LT3799 Datasheet, PDF (13/20 Pages) Linear Technology – Offline Isolated Flyback LED Controller with Active PFC

LT3799
OPERATION
a blanking time of between 600ns and 2.25µs is applied
after the switch turns off, depending on the current limit
shown in the Leakage Inductance Blanking Time vs Cur-
rent Limit curve in the Typical Performance Characteristics
section. The detector looks for 40µA of current through
the DCM pin due to falling voltage on the third winding
when the secondary diode turns off. This detection is
important since the output current is calculated using this
comparator’s output. This is not the optimal time to turn
the switch on because the switch voltage is still close to
V_IN + V_OUT • N and would waste all the energy stored in the
parasitic capacitance on the switch node. Discontinuous
ringing begins when the secondary current reaches zero
and the energy in the parasitic capacitance on the switch
node transfers to the input capacitor. This is a second-
order network composed of the parasitic capacitance on
the switch node and the magnetizing inductance of the
primary winding of the transformer. The minimum volt-
age of the switch node during this discontinuous ring is
V_IN – V_OUT • N. The LT3799 turns the switch back on at
this time, during the discontinuous switch waveform, by
sensing when the slope of the switch waveform goes from
negative to positive using the dv/dt detector. This switching
technique may increase efficiency by 5%.
Sense Resistor Selection
The resistor, R_SENSE, between the source of the external
N-channel MOSFET and GND should be selected to provide
an adequate switch current to drive the application without
exceeding the current limit threshold .
For applications without power factor correction, select a
resistor according to:
R_SENSE
=
2(1− D)N
I_OUT • 42
•
95%
where
D = ^{VOUT • N}
V_OUT • N + V_IN
For applications with power factor correction, select a
resistor according to:
R_SENSE
=
(1− D)N
I_OUT • 42
•
47.5%
where
D = ^{VOUT • N}
V_OUT • N + V_IN
Minimum Current Limit
The LT3799 features a minimum current limit of approxi-
mately 7% of the peak current limit. This is necessary when
operating in critical conduction mode since low current
limits would increase the operating frequency to a very
high frequency. The output voltage sensing circuitry needs
a minimum amount of flyback waveform time to sense the
output voltage on the third winding. The time needed is
350ns. The minimum current limit allows the use of smaller
transformers since the magnetizing primary inductance
does not need to be as high to allow proper time to sample
the output voltage information.
Errors Affecting Current Output Regulation
There are a few factors affecting the regulation of current in
a manufacturing environment along with some systematic
issues. The main manufacturing issues are the winding
turns ratio and the LT3799 control loop accuracy. The
winding turns ratio is well controlled by the transformer
manufacturer’s winding equipment, but most transformers
do not require a tight tolerance on the winding ratio. We
have worked with transformer manufacturers to specify
±1% error for the turns ratio. Just like any other LED driver,
the part is tested and trimmed to eliminate offsets in the
control loop and an error of ±3% is specified at 80% of
the maximum output current. The error grows larger as
the LED current is decreased from the maximum output
current. At half the maximum output current, the error
doubles to ±6%.
There are a number of systematic offsets that may be elimi-
nated by adjusting the control voltage from the ideal voltage.
It is difficult to measure the flyback time with complete
accuracy. If this time is not accurate, the control voltage
needs to be adjusted from the ideal value to eliminate the
offset but this error still causes line regulation errors. If
the supply voltage is lowered, the time error becomes a
smaller portion of the switching cycle period so the offset
becomes smaller and vice versa. This error may be com-
pensated for at the primary supply voltage, but this does
3799p
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