LT8302HS8E-3-WPBF 查看數據表（PDF） - Analog Devices

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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

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

LT8302/LT8302-3 has an additional internal oscillator,

which clamps the maximum switching frequency to be

less than 380kHz. 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 LT8302/

LT8302-3 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 current limit and minimum switch-off

time are necessary to guarantee the correct operation of

specific applications.

As the load gets very light, the LT8302/LT8302-3 starts to

fold back the switching frequency while keeping the min-

imum 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

reducing the effective quiescent current to improve light

load efficiency. In this condition, the LT8302/LT8302-3

runs in low ripple Burst Mode operation. The typical

12kHz minimum switching frequency determines how

often the output voltage is sampled and also the mini-

mum load requirement.

Difference Between LT8302 and LT8302-3

The difference between LT8302 and LT8302-3 is

the boundary detection method. The LT8302 is using the

dv/dt slope on RREF pin, while the LT8302-3 is using the

voltage level on RREF pin. For good transformers with

low leakage inductance, both the LT8302 and LT8302-3

are behaving the same. The LT8302-3 is recommended

for multiple-winding output applications due to its lower

sensitivity to the noise on RREF pin.

Rev. G

10

For more information www.analog.com

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