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LT3799
OPERATION
PEAK V IN_SENSE
R SENSE =
2(1 ? D)N
V OUT ? N
V OUT ? N + V IN
R SENSE =
2(1 ? D)N
V OUT ? N
V OUT ? N + V IN
1.16
1.14
1.12
1.10
1.08
1.06
1.04
1.02
0 0.5 1 1.5
3799 F02
Figure 2. Output Current Correction Factor
where CF is the output current correction factor on the
Y-axis in Figure 2.
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,
a blanking time of between 600ns and 2.25μs is applied
after the switch turns off, depending on the current limit.
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 com-
parator’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:
? 95%
I OUT ? 42
where
D =
For applications with power factor correction, select a
resistor according to:
? 47.5%
I OUT ? 42
where
D =
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
3799fa
12
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