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LT3799
OPERATION
R =
by the switch voltage clamp. Always check the switch
waveform with an oscilloscope to make sure the leakage
inductance voltage spike is below the breakdown voltage
of the MOSFET. A transient voltage suppressor and diode
are slower than the leakage inductance voltage spike,
therefore causing a higher voltage than calculated.
The secondary diode stress may be as much as
V OUT + 2 ? V IN /N PS due to the anode of the diode ringing
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 OUT + V IN /N PS .
With a high N PS and output current greater than 3A, the
I RMS through the diode can become very high and a low
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 CC to the DCM pin adds this current. Up to
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:
10V ? 0.7V
I
where I is equal to the additional current into the DCM pin.
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.
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 IN UVLO to shutdown switching.
The part starts switching again when V IN has reached its
turn-on voltage.
Power Factor Correction/Harmonic Content
The LT3799 attains high power factor and low harmonic
content by making the peak current of the main power
3799fa
15
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