Datasheet LT8304, LT8304-1 (Analog Devices) - 9

LT8304/LT8304-1
OPERATION
The LT8304 is a simple to use micropower isolated fly-
Discontinuous Conduction Mode Operation
back converter housed in a thermally enhanced 8-lead As the load gets lighter, boundary conduction mode in- SO package. The output voltage is programmed with two creases the switching frequency and decreases the switch external resistors. An optional TC resistor provides easy peak current at the same ratio. Running at a higher switching output diode temperature compensation. By integrating frequency up to several MHz increases switching and gate the loop compensation and soft-start inside, the part charge losses. To avoid this scenario, the LT8304 has an reduces the number of external components. As shown additional internal oscillator, which clamps the maximum in the Block Diagram, many of the blocks are similar to switching frequency to be less than 350kHz (TYP). Once those found in traditional switching regulators including the switching frequency hits the internal frequency clamp, reference, regulators, oscillator, logic, current amplifier, the part starts to delay the switch turn-on and operates in current comparator, driver, and power switch. The novel discontinuous conduction mode. sections include a flyback pulse sense circuit, a sample- and-hold error amplifier, and a boundary mode detector,
Low Ripple Burst Mode Operation
as well as the additional logic for boundary conduction mode, discontinuous conduction mode, and low ripple Unlike traditional flyback converters, the LT8304 has to Burst Mode operation. turn on and off at least for a minimum amount of time and with a minimum frequency to allow accurate sampling
Quasi-Resonant Boundary Mode Operation
of the output voltage. The inherent minimum switch cur- rent limit and minimum switch-off time are necessary to The LT8304 features quasi-resonant boundary conduction guarantee the correct operation of specific applications. mode operation at heavy load, where the chip turns on the primary power switch when the secondary current is zero As the load gets very light, the LT8304 starts to fold back and the SW rings to its valley. Boundary conduction mode the switching frequency while keeping the minimum switch is a variable frequency, variable peak-current switching current limit. So the load current is able to decrease while scheme. The power switch turns on and the transformer still allowing minimum switch-off time for the sample-and- primary current increases until an internally controlled peak hold error amplifier. Meanwhile, the part switches between current limit. After the power switch turns off, the voltage sleep mode and active mode, thereby reducing the effec- on the SW pin rises to the output voltage multiplied by tive quiescent current to improve light load efficiency. In the primary-to-secondary transformer turns ratio plus the this condition, the LT8304 runs in low ripple Burst Mode input voltage. When the secondary current through the operation. The typical 11kHz minimum switching frequency output diode falls to zero, the SW pin voltage collapses determines how often the output voltage is sampled and and rings around VIN. A boundary mode detector senses also the minimum load requirement. this event and turns the power switch back on at its valley.
High Step-Up V
Boundary conduction mode returns the secondary current
OUT Applications
to zero every cycle, so parasitic resistive voltage drops Typically, high step-up output applications have excessive do not cause load regulation errors. Boundary conduc- primary inductor current ringing during primary switch tion mode also allows the use of smaller transformers turn-on due to the huge reflected capacitance on SW node. compared to continuous conduction mode and does not Such current ringing can falsely trigger LT8304 current exhibit subharmonic oscillation. comparator after 160ns typical blanking time and create large signal oscillation, especially at high VIN and light load condition. The LT8304-1, specially optimized for high step-up output applications, is more immune to the current ringing without requiring longer blanking time. For any 1:N step-up transformer turns ratio larger than or equal to 5, the LT8304-1 is recommended. 8304fa For more information www.linear.com/LT8304 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts 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 Revision History Typical Application Related Parts