin serves as the compensation point for Due to space limitations, the basics of current mode the control loop. Typically, an external series RC network DC/DC conversion will not be discussed here; instead, the is connected from ITH/RUN to ground and is chosen for reader is referred to the detailed treatment in Application optimal response to load and line transients. The impedance Note 19, or in texts such as Abraham Pressman’s Switch- of this RC network converts the output current of the error ing Power Supply Design. amplifi er to the ITH/RUN voltage which sets the current Please refer to the Block Diagram and the Typical Ap- comparator threshold and commands considerable infl u- plication on the front page of this data sheet. An external ence over the dynamics of the voltage regulation loop. resistive voltage divider presents a fraction of the output
Start-Up/Shutdown
voltage to the VFB pin. The divider must be designed so that when the output is at the desired voltage, the VFB pin The LTC3803 has two shutdown mechanisms to disable voltage will equal the 800mV from the internal reference. and enable operation: an undervoltage lockout on the If the load current increases, the output voltage will de- VCC supply pin voltage, and a forced shutdown whenever crease slightly, causing the VFB pin voltage to fall below external circuitry drives the ITH/RUN pin low. The LTC3803 800mV. The error amplifi er responds by feeding current transitions into and out of shutdown according to the state into the ITH/RUN pin. If the load current decreases, the diagram (Figure 1). VFB voltage will rise above 800mV and the error amplifi er will sink current away from the ITH/RUN pin. The voltage at the ITH/RUN pin commands the pulse-width LTC3803 modulator formed by the oscillator, current comparator SHUT DOWN and RS latch. Specifi cally, the voltage at the ITH/RUN pin sets the current comparator’s trip threshold. The current comparator monitors the voltage across a current sense VITH/RUN > VITHSHDN resistor in series with the source terminal of the external V V CC < VTURNOFF ITH/RUN < VITHSHDN AND V (NOMINALLY 5.7V) (NOMINALLY 0.28V) CC > VTURNON (NOMINALLY 8.7V) MOSFET. The LTC3803 turns on the external power MOSFET when the internal free-running 200kHz oscillator sets the RS latch. It turns off the MOSFET when the current comparator resets the latch or when 80% duty cycle is LTC3803 reached, whichever happens fi rst. In this way, the peak ENABLED current levels through the fl yback transformer’s primary 3803 F01 and secondary are controlled by the ITH/RUN voltage.
Figure 1. Start-Up/Shutdown State Diagram
Since the ITH/RUN voltage is increased by the error ampli- fi er whenever the output voltage is below nominal, and decreased whenever output voltage exceeds nominal, the 3803fc 7 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATIONS RELATED PARTS