lt3799 Linear Technology Corporation, lt3799 Datasheet - Page 12

• Manufacturer Part Number: lt3799
• Description: Lt3799 - Offline Isolated Flyback Led Controller With Active Pfc
• Manufacturer: Linear Technology Corporation

LT3799
The V
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:
where R1 is the resistor connected to the V
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 V
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
V
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
OPERATION
12
IN_SENSE
R1= R2
REF
pin supplies a 2V reference voltage to be used
is low, very little power is being delivered to
Figure 2. Correction Factor in Selecting the
Resistor Divider Resistors
IN_SENSE

 
42 • I
OUT
goes below 65mV and above 65mV
2N
• R
SENSE
− 1

 
REF
pin and the
with AC, the following equation should be used with the
correction factor:
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,
I
R1= R2
OUT
= CTRL •
Figure 3. Output Current Correction Factor
1.16
1.14
1.06
1.02
1.12
1.10
1.08
1.04

 
(42 • I
0
42 • R
OUT
0.5
• R
PEAK V
2N
SENSE
N
SENSE
IN_SENSE
− CF
1
• CF)
− 1
3799 F03
1.5

 
3799p