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

零件编号

产品描述 (功能)

比赛名单

LT3799I
(Rev.:RevA)

Offline Isolated Flyback LED Controller with Active PFC

Linear Technology 

LT3799

Operation

begins to switch. The output voltage information from the

FB pin is sampled but ignored until the CT pin reaches

1.25V. When this occurs, if the FB pin is above 1.25V, the

fault flag pulls low. The FAULT pin is meant to be used

with a large pull-up resistor to the INTVCC pin or another

supply. The CT pin begins to sink 200nA of current. When

the CT pin goes below 240mV, the part will re-enable itself,

begin to switch, and start to source 10µA of current to the

CT pin but not remove the fault condition. When the CT

pin reaches 1.25V and FB is below 1.25V, the FAULT pin

will no longer pull low and switching will continue. If not

below 1.25V, the process repeats itself.

Programming Output Current

The maximum output current depends on the supply

voltage and the output voltage in a flyback topology.

With the VIN_SENSE pin connected to 1V and a DC supply

voltage, the maximum output current is determined at

the minimum supply voltage, and the maximum output

voltage using the following equation:

IO U T(M A X )

=

2

•

(1−

D)

•

42

•

N

RSENSE

where

D = VOUT • N

VOUT • N + VIN

The maximum control voltage to achieve this maximum

output current is 2V • (1-D).

It is suggested to operate at 95% of these values to give

margin for the part’s tolerances.

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.

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 setting 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 fol-

lowing 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

3799fa

11

Share Link: