Datasheet LTC3803-3 (Analog Devices) - 7

LTC3803-3
OPERATION
The LTC3803-3 is a constant frequency current mode con- voltage regulation loop is closed. For example, whenever troller for fl yback and DC/DC boost converter applications the load current increases, output voltage will decrease in a tiny ThinSOT package. The LTC3803-3 is designed so slightly, and sensing this, the error amplifi er raises the that none of its pins need to come in contact with the input ITH/RUN voltage by sourcing current into the ITH/RUN pin, or output voltages of the power supply circuit of which it raising the current comparator threshold, thus increasing is a part, allowing the conversion of voltages well beyond the peak currents through the transformer primary and the LTC3803-3’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 DC/DC the control loop. Typically, an external series RC network conversion will not be discussed here; instead, the reader is connected from ITH/RUN to ground and is chosen for is referred to the detailed treatment in Application Note 19, optimal response to load and line transients. The impedance or in texts such as Abraham Pressman’s Switching Power of this RC network converts the output current of the error 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 LTC3803-3 has two shutdown mechanisms to disable voltage will equal the 800mV from the internal reference. and enable operation: an undervoltage lockout on the VCC If the load current increases, the output voltage will de- supply pin voltage, and a forced shutdown whenever ex- crease slightly, causing the VFB pin voltage to fall below ternal circuitry drives the ITH/RUN pin low. The LTC3803-3 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 modulator formed by the oscillator, current comparator LTC3803-3 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 LTC3803-3 turns on the external power MOSFET when the internal free-running 300kHz oscillator sets the RS latch. It turns off the MOSFET when the cur- rent comparator resets the latch or when 80% duty cycle LTC3803-3 is reached, whichever happens fi rst. In this way, the peak ENABLED current levels through the fl yback transformer’s primary 38033 F01 and secondary are controlled by the ITH/RUN voltage. Since the I
Figure 1. Start-Up/Shutdown State Diagram
TH/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 38033fd 7 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS