LT3799I(RevA) 查看數據表（PDF） - Linear Technology

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LT3799I
(Rev.:RevA)

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799

Operation

1.16

1.14

1.12

1.10

1.08

1.06

1.04

1.02

0

0.5

1

PEAK VIN_SENSE

1.5

3799 F02

Figure 2. Output Current Correction Factor

where CF is the output current correction factor on the

Y-axis in Figure 2.

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 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 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%.

Sense Resistor Selection

The resistor, RSENSE, between the source of the external

N-channel MOSFET and GND should be selected to provide

an adequate switch current to drive the application without

exceeding the current limit threshold .

For applications without power factor correction, select a

resistor according to:

RSENSE

=

2(1− D)N

IOUT • 42

•

95%

where

D = VOUT • N

VOUT • N + VIN

For applications with power factor correction, select a

resistor according to:

RSENSE

=

2(1− D)N

IOUT • 42

•

47.5%

where

D = VOUT • N

VOUT • N + VIN

Minimum Current Limit

The LT3799 features a minimum current limit of approxi-

mately 7% of the peak current limit. This is necessary when

operating in critical conduction mode since low current

limits would increase the operating frequency to a very

high frequency. The output voltage sensing circuitry needs

a minimum amount of flyback waveform time to sense the

output voltage on the third winding. The time needed is

350ns. The minimum current limit allows the use of smaller

3799fa

12

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