LT3757 datasheet(7/24 Pages) AD

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LT8300

Rev. A

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OPERATION

The LT8300 is a current mode switching regulator IC

designed specially for the isolated flyback topology. The

key problem in isolated topologies is how to communicate

the output voltage information from the isolated secondary

side of the transformer to the primary side for regulation.

Historically, opto-isolators or extra transformer windings

communicate this information across the isolation bound-

ary. Opto-isolator circuits waste output power, and the extra

components increase the cost and physical size of the

power supply. Opto-isolators can also cause system issues

due to limited dynamic response, nonlinearity, unit-to-unit

variation and aging over lifetime. Circuits employing extra

transformer windings also exhibit deficiencies, as using an

extra winding adds to the transformer’s physical size and

cost, and dynamic response is often mediocre.

The LT8300 samples the isolated output voltage through

the primary-side flyback pulse waveform. In this man-

ner, neither opto-isolator nor extra transformer winding

is required for regulation. Since the LT8300 operates

in either boundary conduction mode or discontinuous

conduction mode, the output voltage is always sampled

on the SW pin when the secondary current is zero. This

method improves load regulation without the need of

external load compensation components.

The LT8300 is a simple to use micropower isolated fly-

back converter housed in a 5-lead TSOT-23 package. The

output voltage is programmed with a single external resis-

tor. By integrating the loop compensation and soft-start

inside, the part further reduces the number of external

components. As shown in the Block Diagram, many of

the blocks are similar to those found in traditional switch-

ing regulators including reference, regulators, oscillator,

logic, current amplifier, current comparator, driver, and

power switch. The novel sections include a flyback pulse

sense circuit, a sample-and-hold error amplifier, and a

boundary mode detector, as well as the additional logic for

boundary conduction mode, discontinuous conduction

mode, and low ripple Burst Mode operation.

Boundary Conduction Mode Operation

The LT8300 features boundary conduction mode opera-

tion at heavy load, where the chip turns on the primary

power switch when the secondary current is zero.

Boundary conduction mode is a variable frequency, vari-

able peak-current switching scheme. The power switch

turns on and the transformer primary current increases

until an internally controlled peak current limit. After the

power switch turns off, the voltage on the SW pin rises to

the output voltage multiplied by the primary-to-secondary

transformer turns ratio plus the input voltage. When the

secondary current through the output diode falls to zero,

the SW pin voltage collapses and rings around VIN. A

boundary mode detector senses this event and turns the

power switch back on.

Boundary conduction mode returns the secondary current

to zero every cycle, so parasitic resistive voltage drops

do not cause load regulation errors. Boundary conduc-

tion mode also allows the use of smaller transformers

compared to continuous conduction mode and does not

exhibit subharmonic oscillation.

Discontinuous Conduction Mode Operation

As the load gets lighter, boundary conduction mode

increases the switching frequency and decreases the

switch peak current at the same ratio. Running at a higher

switching frequency up to several MHz increases switch-

ing and gate charge losses. To avoid this scenario, the

LT8300 has an additional internal oscillator, which clamps

the maximum switching frequency to be less than 750kHz.

Once the switching frequency hits the internal frequency

clamp, the part starts to delay the switch turn-on and

operates in discontinuous conduction mode.

Low Ripple Burst Mode Operation

Unlike traditional flyback converters, the LT8300 has to

turn on and off at least for a minimum amount of time

and with a minimum frequency to allow accurate sampling

of the output voltage. The inherent minimum switch cur-

rent limit and minimum switch-off time are necessary to

guarantee the correct operation of specific applications.

As the load gets very light, the LT8300 starts to fold

back the switching frequency while keeping the mini-

mum switch current limit. So the load current is able to

decrease while still allowing minimum switch-off time for

the sample-and-hold error amplifier. Meanwhile, the part

switches between sleep mode and active mode, thereby

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