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X-11181-002 Datasheet(PDF) 11 Page - Linear Technology

Part No. X-11181-002
Description  Isolated No Opto-Coupler Flyback Controller
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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 11 page
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LT3798
11
3798f
Programming Output Current
The maximum output current depends on the supply volt-
age and the output voltage in a flyback topology. With the
VIN_SENSE pin connected to 100μA current source and a DC
supply voltage, the maximum output current is determined
at the minimum supply voltage, and the maximum output
voltage using the following equation:
IOUT(MAX) = 2•(1– D)•
NPS
42 • RSENSE
where
D
=
VOUT •NPS
VOUT •NPS + VIN
The maximum control voltage to achieve this maximum
output current is 2V • (1-D).
It is suggested to operate at 95% of these values to give
margin for the part’s tolerances.
When designing for power factor correction, the output
current waveform is going to have a half sine wave squared
shape and will no longer be able to provide the above
currents. By taking the integral of a sine wave squared
over half a cycle, the average output current is found to
be half the value of the peak output current. In this case,
the recommended maximum average output current is
as follows:
IOUT(MAX) = 2•(1−D) •
NPS
42 • RSENSE
• 47.5%
where
D
=
VOUT •NPS
VOUT •NPS + VIN
The maximum control voltage to achieve this maximum
output current is (1-D) • 47.5%.
For control voltages below the maximum, the output cur-
rent is equal to the following equation:
IOUT = CTRL •
NPS
42 • RSENSE
The VREF pin supplies a 2V reference voltage to be used
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:
R1
= R2
2NPS
42 •IOUT •RSENSE
–1
where R1 is the resistor connected to the VREF pin and the
CTRL pin and R2 is the resistor connected to the CTRL
pin and ground.
Setting VIN_SENSE Resistor
The VIN_SENSE resistor sets the current feeding the internal
multiplier that modulates the current limit for power factor
correction. At the maximum line voltage, VMAX, the current
is set to 360μA. Under this condition, the resistor value is
equal to (VMAX/360μA).
For DC input or non-PFC AC input applications, connect
a 25k resistor from VIN_SENSE to INTVCC instead of the
AC line voltage.
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 LT3798 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 trip-
ping due to leakage inductance ringing, a blanking time of
between 600ns and 2μs is applied after the switch turns off,
depending on the current limit shown in the Leakage In-
ductance Blanking Time vs SENSE Current Limit Threshold
curve in the Typical Performance Characteristics section.
The detector looks for 80μA 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 VIN + VOUT • NPS
and would waste all the energy stored in the parasitic ca-
pacitance 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
OPERATION




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