lt3799 Linear Technology Corporation, lt3799 Datasheet - Page 11

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

OPERATION
MOSFET off when the TRIAC is off, this power device is
kept on and sinks the current to properly load the TRIAC.
When the TRIAC turns on, the V
and enables the loop, but the current comparator is always
enabled and turns the switch off if it is tripped.
Start-Up
The LT3799 uses a hysteretic start-up to operate from
high offline voltages. A resistor connected to the supply
voltage protects the part from high voltages. This resis-
tor is connected to the V
capacitor. When the resistor charges the part up to 23V and
INTV
CT pin to 340mV and then starts to switch. The resistor
does not provide power for the part in steady state, but
relies on the capacitor to start-up the part, then the third
winding begins to provide power to the V
the resistor. An internal voltage clamp is attached to the
V
to go above the absolute maximum voltage of the pin.
The internal clamp is set at 25V and is capable of 28mA
(typical) of current at room temperature. But, ideally, the
resistor connected between the input supply and the V
pin should be chosen so that less than 10mA is being
shunted by this internal clamp.
CT Pin and Faults
The CT pin is a timing pin for the fault circuitry. When the
input voltages are at the correct levels, the CT pin sources
10µA of current. When the CT pin reaches 340mV, the part
begins to switch. The output voltage information from the
FB pin is sampled but ignored until the CT pin reaches
1.25V. When this occurs, if the FB pin is above 1.25V, the
fault flag pulls low. The FAULT pin is meant to be used
with a large pull-up resistor to the INTV
supply. The CT pin begins to sink 200nA of current. When
the CT pin goes below 240mV, the part will re-enable itself,
begin to switch, and start to source 10µA of current to the
CT pin but not remove the fault condition. When the CT
pin reaches 1.25V and FB is below 1.25V, the FAULT pin
will no longer pull low and switching will continue. If not
below 1.25V, the process repeats itself.
IN
pin to prevent the resistor current from allowing V
CC
is in regulation at 10V, the part begins to charge the
IN
pin on the part and also to a
IN_SENSE
CC
IN
pin detects this
pin or another
pin along with
IN
IN
Programming Output Current
The maximum output current depends on the supply
voltage and the output voltage in a flyback topology.
With the V
voltage, the maximum output current is determined at
the minimum supply voltage, and the maximum output
voltage using the following equation:
where
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:
where
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:
I
I
I
D =
D =
OUT(MAX)
OUT(MAX)
OUT
V
V
= CTRL •
OUT
OUT
IN_SENSE
V
V
OUT
OUT
• N + V
• N + V
= 2 • (1− D) •
= (1− D) •
• N
• N
42 • R
pin connected to 1V and a DC supply
IN
IN
N
42 • R
SENSE
42 • R
N
SENSE
N
SENSE
• 47.5%
LT3799
11
3799p