lt3799 Linear Technology Corporation, lt3799 Datasheet - Page 16

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

The secondary diode stress may be as much as
V
with the secondary leakage inductance. An RC snubber
in parallel with the diode eliminates this ringing, so that
the reverse voltage stress is limited to V
With a high N
I
forward drop Schottky is recommended.
Discontinuous Mode Detection
The discontinuous mode detector uses AC-coupling to
detect the ringing on the third winding. A 10pF capacitor
with a 500Ω resistor in series is recommended in most
designs. Depending on the amount of leakage inductance
ringing, an additional current may be needed to prevent
false tripping from the leakage inductance ringing. A resis-
tor from INTV
an additional 100µA of current may be needed in some
cases. The DCM pin is roughly 0.7V, therefore the resistor
value is selected using the following equation:
where I is equal to the additional current into the DCM pin.
Power Factor Correction/Harmonic Content
The LT3799 attains high power factor and low harmonic
content by making the peak current of the main power
switch proportional to the line voltage by using an internal
multiplier. A power factor of >0.97 is easily attainable for
most applications by following the design equations in this
datasheet. With proper design, LT3799 applications meet
IEC 6100-3-2 Class C harmonic standards.
LT3799
OPERATION
16
RMS
OUT
R =
through the diode can become very high and a low
+ 2 • V
10V − 0.7V
IN
I
PS
CC
/N
PS
to the DCM pin adds this current. Up to
and output current greater than 3A, the
due to the anode of the diode ringing
OUT
+ V
IN
/N
PS
.
Protection from Open LED and Shorted LED Faults
The LT3799 detects output overvoltage conditions by look-
ing at the voltage on the third winding. The third winding
voltage is proportional to the output voltage when the main
power switch is off and the secondary diode is conducting
current. Sensing the output voltage requires delivering
power to the output. Using the CT pin, the part turns off
switching when a overvoltage condition occurs and re-
checks to see if the overvoltage condition has cleared, as
described in “CT Pin and Faults” in the Operation section.
This greatly reduces the output current delivered to the
output but a Zener is required to dissipate 2% of the set
output current during an open LED condition. The Zener
diode’s voltage needs to be 10% higher than the output
voltage set by the resistor divider connected to the FB pin.
Multiple Zener diodes in series may be needed for higher
output power applications to keep the Zener’s temperature
within the specification.
During a shorted LED condition, the LT3799 operates at
the minimum operating frequency. In normal operation,
the third winding provides power to the IC, but the third
winding voltage is zero during a shorted LED condition.
This causes the part’s V
The part starts switching again when V
turn-on voltage.
IN
UVLO to shutdown switching.
IN
has reached its
3799p