Datasheet LT8316 (Analog Devices) - 9

link to page 8 LT8316
OPERATION
The LT8316 is a high-voltage current-mode switching
Boundary Mode Operation
controller designed for the isolated flyback topology. The Boundary mode is a variable frequency, current-mode problem normally encountered in such circuits is that switching scheme. The external N-channel MOSFET turns information relating to the output voltage on the isolated on and the inductor current increases until it reaches the secondary side of the transformer must be communicated limit determined by the voltage on the V to the primary side in order to achieve regulation. This is C pin and the sense resistor’s value. After the MOSFET turns off, the often performed by opto-isolator circuits, which waste voltage on the tertiary winding rises to the output voltage output power, require extra components that increase the multiplied by the transformer tertiary-to-secondary turns cost and physical size of the power supply, and exhibit ratio. After the current through the output diode falls to trouble due to limited dynamic response, nonlinearity, zero, the voltage on the tertiary winding falls. A boundary unit-to-unit variation, and aging over their life. mode detection comparator on the DCM pin detects the The LT8316 does not need an opto-isolator because it negative dV/dt associated with the falling voltage and trig- derives information about the isolated output voltage by gers the sample-and-hold circuit to sample the FB voltage. examining the flyback pulse waveform appearing on a When the tertiary voltage reaches its minimum and stops tertiary winding on the transformer. The output voltage is falling, the boundary mode comparator turns the internal easily programmed with two resistors. MOSFET back on for minimal switching energy loss. The LT8316 features a boundary mode control method Boundary mode operation returns the secondary current (also called critical conduction mode), where the part to zero every cycle, so parasitic resistive voltage drops operates at the boundary between continuous conduc- do not cause load regulation errors. Boundary mode also tion mode and discontinuous conduction mode. Due allows the use of a smaller transformer compared to con- to boundary mode operation, the output voltage can be tinuous conduction mode and does not exhibit subhar- determined from the tertiary winding’s voltage when the monic oscillation. secondary current is almost zero. This method improves load regulation without extra resistors and capacitors.
Discontinuous Conduction Mode Operation
The Block Diagram shows an overall view of the system. As the load gets lighter, the peak switch current decreases. Many of the blocks are similar to those found in traditional Maintaining boundary mode requires the switching fre- switching regulators, including a current comparator, quency to increase. An excessive switching frequency internal reference, LDO, logic, timers and a MOSFET gate increases switching and gate charge losses. To limit these driver. The novel sections include a special sampling error losses, the LT8316 features an internal oscillator which amplifier, a temperature compensation circuit, an output limits the maximum switching frequency to 140kHz. Once current regulator, and a depletion-mode startup FET. the switching frequency hits this limit, the part starts to reduce its switching frequency and operates in discon-
Depletion Startup FET
tinuous conduction mode. The LT8316 features an internal depletion mode MOSFET.
Low Ripple Burst Mode Operation
At startup, this transistor charges the INTVCC capacitor so that the LT8316 has power to begin switching. This Unlike traditional flyback converters, the MOSFET has removes the need for an external bleeder resistor or other to turn on and off to generate a flyback pulse in order components. to update the sampled output voltage. The duration of a well-formed flyback pulse must exceed the minimum-off time for proper sampling. To this end, a minimum switch turn-off current is necessary to ensure a flyback pulse of sufficient duration. Rev. 0 For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts