LT3574 datasheet(7/24 Pages) AD

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LT8301

Rev. B

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OPERATION

The LT8301 is a current mode switching regulator IC

designed specially for the isolated flyback topology. The

key problem in isolated topologies is how to commu-

nicate the output voltage information from the isolated

secondary side of the transformer to the primary side

for regulation. Historically, opto-isolators or extra trans-

former windings communicate this information across

the isolation boundary. 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 life-

time. 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 LT8301 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 LT8301 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 LT8301 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 LT8301 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 sub-harmonic 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

LT8301 has an additional internal oscillator, which clamps

the maximum switching frequency to be less than 430kHz

(typ). Once the switching frequency hits the internal fre-

quency 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 LT8301 has to

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

and with a minimum frequency to allow accurate sampling

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