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LT8302HS8E-3-WPBF

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch

Analog Devices 

LT8302/LT8302-3

OPERATION

The LT8302/LT8302-3 is a current mode switching regula-

tor 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 LT8302/LT8302-3 samples the isolated output voltage

through the primary-side flyback pulse waveform. In this

manner, neither opto-isolator nor extra transformer wind-

ing is required for regulation. Since the LT8302/LT8302-3

operates in either boundary conduction mode or discon-

tinuous 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 LT8302/LT8302-3 is a simple to use micropower iso-

lated flyback converter housed in a thermally enhanced

8-lead SO package. The output voltage is programmed

with two external resistors. An optional TC resistor

provides easy output diode temperature compensation.

By integrating the loop compensation and soft-start

inside, the part reduces the number of external compo-

nents. As shown in the Block Diagram, many of the blocks

are similar to those found in traditional switching reg-

ulators 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.

Quasi-Resonant Boundary Mode Operation

The LT8302/LT8302-3 features quasi-resonant bound-

ary conduction mode operation at heavy load, where

the chip turns on the primary power switch when the

secondary current is zero and the SW rings to its valley.

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 at its valley.

Rev. G

For more information www.analog.com

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