LT3799EMSE 查看數據表（PDF） - Linear Technology

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LT3799EMSE

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799

OPERATION

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.

When used with an AC input voltage, the LT3799 senses

when the VIN_SENSE goes below 65mV and above 65mV

for detecting when the TRIAC is off. During this low input

voltage time, the output current regulation loop is off but the

part still switches. This helps with output current regulation

with a TRIAC but introduces a line regulation error. When

VIN_SENSE is low, very little power is being delivered to

the output and since the output current regulation loop

is off, this time period needs to be accounted for in set-

ting the output current. This time period slightly varies

with line voltage. Figure 2 shows the correction factor

in selecting the resistor divider resistors. When used

with AC, the following equation should be used with the

correction factor:

IO U T

=

CTRL

•

42

N

• RSENSE

−

CF

R1 =

R2



 (42

•

IO U T

2N

• RSENSE

•

CF)

− 1

where CR is the output current correction factor on the

Y-axis in Figure 3.

Setting Control Voltages for LED Over Temperature

and Brownout Conditions

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,

Figure 2. Correction Factor in Selecting the

Resistor Divider Resistors

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 F03

Figure 3. Output Current Correction Factor

3799p

12

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