LT3799IMSE-1(RevA) 查看數據表（PDF） - Linear Technology

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LT3799IMSE-1
(Rev.:RevA)

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799-1

Operation

When designing for power factor correction, the output

current waveform is going to have a half sine wave squared

shape and will no longer be able to provide the above

currents. By taking the integral of a sine wave squared

over half a cycle, the average output current is found to

be half the value of the peak output current. In this case,

the recommended maximum average output current is

as follows:

IO U T(M A X )

= 2•(1−D) •

N

42 •RSENSE

• 47.5%

where

D = VOUT • N

VOUT • N+ VIN

The maximum control voltage to achieve this maximum

output current is (1-D) • 47.5%.

For control voltages below the maximum, the output cur-

rent is equal to the following equation:

IO U T

=

CTRL

•

42

N

• RSENSE

The VREF pin supplies a 2V reference voltage to be used

with the control pins. To set an output current, a resistor

divider is used from the 2V reference to one of the control

pins. The following equation sets the output current with

a resistor divider:

R1=

R2

⎛

⎜

⎝

4

2

•

2N

IOUT •

RSE

N

SE

⎞

− 1⎟

⎠

where R1 is the resistor connected to the VREF pin and the

CTRL pin and R2 is the resistor connected to the CTRL

pin and ground.

Critical Conduction Mode Operation

Critical conduction mode is a variable frequency switching

scheme that always returns the secondary current to zero

with every cycle. The LT3799-1 relies on boundary mode

and discontinuous mode to calculate the critical current

because the sensing scheme assumes the secondary

current returns to zero with every cycle. The DCM pin

uses a fast current input comparator in combination with

a small capacitor to detect dv/dt on the third winding. To

eliminate false tripping due to leakage inductance ringing,

a blanking time of between 600ns and 2.25µs is applied

after the switch turns off, depending on the current limit.

The detector looks for 40µA of current through the DCM

pin due to falling voltage on the third winding when the

secondary diode turns off. This detection is important

since the output current is calculated using this com-

parator’s output. This is not the optimal time to turn the

switch on because the switch voltage is still close to VIN

+ VOUT • N and would waste all the energy stored in the

parasitic capacitance on the switch node. Discontinuous

ringing begins when the secondary current reaches zero

and the energy in the parasitic capacitance on the switch

node transfers to the input capacitor. This is a second-

order network composed of the parasitic capacitance on

the switch node and the magnetizing inductance of the

primary winding of the transformer. The minimum volt-

age of the switch node during this discontinuous ring is

VIN – VOUT • N. The LT3799-1 turns the switch back on

at this time, during the discontinuous switch waveform,

by sensing when the slope of the switch waveform goes

from negative to positive using the dv/dt detector. This

switching technique may increase efficiency by 5%.

12

For more information www.linear.com/LT3799-1

37991fa

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