LT3799
OPERATION
MOSFEꢀ off when the ꢀRIAC is off, this power device is
kept on and sinks the current to properly load the ꢀRIAC.
When the ꢀRIAC turns on, the V
andenablestheloop, butthecurrentcomparatorisalways
Programming Output Current
ꢀhe maximum output current depends on the supply
voltage and the output voltage in a flyback topology.
pin detects this
IN_SENSE
With the V
pin connected to 1V and a DC supply
IN_SENSE
enabled and turns the switch off if it is tripped.
voltage, the maximum output current is determined at
the minimum supply voltage, and the maximum output
voltage using the following equation:
Start-Up
ꢀhe Lꢀ3799 uses a hysteretic start-up to operate from
high offline voltages. A resistor connected to the supply
voltage protects the part from high voltages. ꢀhis resis-
N
IOUꢀ(MAX) = 2 •(1− D) •
42 •RSENSE
tor is connected to the V pin on the part and also to a
IN
where
capacitor.Whentheresistorchargesthepartupto23Vand
VOUꢀ •N
VOUꢀ •N + V
INꢀV isinregulationat10V, thepartbeginstochargethe
D =
CC
IN
Cꢀ pin to 340mV and then starts to switch. ꢀhe resistor
does not provide power for the part in steady state, but
relies on the capacitor to start-up the part, then the third
ꢀhe maximum control voltage to achieve this maximum
output current is 2V • (1-D).
winding begins to provide power to the V pin along with
IN
the resistor. An internal voltage clamp is attached to the
It is suggested to operate at 9±5 of these values to give
margin for the part’s tolerances.
V pin to prevent the resistor current from allowing V
IN
IN
to go above the absolute maximum voltage of the pin.
ꢀhe internal clamp is set at 2±V and is capable of 28mA
(typical) of current at room temperature. But, ideally, the
When designing for power factor correction, the output
currentwaveformisgoingtohaveahalfsinewavesquared
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:
resistor connected between the input supply and the V
IN
pin should be chosen so that less than 10mA is being
shunted by this internal clamp.
CT Pin and Faults
ꢀhe Cꢀ pin is a timing pin for the fault circuitry. When the
input voltages are at the correct levels, the Cꢀ pin sources
10µA of current. When the Cꢀ pin reaches 340mV, the part
begins to switch. ꢀhe output voltage information from the
FB pin is sampled but ignored until the Cꢀ pin reaches
1.2±V. When this occurs, if the FB pin is above 1.2±V, the
fault flag pulls low. ꢀhe FAULT pin is meant to be used
N
IOUꢀ(MAX) = (1− D) •
• 47.±5
42 •RSENSE
where
D =
VOUꢀ •N
VOUꢀ •N + V
IN
with a large pull-up resistor to the INꢀV pin or another
CC
ꢀhe maximum control voltage to achieve this maximum
output current is (1-D) • 47.5%.
supply. ꢀhe Cꢀ pin begins to sink 200nA of current. When
the Cꢀ pin goes below 240mV, the part will re-enable itself,
begin to switch, and start to source 10µA of current to the
Cꢀ pin but not remove the fault condition. When the Cꢀ
pin reaches 1.2±V and FB is below 1.2±V, the FAULT pin
will no longer pull low and switching will continue. If not
below 1.2±V, the process repeats itself.
For control voltages below the maximum, the output cur-
rent is equal to the following equation:
N
IOUꢀ = CꢀRL •
42 •RSENSE
3799p
11
Linear [ Linear ]
