LT3799EMSE-1-PBF Просмотр технического описания (PDF) - Linear Technology

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LT3799EMSE-1-PBF

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799-1

OPERATION

Discontinuous Mode Detection

The discontinuous mode detector uses AC-coupling to

detect the ringing on the third winding. A 10pF capacitor

with a 500Ω resistor in series is recommended in most

designs. Depending on the amount of leakage inductance

ringing, an additional current may be needed to prevent

false tripping from the leakage inductance ringing. A resis-

tor from INTVCC to the DCM pin adds this current. Up to

an additional 100μA of current may be needed in some

cases. The DCM pin is roughly 0.7V, therefore the resistor

value is selected using the following equation:

R = 10V − 0.7V

I

where I is equal to the additional current into the DCM pin.

Power Factor Correction/Harmonic Content

The LT3799-1 attains high power factor and low harmonic

content by making the peak current of the main power

switch proportional to the line voltage by using an internal

multiplier. A power factor of >0.97 is easily attainable for

most applications by following the design equations in

this data sheet. With proper design, LT3799-1 applications

meet IEC 6100-3-2 Class C harmonic standards.

Protection from Open LED and Shorted LED Faults

The LT3799-1 detects output overvoltage conditions by

looking at the voltage on the third winding. The third

winding voltage is proportional to the output voltage when

the main power switch is off and the secondary diode is

conducting current. Sensing the output voltage requires

delivering power to the output. Using the CT pin, the part

turns off switching when a overvoltage condition occurs

and rechecks to see if the overvoltage condition has cleared,

as described in “CT Pin and Faults” in the Operation section.

This greatly reduces the output current delivered to the

output but a Zener is required to dissipate 2% of the set

output current during an open LED condition. The Zener

diode’s voltage needs to be 10% higher than the output

voltage set by the resistor divider connected to the FB pin.

Multiple Zener diodes in series may be needed for higher

output power applications to keep the Zener’s temperature

within the specification.

During a shorted LED condition, the LT3799-1 operates at

the minimum operating frequency. In normal operation,

the third winding provides power to the IC, but the third

winding voltage is zero during a shorted LED condition.

This causes the part’s VIN UVLO to shutdown switching.

The part starts switching again when VIN has reached its

turn-on voltage.

37991f

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